The shapes of the extreme wings of self-broadened CO₂(lines have been investigated)in three spectral regions near 7000, 3800, and 2400 cm^-1. Absorption measurements have been made on the high-wavenumber sides of band heads where much of the absorption by samples at a few atm is due to the extreme wings of strong lines whose centers occur below the band heads. New information has been obtained about the shapes of self-broadened CO₂ lines as well as CO₂ lines broadened by N₂, O₂, Ar, He, and H₂. Beyond a few cm^-1 from the line centers, all of the lines absorb less than Lorentz-shaped lines having the same half-widths. The deviation from the Lorentz shape decreases with increasing wavenumber, from one of the three spectral regions to the next. The absorption by the wings of H₂- and He-broadened lines is particularly low, and the absorption decreases with increasing temperature at a rate faster than predicted by existing theories.

The shapes of the extreme wings of self-broadened CO₂(lines have been investigated)in three spectral regions near 7000, 3800, and 2400 cm^-1. Absorption measurements have been made on the high-wavenumber sides of band heads where much of the absorption by samples at a few atm is due to the extreme wings of strong lines whose centers occur below the band heads. New information has been obtained about the shapes of self-broadened CO₂ lines as well as CO₂ lines broadened by N₂, O₂, Ar, He, and H₂. Beyond a few cm^-1 from the line centers, all of the lines absorb less than Lorentz-shaped lines having the same half-widths. The deviation from the Lorentz shape decreases with increasing wavenumber, from one of the three spectral regions to the next. The absorption by the wings of H₂- and He-broadened lines is particularly low, and the absorption decreases with increasing temperature at a rate faster than predicted by existing theories.

The collision‐induced absorption of the (0–0) and (1–0) bands of the ¹Δ_g ← ³Σ_g⁻ transition of oxygen has been studied in the gas phase from 87°–300°K. Low pressures were used and only binary collisions are important in inducing transitions. At low temperatures the absorption is enhanced because the attractive intermolecular potential causes clustering in the gas. However, no direct evidence for bound‐state (O₂)₂ molecules was found. The frequency maxima of the absorption of collision pairs were only slightly shifted from those calculated from monomer oxygen. This suggests that the shapes of the internuclear potentials and separation of the ¹Δ_g and ³Σ_g electronic states are little affected by these binary collisions. A near‐Boltzmann relation of the broad band shapes, moreover, suggests that the intermolecular potentials for ¹Δ_g⋅⋅⋅³Σ_g⁻and³Σ_g⁻⋅⋅⋅³Σ_g⁺ interactions are nearly the same. The mechanisms of the induced absorption collisions are discussed.

The collision‐induced absorption spectra have been measured at room temperature and at 87°K for bands in the ¹Δ_g + ¹Δ_g ← ³Σ_g⁻ + ³Σ_g⁻ and ¹Δ_g + ¹Σ_g⁺ ← ³Σ_g⁻ + ³Σ_g⁻ simultaneous electronic systems for oxygen. The binary absorption coefficients were found to increase with decreasing temperature for ¹Δ_g + ¹Δ_g ← ³Σ_g⁻ + ³Σ_g⁻. The band shapes for this system suggest that the Hamiltonian which is responsible for intensity borrowing depends on the angular orientation of the O₂ molecules in the collision pair since ΔK  =  0,± 2,± 4 selection rules are needed to account for the Δν_1 / 2  ∼  200cm⁻¹ bandwidth. The relative intensity of the (1–0) and (0–0) bands indicates that the interaction Hamiltonian is also strongly modulated by the vibrational coordinates of O₂. The frequency shift of this simultaneous transition indicates that the intermolecular distance parameter for ¹Δ_g⋅⋅⋅¹Δ_g is 3% larger than for ³Σ_g⁻⋅⋅⋅³Σ_g⁻. The unusual band shape for the ¹Δ_g + ¹Σ_g⁺ ← ³Σ_g⁻ + ³Σ_g⁻ band is interpreted in terms of an exiton interaction for the ¹Δ_g⋅⋅⋅¹Σ_g^>+ combination. Although bound state (O₂)₂ molecules undoubtedly exist at low temperatures these data provide no unambiguous spectroscopic evidence of their presence.

The collision‐induced absorption spectra have been measured at room temperature and at 87°K for bands in the ¹Δ_g + ¹Δ_g ← ³Σ_g⁻ + ³Σ_g⁻ and ¹Δ_g + ¹Σ_g⁺ ← ³Σ_g⁻ + ³Σ_g⁻ simultaneous electronic systems for oxygen. The binary absorption coefficients were found to increase with decreasing temperature for ¹Δ_g + ¹Δ_g ← ³Σ_g⁻ + ³Σ_g⁻. The band shapes for this system suggest that the Hamiltonian which is responsible for intensity borrowing depends on the angular orientation of the O₂ molecules in the collision pair since ΔK  =  0,± 2,± 4 selection rules are needed to account for the Δν_1 / 2  ∼  200cm⁻¹ bandwidth. The relative intensity of the (1–0) and (0–0) bands indicates that the interaction Hamiltonian is also strongly modulated by the vibrational coordinates of O₂. The frequency shift of this simultaneous transition indicates that the intermolecular distance parameter for ¹Δ_g⋅⋅⋅¹Δ_g is 3% larger than for ³Σ_g⁻⋅⋅⋅³Σ_g⁻. The unusual band shape for the ¹Δ_g + ¹Σ_g⁺ ← ³Σ_g⁻ + ³Σ_g⁻ band is interpreted in terms of an exiton interaction for the ¹Δ_g⋅⋅⋅¹Σ_g^>+ combination. Although bound state (O₂)₂ molecules undoubtedly exist at low temperatures these data provide no unambiguous spectroscopic evidence of their presence.

The two components of the (ν₁, 2ν₂) Fermi doublet of gaseous CO₂ in an absorption path length of 56 m at 192 °K show a complex structure. When the allowed C¹⁶O¹⁸O absorption and the pressure-induced CO₂ absorption are removed by computational procedures, the residual spectrum consists of two similar symmetric patterns of five maxima. These are interpreted in terms of the rotation and vibration of (CO₂)₂ dimers held by quadrupole–quadrupole interaction in the locked T position at an intermolecular distance of 4.1 Å.

By means of a photoelectrical technique used in conjunction with a 3 m vacuum monochromator, the absorption cross section of the O₂ continuum in the region 2350–1814 Å and the absorption cross section of CO₂ in the region 2160–1718 Å have been measured. The cross section of the O₂ continuum is 3.8 × 10⁻²⁴ cm² at 2350 Å; it slowly increases towards shorter wavelengths and reaches 10.7 × 10⁻²⁴ cm² at about 1980 Å, then increases very rapidly and reaches 7.1 × 10⁻²² cm² at about 1814 Å. In the case of CO₂, numerous discrete bands were found overlapping a weak continuum in the wavelength region below 1980 Å. The absorption cross section of the CO₂ continuum is about 2 × 10⁻²⁴ cm² at 2100 Å; it gradually increases toward the shorter wavelength side, and reaches about 4 × 10⁻²⁴ cm² at 2000 Å. The continuum rises rapidly at 2000 Å and its value is 1.19 × 10⁻²⁰ cm² at 1718 Å. Origins of the continua of both O₂ and CO₂ are briefly discussed.

By means of a photoelectrical technique used in conjunction with a 3 m vacuum monochromator, the absorption cross section of the O₂ continuum in the region 2350–1814 Å and the absorption cross section of CO₂ in the region 2160–1718 Å have been measured. The cross section of the O₂ continuum is 3.8 × 10⁻²⁴ cm² at 2350 Å; it slowly increases towards shorter wavelengths and reaches 10.7 × 10⁻²⁴ cm² at about 1980 Å, then increases very rapidly and reaches 7.1 × 10⁻²² cm² at about 1814 Å. In the case of CO₂, numerous discrete bands were found overlapping a weak continuum in the wavelength region below 1980 Å. The absorption cross section of the CO₂ continuum is about 2 × 10⁻²⁴ cm² at 2100 Å; it gradually increases toward the shorter wavelength side, and reaches about 4 × 10⁻²⁴ cm² at 2000 Å. The continuum rises rapidly at 2000 Å and its value is 1.19 × 10⁻²⁰ cm² at 1718 Å. Origins of the continua of both O₂ and CO₂ are briefly discussed.

By means of a photoelectrical technique used in conjunction with a 3 m vacuum monochromator, the absorption cross section of the O₂ continuum in the region 2350–1814 Å and the absorption cross section of CO₂ in the region 2160–1718 Å have been measured. The cross section of the O₂ continuum is 3.8 × 10⁻²⁴ cm² at 2350 Å; it slowly increases towards shorter wavelengths and reaches 10.7 × 10⁻²⁴ cm² at about 1980 Å, then increases very rapidly and reaches 7.1 × 10⁻²² cm² at about 1814 Å. In the case of CO₂, numerous discrete bands were found overlapping a weak continuum in the wavelength region below 1980 Å. The absorption cross section of the CO₂ continuum is about 2 × 10⁻²⁴ cm² at 2100 Å; it gradually increases toward the shorter wavelength side, and reaches about 4 × 10⁻²⁴ cm² at 2000 Å. The continuum rises rapidly at 2000 Å and its value is 1.19 × 10⁻²⁰ cm² at 1718 Å. Origins of the continua of both O₂ and CO₂ are briefly discussed.

By means of a photoelectrical technique used in conjunction with a 3 m vacuum monochromator, the absorption cross section of the O₂ continuum in the region 2350–1814 Å and the absorption cross section of CO₂ in the region 2160–1718 Å have been measured. The cross section of the O₂ continuum is 3.8 × 10⁻²⁴ cm² at 2350 Å; it slowly increases towards shorter wavelengths and reaches 10.7 × 10⁻²⁴ cm² at about 1980 Å, then increases very rapidly and reaches 7.1 × 10⁻²² cm² at about 1814 Å. In the case of CO₂, numerous discrete bands were found overlapping a weak continuum in the wavelength region below 1980 Å. The absorption cross section of the CO₂ continuum is about 2 × 10⁻²⁴ cm² at 2100 Å; it gradually increases toward the shorter wavelength side, and reaches about 4 × 10⁻²⁴ cm² at 2000 Å. The continuum rises rapidly at 2000 Å and its value is 1.19 × 10⁻²⁰ cm² at 1718 Å. Origins of the continua of both O₂ and CO₂ are briefly discussed.

The infrared fundamental band and the five strongest near-infrared and visible electronic bands of gaseous oxygen were studied from 90 to 115 K with path lengths up to 140 m in two low-temperature multiple-traversal absorption cells. The profile of the fundamental band is in good agreement with the theory of quadrupole-induced absorption except for a low-intensity residual in the Q-branch region. Although the electronic bands are less amenable to complete analysis, the general validity of a Boltzmann relation in their intensity distributions confirms their collision-induced nature. The temperature variation of the integrated band intensities is indicative of quadrupole induction for the fundamental and of overlap induction for the electronic bands; a somewhat too sharp rise at low temperatures may be due to the neglect of the quadrupole–quadrupole coupling in evaluating the pair distribution function.

The infrared fundamental band and the five strongest near-infrared and visible electronic bands of gaseous oxygen were studied from 90 to 115 K with path lengths up to 140 m in two low-temperature multiple-traversal absorption cells. The profile of the fundamental band is in good agreement with the theory of quadrupole-induced absorption except for a low-intensity residual in the Q-branch region. Although the electronic bands are less amenable to complete analysis, the general validity of a Boltzmann relation in their intensity distributions confirms their collision-induced nature. The temperature variation of the integrated band intensities is indicative of quadrupole induction for the fundamental and of overlap induction for the electronic bands; a somewhat too sharp rise at low temperatures may be due to the neglect of the quadrupole–quadrupole coupling in evaluating the pair distribution function.

Collision induced microwave absorption is reported in pure CO₂, and CO₂-Ar, CO₂-CH₄ mixtures in the 70 G H z (2.3 cm^-1) region at a temperature of 22°C, using a sensitive cavity technique previously described. The results in pure CO₂ in the very low density region from 5 to 30 amagat accurately establish the dependence of the loss on the square and cube of the density, and the relaxation times are calculated. The experimental results agree well with previously reported lower frequency data at 0.3-0.8 cm^-1 which establishes the linear dependence on frequency of the absorption up to 2.3 cm^-1. There is also good agreement with an extrapolation of higher frequency infrared results of Ho et al. The relaxation times associated with the two and three body collisions are shown to be nearly equal at room temperatures with T~ = 0.84 x 10^-12 s and T~ = 1.0 x 10^-12 S. Higher order dependence on the density is observed for the CO₂-Ar and CO₂-CH₄ mixtures. The results are compared with earlier low frequency measurements at 0.8 cm^-1 and with the theory of Maryott and Kryder, taking account of correction terms in the dielectric virial coefficient according to Bose and Cole.

The continuum absorption by H₂O has several characteristics that are common throughout the windows in the infrared and millimeter-wave regions. Values of the continuum absorption coefficient calculated on the basis of simple, widely used line shapes may differ greatly from observed values in the windows between strong absorption lines. The temperature dependence of this absorption is also not predictable from present day understanding of line shapes or of dieters, which may also contribute. The shapes of self-broadened H₂O lines are quite different from those of N₂-broadened lines, and the difference increases with increasing distance from the centers of the lines. Data obtained from laboratory samples and from atmospheric paths are presented to compare the various windows in the infrared and millimeter regions. 

Absorptions of the ρ_στ, 0.8 μm and a bands of water vapor were measured using a long pathlength, multiple-traversal absorption cell. The band intensities were found to be 842.3, 53.2, and 50.12 cm^-1/g-cm^-2 at 334 K for the ρ_στ, 0.8 μm and a bands, respectively. The measured equivalent widths gradually exceed calculated values for the Lorentz line profile and the self-broadening coefficients of 5 with increasing absorber amounts. Possible causes of this excess absorption are discussed.

Absorption of 9.6-µm CO₂ laser radiation by CO₂ at temperatures between 296 and 625 K has been measured at a pressure of 200 Torr. Experimental results for the R10—R26 and P10—P28 transitions have been obtained and compared with computed values of absorption. The relative optical broadening coefficients due to He and N₂ have been measured on the R16—R22 and P16—P22 transitions over the same temperature range.

Using a tunable source of monochromatic radiation (BWO) and a pneumatic detector, the field and laboratory investigations have been performed to obtain spectral distribution of atmospheric water vapor absorption coefficient in transparent windows centered at the wavelengths of 0.88 and 0.73 mm.

Using a tunable source of monochromatic radiation (BWO) and a pneumatic detector, the field and laboratory investigations have been performed to obtain spectral distribution of atmospheric water vapor absorption coefficient in transparent windows centered at the wavelengths of 0.88 and 0.73 mm.

The collision-induced absorption (CIA) spectrum for nitrogen has been measured in the spectral region below 360 cm⁻¹ at 126, 149, 179, and 212 K. The measurements have been obtained using Fourier transform infrared (FTIR) techniques, a far infrared (FIR) laser system operating at 84.2 and 15.1 cm⁻¹, and microwave cavity techniques. The experimental line shapes have been compared with the theoretical predictions of Joslin, based on Mori theory, and of Joslin and Gray, based on information theory alone. The data have been used to determine the quadrupole moment employing various intermolecular potentials. One Lennard–Jones potential has resulted in a quadrupole moment of 1.51 B, the value that was used in generating the theoretical line shapes. These results, when combined with our forthcoming measurements on nitrogen mixed with methane and argon, may be helpful in determining the role of CIA in calculating the opacity of some planetary atmospheres.

A practical atmospheric Millimeter‐Wave Propagation Model (MPM) is formulated that predicts attenuation, delay, and noise properties of moist air for frequencies up to 1000 GHz. Input variables are height distributions (0–30 km) of pressure, temperature, humidity, and suspended droplet concentration along an anticipated radio path. Spectroscopic data consist of more than 450 parameters describing local O₂ and H₂O absorption lines complemented by continuum spectra for dry air, water vapor, and hydrosols. For a model (MPM^*) limited to frequencies below 300 GHz, the number of spectroscopic parameters can be reduced to less than 200. Recent laboratory measurements by us at 138 GHz of absolute attenuation rates for simulated air with water vapor pressures up to saturation allow the formulation of an improved, though empirical water vapor continuum. Model predictions are compared with selected (2.5–430 GHz) data from both laboratory and field experiments. In general, good agreement is obtained.

The temperature dependence of the high frequency far wings of the self-broadened CO₂ lines has been investigated in the 2400–2600-cm^-1 spectral region. The temperature dependence of the corrective shape factor X(σ,T) is demonstrated for the first time.

The temperature dependence of the high frequency far wings of the self-broadened CO₂ lines has been investigated in the 2400–2600-cm^-1 spectral region. The temperature dependence of the corrective shape factor X(σ,T) is demonstrated for the first time.

Existing laboratory measurements of the far-infrared collision-induced spectra of gaseous nitrogen at temperatures from 124 to 300 K are analyzed on the basis of quantum line shapes computed from a suitable, advanced isotropic potential and multipole-induced dipole functions. The input is chosen to represent most closely the measurements at all temperatures and over the full range of frequencies. Simple analytical expressions are specified which represent the spectral profiles closely. It is thus possible to reproduce the collision-induced absorption spectra of nitrogen effortlessly in seconds at temperatures from 50 to 300 K on small computers, even in the far wings which never have been modeled from a quantum formalism before. The work thus gives new and reliable spectral intensities and their temperature dependence for a detailed analysis of the Voyager IRIS spectra of Titan's atmosphere.

The rototranslational absorption spectrum of gaseous methane has been measured at seven different temperatures from 296 to 140 K. We have analyzed both the spectral moments and the experimental absorption shapes, assuming that only octupolar and hexadecapolar induction mechanisms contribute to the absorption. This assumption allows us to parameterize the temperature dependence of both the intensity and the shape of the absorption band. The results obtained indicate that other contributions to absorption are not negligible.

Existing rototranslational collision-induced absorption (CIA) spectra of methane pairs are analyzed with the help of spectral profiles computed from quantum mechanics. Dipoles induced by octopolar and hexadecapolar fields, hexadecapolar overlap, and the gradient of the octopolar field are considered. The spectral contributions of both bound and free pairs of molecules are accounted for. The analysis which suggests a centrifugal distortion of rotating methane molecules permits one to reproduce from theory the measured CIA spectra at all temperatures (126–300 K) and over the full range of frequencies (> 700 cm⁻¹ at high temperatures) with rms deviations that are smaller than the experimental uncertainties. The values of the octopole and hexadecapole moments of (nonrotating) CH₄ molecules needed for that purpose are consistent with state-of-the-art ab initio computations for the first time in such work.

In previous work (Borysow and Frommhold, 1986) rigorous quantum computations of the rototranslational absorption spectra of CH4-CH4 pairs have been communicated which closely reproduce existing laboratory measurements at temperatures from 124 to 300 K and over a range of frequencies from 0 to 750 per cm. Since the computations are complex, this work shows how the spectra can be reproduced from simple, analytical functions that closely model the quantum profiles and, thus, the laboratory measurements. For temperatures from 50 to 300 K, the rototranslational, collision-induced spectra of methane pairs can thus be obtained accurately on small computers in seconds. No measurements exist for comparison with the computational results at the lower temperatures (less than 124 K) but these data are believed to constitute the most dependable theoretical prediction that can presently be made.

The rototranslational absorption spectrum of gaseous N₂ is analyzed, considering quadrupolar and hexadecapolar induction mechanisms. The available experimental data are accounted for by using a line-shape analysis in which empirical profiles describe the single-line translational profiles. We thus derive the simple procedure that allows one to predict the N₂ spectrum at any temperature. On the basis of the results obtained for the pure gas, we also propose a procedure to compute the far-infrared spectrum of the N₂–Ar gaseous mixture. The good agreement between computed and experimental N₂–Ar data indicates that it is possible to predict the far-infrared absorption induced by N₂ on the isotropic polarizability of any interacting partner.

The rototranslational absorption spectrum of gaseous N₂ is analyzed, considering quadrupolar and hexadecapolar induction mechanisms. The available experimental data are accounted for by using a line-shape analysis in which empirical profiles describe the single-line translational profiles. We thus derive the simple procedure that allows one to predict the N₂ spectrum at any temperature. On the basis of the results obtained for the pure gas, we also propose a procedure to compute the far-infrared spectrum of the N₂–Ar gaseous mixture. The good agreement between computed and experimental N₂–Ar data indicates that it is possible to predict the far-infrared absorption induced by N₂ on the isotropic polarizability of any interacting partner.

A spectrum of the carbon dioxide trimer van der Waals species has been recorded near 3614 cm⁻¹ at sub‐Doppler resolution using an optothermal (bolometer‐detected) molecular‐beam color‐center laser spectrometer. A planar, cyclic structure with C_3h symmetry has been determined for the complex with a carbon–carbon separation of 4.0382(3) Å. The observed perpendicular band, corresponding to an in‐plane E^′‐symmetry vibration of the trimer, has been attributed to a localized excitation of the 2ν⁰₂ +ν₃ combination mode of a CO₂ subunit by virtue of its small blue shift (∼0.98 cm⁻¹) from that of the isolated monomer.

This is a comprehensive study of water-vapor line and continuum absorption in the 8–14μm atmospheric window by laser-photoacoustic spectroscopy. The characteristics of laser-photoacoustic spectroscopy and detectors are discussed with results on continuum and line absorption at selected CO₂-laser wavelengths.

We have assigned four weak absorption lines which occur at the CO₂-laser emissions 10P(40), 10R(20), 9P(38) and 9R(36) to pure rotational transitions of H₂O, and have determined the dependence of the continuum water-vapor absorption over the temperature range +70°C and −20°C. The measured negative temperature coefficient of the continuum is consistent with both monomer and dimer models, yet not with predictions of larger water clusters.

Experiments with supersaturated water vapor indicate that for S ⩾ 1 collision broadening of distant strong lines as well as water dimer absorption contribute to the continuum. However, the dimer absorption is an order of magnitude too small to cause a significant contribution at ambient atmospheric conditions.

We have investigated the effect of UV-radiation on the 8–14μm absorption of water vapor, buffered either with N₂ or synthetic air. The observed changes are explained by UV-photodissociation of H₂O molecules and by ozone production. There is no evidence in favor of a cluster model.

Finally, we compared our measured spectra with LOWTRAN 6 and HITRAN models. The LOWTRAN yields a stronger negative temperature dependence than observed while the HITRAN does not predict the observed continuum absorption.

This is a comprehensive study of water-vapor line and continuum absorption in the 8–14μm atmospheric window by laser-photoacoustic spectroscopy. The characteristics of laser-photoacoustic spectroscopy and detectors are discussed with results on continuum and line absorption at selected CO₂-laser wavelengths.

We have assigned four weak absorption lines which occur at the CO₂-laser emissions 10P(40), 10R(20), 9P(38) and 9R(36) to pure rotational transitions of H₂O, and have determined the dependence of the continuum water-vapor absorption over the temperature range +70°C and −20°C. The measured negative temperature coefficient of the continuum is consistent with both monomer and dimer models, yet not with predictions of larger water clusters.

Experiments with supersaturated water vapor indicate that for S ⩾ 1 collision broadening of distant strong lines as well as water dimer absorption contribute to the continuum. However, the dimer absorption is an order of magnitude too small to cause a significant contribution at ambient atmospheric conditions.

We have investigated the effect of UV-radiation on the 8–14μm absorption of water vapor, buffered either with N₂ or synthetic air. The observed changes are explained by UV-photodissociation of H₂O molecules and by ozone production. There is no evidence in favor of a cluster model.

Finally, we compared our measured spectra with LOWTRAN 6 and HITRAN models. The LOWTRAN yields a stronger negative temperature dependence than observed while the HITRAN does not predict the observed continuum absorption.

This is a comprehensive study of water-vapor line and continuum absorption in the 8–14μm atmospheric window by laser-photoacoustic spectroscopy. The characteristics of laser-photoacoustic spectroscopy and detectors are discussed with results on continuum and line absorption at selected CO₂-laser wavelengths.

We have assigned four weak absorption lines which occur at the CO₂-laser emissions 10P(40), 10R(20), 9P(38) and 9R(36) to pure rotational transitions of H₂O, and have determined the dependence of the continuum water-vapor absorption over the temperature range +70°C and −20°C. The measured negative temperature coefficient of the continuum is consistent with both monomer and dimer models, yet not with predictions of larger water clusters.

Experiments with supersaturated water vapor indicate that for S ⩾ 1 collision broadening of distant strong lines as well as water dimer absorption contribute to the continuum. However, the dimer absorption is an order of magnitude too small to cause a significant contribution at ambient atmospheric conditions.

We have investigated the effect of UV-radiation on the 8–14μm absorption of water vapor, buffered either with N₂ or synthetic air. The observed changes are explained by UV-photodissociation of H₂O molecules and by ozone production. There is no evidence in favor of a cluster model.

Finally, we compared our measured spectra with LOWTRAN 6 and HITRAN models. The LOWTRAN yields a stronger negative temperature dependence than observed while the HITRAN does not predict the observed continuum absorption.

An F‐center laser–molecular beam spectrometer has been used to obtain a sub‐Doppler resolution infrared spectrum of the carbon dioxide dimer. The vibrational mode investigated in this study corresponds to the ν₁+ν₃ combination mode of the monomer located at 3716 cm⁻¹. A qualitative assignment of the spectrum shows unambiguously that the equilibrium structure of the dimer is the slipped parallel, rather than the T‐shaped, geometry. The observed spectrum cannot be fit to within experimental error using conventional asymmetric rotor formalism. This may be due to a number of factors such as Fermi resonance between the upper state levels of the band and nearby levels of the dimer, such as seen in the monomer, or it could arise from tunneling effects arising from the two large amplitude motions in the dimer.

Existing measurements of the collision-induced rototranslational absorption spectrum of gaseous mixtures of helium and methane are analyzed using quantum profiles computed from recent interaction potentials and various assumptions concerning the interaction-induced dipole components. The computed profiles are expressed in terms of simple functions that reproduce closely both the computed spectra at temperatures from 40 to 350 K and the existing measurements at temperatures from 150 to 353 K. Such a simple analytical representation of the HeCH₄ rototranslational spectra facilitates accurate modeling of planetary atmospheres in the far-infrared region of the spectrum.

The absorption by pure CO₂ beyond the ν₃ bandhead has been measured with a grating spectrometer. Experiments have been made in the 0–60-bar and 291–751-K pressure and temperature ranges. Our room temperature determinations are in good agreement with previous ones and the measured temperature dependence above room temperature is consistent with recent determinations below 300 K. Lorentzian calculations, modified by the introduction of a line shape corrective factor x, are presented. Good agreement between the observed and calculated spectra is obtained when a temperature independent x factor, determined by Cousin et al. at 296 K, is used.

We present experimental results on the absorption by CO₂-N₂ mixtures in the 4.3 μm region. Measurements have been made in the 300–800 K and 0–60 bar ranges; these are in good agreement with previous determinations below 296 K. Frequency- and temperature-dependent factors χ are introduced in order to account for the subLorentzian behavior of the CO₂ far line-wings. Their dependences on temperature are deduced from experimental results beyond the 4.3μm band-head in the 193–773 K range for both CO₂-CO₂ and CO₂-N₂ and fitted by simple analytical functions. Comparisons are presented between experimental and theoretical spectra on both the low- and high-frequency sides of the band (2100–2600 cm^-1 range). It is shown that calculations using χ factors are inaccurate near line-centers at elevated pressures.

An attempt is made to describe the observed band shape deformations of a number of CO₂ vibro-rotational absorption bands (1.4, 1.6 and 2.0 pm) making use of the ideas on the associative equilibrium in compressed gas. It is shown that the model of monomer-dimer equilibrium permits representation of the IR absorption band shapes within an experimental accuracy up to 5.0 MPa. The comparison of the found dimeric band shape with the calculated synthetic spectra of P- and T-isomers is also discussed.

Spectra of the weakly bound van der Waals complexes H₂–N₂ and H₂–CO have been studied in the mid‐ and far‐infrared regions corresponding to H₂ vibrational and pure rotational frequencies, respectively. The experiments were done using a Fourier transform infrared spectrometer to study equilibrium gas samples at low temperature (77 K) with a long absorption path. The spectra of the complexes appear as fine structure located near the peaks of the lines in the collision‐induced spectrum of hydrogen. Compared to earlier studies of these species, the present results have more complete coverage of the various possible transitions, better resolution, and better signal to noise. New structure is reported which corresponds to hindered rotational transitions of the N₂ component of an H₂–N₂ complex and is reminiscent of patterns observed for the N₂–Ar and (N₂)₂ complexes. The relation of these results to far‐infrared measurements of Saturn’s satellite Titan, made by the Voyager spacecraft, is discussed.

Thermal emission from the deep atmosphere of Venus can be detected on the nightside around 2.3 μm. The analysis of this radiation requires a reliable knowledge of the absorption in the far wings of the nearby allowed CO2 bands and of the absorption due to collision‐induced bands. We measured absorption coefficients for pure CO2 at pressures varying from 30 to 60 bars in the frequency range 3910–4570 cm−1 at 297.5 K. Values between 1.0 and 1.6 × 10−7 cm−1 amagat−2 are found in the 4100–4500 cm−1 interval where emission from the Venus nightside occurs. The comparison of experimental results with synthetic spectra calculated from a line by line code demonstrates that the Lorentzian line shape strongly overestimates the observed absorption, whereas the use of a χ factor extrapolated from the 3800–4000 cm−1 region does not provide enough opacity.

The shape of the far wing of self- and N₂-broadened CO₂ lines has been investigated in the 2150–2250-cm⁻¹ spectral region, i.e., on the low wavenumber side of the lines of the very intense v₃ band of ¹²C¹⁶O₂ in a temperature range of atmospheric interest (200–300 K). The experimental results have been compared to calculated values based on the AFGL 1986 compilation. It appears that a symmetrical sub-Lorentzian line shape based on experiments made on the high wavenumber side cannot reproduce experiments. Comparison with experiments made at room temperature shows that the asymmetry of the correcting line shape factor χ strongly increases when decreasing the temperature.

The shape of the far wing of self- and N₂-broadened CO₂ lines has been investigated in the 2150–2250-cm⁻¹ spectral region, i.e., on the low wavenumber side of the lines of the very intense v₃ band of ¹²C¹⁶O₂ in a temperature range of atmospheric interest (200–300 K). The experimental results have been compared to calculated values based on the AFGL 1986 compilation. It appears that a symmetrical sub-Lorentzian line shape based on experiments made on the high wavenumber side cannot reproduce experiments. Comparison with experiments made at room temperature shows that the asymmetry of the correcting line shape factor χ strongly increases when decreasing the temperature.

We present a simple model which allows for the computation of the rototranslational band of the collision induced absorption spectra of N₂-CH₄ pairs at temperatures between 70 and 300 K and at frequencies of up to ~550 cm^-1. The agreement with current experimental data has been obtained by adjusting the classical, multipole induced dipolar components by adding semiempirical quantum corrections. We have included a Q₆-induction term, never considered before, which we believe is essential for the agreement we have obtained. With the set of temperature-independent parameters derived from fitting the experimental data, our model reproduces recent laboratory measurements within 124-300 K temperature and the 10-600 cm^-1 frequency range, within few percent root mean square. The method provides reliable temperature dependence of the absorption coefficient as a function of frequency at temperatures as low as 70 K, which are much lower than those at which laboratory measurements are taken. The work is of significance for modeling the infrared opacity of Titan's atmosphere.

We present a simple model which allows for the computation of the rototranslational band of the collision induced absorption spectra of N₂-CH₄ pairs at temperatures between 70 and 300 K and at frequencies of up to ~550 cm^-1. The agreement with current experimental data has been obtained by adjusting the classical, multipole induced dipolar components by adding semiempirical quantum corrections. We have included a Q₆-induction term, never considered before, which we believe is essential for the agreement we have obtained. With the set of temperature-independent parameters derived from fitting the experimental data, our model reproduces recent laboratory measurements within 124-300 K temperature and the 10-600 cm^-1 frequency range, within few percent root mean square. The method provides reliable temperature dependence of the absorption coefficient as a function of frequency at temperatures as low as 70 K, which are much lower than those at which laboratory measurements are taken. The work is of significance for modeling the infrared opacity of Titan's atmosphere.

We present a simple model which allows for the computation of the rototranslational band of the collision induced absorption spectra of N₂-CH₄ pairs at temperatures between 70 and 300 K and at frequencies of up to ~550 cm^-1. The agreement with current experimental data has been obtained by adjusting the classical, multipole induced dipolar components by adding semiempirical quantum corrections. We have included a Q₆-induction term, never considered before, which we believe is essential for the agreement we have obtained. With the set of temperature-independent parameters derived from fitting the experimental data, our model reproduces recent laboratory measurements within 124-300 K temperature and the 10-600 cm^-1 frequency range, within few percent root mean square. The method provides reliable temperature dependence of the absorption coefficient as a function of frequency at temperatures as low as 70 K, which are much lower than those at which laboratory measurements are taken. The work is of significance for modeling the infrared opacity of Titan's atmosphere.

We present experimental and theoretical studies of medium infrared absorption by pure water vapor. Measurements have been made in the 1900–2600 cm^-1 and 3900–4600 cm^-1 regions, for temperatures and pressures in the 500–900 K and 0–70 atm ranges, respectively. They are consistent with available data and enable the determination of continuum absorption parameters. It is shown that calculations with line shapes derived from the impact approximation are very inaccurate. Models accounting for the finite durations of collisions and line-mixing through wave-number dependent effective broadening parameters are introduced. The latter have been determined using two different approaches, which are (i) empirical determinations from fits of experimental data and (ii) direct predictions from first principles using a statistical approach. Effective broadening parameters obtained using these two different approaches are in satisfactory agreement for both the temperature and wavenumber dependencies. These data are tested by calculations of continua in various spectral regions and the agreement with measured values is satisfactory. The remaining discrepancies probably result from the influence of the internal structures of the absorption bands considered and thus from the influence of line-mixing. Nevertheless, accurate predictions are obtained in wide temperature and spectral ranges when the total absorption at elevated density is considered. This agreement, which is due to the relatively weak continuum absorption and large contributions of nearby lines, makes the present models suitable for most practical applications involving elevated densities.

We present experimental and theoretical studies of medium infrared absorption by pure water vapor. Measurements have been made in the 1900–2600 cm^-1 and 3900–4600 cm^-1 regions, for temperatures and pressures in the 500–900 K and 0–70 atm ranges, respectively. They are consistent with available data and enable the determination of continuum absorption parameters. It is shown that calculations with line shapes derived from the impact approximation are very inaccurate. Models accounting for the finite durations of collisions and line-mixing through wave-number dependent effective broadening parameters are introduced. The latter have been determined using two different approaches, which are (i) empirical determinations from fits of experimental data and (ii) direct predictions from first principles using a statistical approach. Effective broadening parameters obtained using these two different approaches are in satisfactory agreement for both the temperature and wavenumber dependencies. These data are tested by calculations of continua in various spectral regions and the agreement with measured values is satisfactory. The remaining discrepancies probably result from the influence of the internal structures of the absorption bands considered and thus from the influence of line-mixing. Nevertheless, accurate predictions are obtained in wide temperature and spectral ranges when the total absorption at elevated density is considered. This agreement, which is due to the relatively weak continuum absorption and large contributions of nearby lines, makes the present models suitable for most practical applications involving elevated densities.

The shape of the 00⁰3–00⁰0 CO₂ band in helium has been investigated at room temperature over an extended range of perturber pressures (0–140 atm). Various and strong deviations from an additive superposition of Lorentzian lines have been observed, due to important line mixing effects enhanced by the specific structure of the R branch in this band.

The shape of the 00⁰3–00⁰0 CO₂ band in helium has been investigated at room temperature over an extended range of perturber pressures (0–140 atm). Various and strong deviations from an additive superposition of Lorentzian lines have been observed, due to important line mixing effects enhanced by the specific structure of the R branch in this band.

The shape of the 00⁰3–00⁰0 CO₂ band in helium has been investigated at room temperature over an extended range of perturber pressures (0–140 atm). Various and strong deviations from an additive superposition of Lorentzian lines have been observed, due to important line mixing effects enhanced by the specific structure of the R branch in this band.

In paper I of this series, important deviations from an additive superposition of Lorentzian profiles were experimentally evidenced in the 00°3–00°0 band of CO₂ in He. All the observed deviations are explained by the collision‐induced line mixing effects which schematically transfer intensity from the wing of the band to its central part. The IOS approximation has been found to be insufficient while, the ECS approximation leads to theoretical predictions in good agreement with the experimental data over extended ranges of frequency and perturber pressure. However it must be emphasized that it has been necessary to resort to the method in current use for the determination of the fundamental rates, an ad hoc adjustement starting from the observed linewidths.

In paper I of this series, important deviations from an additive superposition of Lorentzian profiles were experimentally evidenced in the 00°3–00°0 band of CO₂ in He. All the observed deviations are explained by the collision‐induced line mixing effects which schematically transfer intensity from the wing of the band to its central part. The IOS approximation has been found to be insufficient while, the ECS approximation leads to theoretical predictions in good agreement with the experimental data over extended ranges of frequency and perturber pressure. However it must be emphasized that it has been necessary to resort to the method in current use for the determination of the fundamental rates, an ad hoc adjustement starting from the observed linewidths.

In paper I of this series, important deviations from an additive superposition of Lorentzian profiles were experimentally evidenced in the 00°3–00°0 band of CO₂ in He. All the observed deviations are explained by the collision‐induced line mixing effects which schematically transfer intensity from the wing of the band to its central part. The IOS approximation has been found to be insufficient while, the ECS approximation leads to theoretical predictions in good agreement with the experimental data over extended ranges of frequency and perturber pressure. However it must be emphasized that it has been necessary to resort to the method in current use for the determination of the fundamental rates, an ad hoc adjustement starting from the observed linewidths.

We present high density experimental and theoretical results on CO₂–He absorption in the v₃ and 3v₃ infrared bands. Measurements have been made at room temperature for pressures up to 1000 bar in both the central and wing regions of the bands. Computations are based on an impact line-mixing approach in which the relaxation operator is modeled with the energy corrected sudden (ECS) approximation. Comparisons between experimental and calculated results demonstrate the accuracy of the ECS approach when applied to band wings and band centers at moderate densities. On the other hand, small but significant discrepancies appear at very high pressures. They are attributed to a number of reasons which include nonlinear density dependence due to the finite volume of the molecules, neglected contributions of vibration to the relaxation matrix, and incorrect modeling of interbranch mixing.

We present high density experimental and theoretical results on CO₂–He absorption in the v₃ and 3v₃ infrared bands. Measurements have been made at room temperature for pressures up to 1000 bar in both the central and wing regions of the bands. Computations are based on an impact line-mixing approach in which the relaxation operator is modeled with the energy corrected sudden (ECS) approximation. Comparisons between experimental and calculated results demonstrate the accuracy of the ECS approach when applied to band wings and band centers at moderate densities. On the other hand, small but significant discrepancies appear at very high pressures. They are attributed to a number of reasons which include nonlinear density dependence due to the finite volume of the molecules, neglected contributions of vibration to the relaxation matrix, and incorrect modeling of interbranch mixing.

We present high density experimental and theoretical results on CO₂–He absorption in the v₃ and 3v₃ infrared bands. Measurements have been made at room temperature for pressures up to 1000 bar in both the central and wing regions of the bands. Computations are based on an impact line-mixing approach in which the relaxation operator is modeled with the energy corrected sudden (ECS) approximation. Comparisons between experimental and calculated results demonstrate the accuracy of the ECS approach when applied to band wings and band centers at moderate densities. On the other hand, small but significant discrepancies appear at very high pressures. They are attributed to a number of reasons which include nonlinear density dependence due to the finite volume of the molecules, neglected contributions of vibration to the relaxation matrix, and incorrect modeling of interbranch mixing.

 High resolution infrared spectra of a previously unidentified noncyclic isomer of (CO2)3 have been obtained via direct absorption of a 4.3 μm diode laser in a slit jet supersonic expansion. Two vibrational bands (labeled νI and νIII) are observed, corresponding to the two most infrared active linear combinations of the three constituent CO2 monomer asymmetric stretches: νI is redshifted −5.85 cm−1 from the monomer vibrational origin and is predominately a c‐type band of an asymmetric top, while νIII is blueshifted +3.58 cm−1 and is predominately an a‐type band. Transitions with Ka+Kc=odd (even) in the ground (excited) state are explicitly absent from the spectra due to the zero nuclear spin of CO2; this rigorously establishes that the noncyclic isomer has a C2 symmetry axis. The vibrational shifts and relative intensities of the bands are interpreted via a resonant dipole interaction model between the high‐frequency stretches of the CO2 monomers. Rotational constants are determined by fits of transition frequencies to an asymmetric top Hamiltonian. These results are used to determine vibrationally averaged structural parameters for the complex, which is found to be stacked asymmetric but with C2 symmetry about the b inertial axis. The structural parameters are then used to test several trial CO2–CO2interaction potentials.

 High resolution infrared spectra of a previously unidentified noncyclic isomer of (CO2)3 have been obtained via direct absorption of a 4.3 μm diode laser in a slit jet supersonic expansion. Two vibrational bands (labeled νI and νIII) are observed, corresponding to the two most infrared active linear combinations of the three constituent CO2 monomer asymmetric stretches: νI is redshifted −5.85 cm−1 from the monomer vibrational origin and is predominately a c‐type band of an asymmetric top, while νIII is blueshifted +3.58 cm−1 and is predominately an a‐type band. Transitions with Ka+Kc=odd (even) in the ground (excited) state are explicitly absent from the spectra due to the zero nuclear spin of CO2; this rigorously establishes that the noncyclic isomer has a C2 symmetry axis. The vibrational shifts and relative intensities of the bands are interpreted via a resonant dipole interaction model between the high‐frequency stretches of the CO2 monomers. Rotational constants are determined by fits of transition frequencies to an asymmetric top Hamiltonian. These results are used to determine vibrationally averaged structural parameters for the complex, which is found to be stacked asymmetric but with C2 symmetry about the b inertial axis. The structural parameters are then used to test several trial CO2–CO2interaction potentials.

The ν₂ and 3ν₃ bands of CO₂ in helium baths at 193 K have studied with a Fourier transform interferometer. The behavior of the band shapes has been explored at moderate densities. The energy corrected sudden (ECS) approximation is used to model the relaxation matrix in order to account for line mixing effects. The basis cross-sections were calculated with the simple power law (P). Computed spectra are in good agreement with the observed ones. Measured broadening coefficients are also comparable with the ones derived from the ECS-P model.

We present an experimental study of the self- and N₂-broadened H₂ O continuum in microwindows within the ν₂ fundamental centered at ~1600 cm⁻¹. The continuum is derived from transmission spectra recorded at room temperature with a BOMEM Fourier transform spectrometer at a resolution of ~0.040 cm⁻¹. Although we find general agreement with previous studies, our results suggest that there is significant near-wing super-Lorentzian behavior that produces a highly wave-number-dependent structure in the continuum as it is currently defined.

The shapes of Q-branches in CO₂ spectra in the 580–850 cm⁻¹ region for pure gas and mixtures CO₂---He and CO₂---Ar have been studied at resolutions up to 0.002cm⁻¹. The branch broadening coefficients were measured in the pressure range from a few torr to 50 atm. Those for 11¹02-02²01 Π-Δ transition are different for pressures below and above 5 atm. At higher pressures the branch broadening coefficients are similar for all Q-branches. The observed Q-branch shape transformation is explained by line mixing effects.

The shapes of Q-branches in CO₂ spectra in the 580–850 cm⁻¹ region for pure gas and mixtures CO₂---He and CO₂---Ar have been studied at resolutions up to 0.002cm⁻¹. The branch broadening coefficients were measured in the pressure range from a few torr to 50 atm. Those for 11¹02-02²01 Π-Δ transition are different for pressures below and above 5 atm. At higher pressures the branch broadening coefficients are similar for all Q-branches. The observed Q-branch shape transformation is explained by line mixing effects.

The shapes of Q-branches in CO₂ spectra in the 580–850 cm⁻¹ region for pure gas and mixtures CO₂---He and CO₂---Ar have been studied at resolutions up to 0.002cm⁻¹. The branch broadening coefficients were measured in the pressure range from a few torr to 50 atm. Those for 11¹02-02²01 Π-Δ transition are different for pressures below and above 5 atm. At higher pressures the branch broadening coefficients are similar for all Q-branches. The observed Q-branch shape transformation is explained by line mixing effects.

The shapes of Q-branches in CO₂ spectra in the 580–850 cm⁻¹ region for pure gas and mixtures CO₂---He and CO₂---Ar have been studied at resolutions up to 0.002cm⁻¹. The branch broadening coefficients were measured in the pressure range from a few torr to 50 atm. Those for 11¹02-02²01 Π-Δ transition are different for pressures below and above 5 atm. At higher pressures the branch broadening coefficients are similar for all Q-branches. The observed Q-branch shape transformation is explained by line mixing effects.

Measurements of pure CO₂ absorption in the 2.3-μm region are presented. The 3800–4700-cm⁻¹ range has been investigated at room temperature for pressures in the 10–50-atm range by using long optical paths. Phenomena that contribute to absorption are listed and analyzed, including the contribution of far line wings as well as those of the central region of both allowed and collision-induced absorption bands. The presence of simultaneous transitions is also discussed. Simple and practical approaches are proposed for the modeling of absorption, which include a line-shape correction factor χ that extends to approximately 600 cm⁻¹ from line centers.

Measurements of absorption coefficients in the 3v₃ band of CO₂ at 1.44 μm perturbed by Ar up to 146 bar have been analyzed by using two line-mixing theoretical calculations within the impact approximation. In the first approach, the relaxation operator is treated semi-classically with adiabatic corrections. In the second, the relaxation operator is modelled with the Energy Corrected Sudden (ECS) approximation associated with a power fitting law providing the basic rotational state-to-state rates. Although the line-coupling spectroscopic cross-sections of the two models are significantly different, they both lead to satisfactory agreement with bandshapes at moderate densities (< 100 Amagat). Significant deviations between experimental and calculated spectra appear at higher densities. They are mainly attributed to the probable breakdown of the impact and binary collision approximations and to a number of reasons including an incorrect ECS calculation of the interbranch coupling, the nonlinear density dependence due to the finite volume of the molecules, and the neglect of the unknown imaginary part of the off-diagonal elements in the calculated relaxation matrix.

Measurements of absorption coefficients in the 3v₃ band of CO₂ at 1.44 μm perturbed by Ar up to 146 bar have been analyzed by using two line-mixing theoretical calculations within the impact approximation. In the first approach, the relaxation operator is treated semi-classically with adiabatic corrections. In the second, the relaxation operator is modelled with the Energy Corrected Sudden (ECS) approximation associated with a power fitting law providing the basic rotational state-to-state rates. Although the line-coupling spectroscopic cross-sections of the two models are significantly different, they both lead to satisfactory agreement with bandshapes at moderate densities (< 100 Amagat). Significant deviations between experimental and calculated spectra appear at higher densities. They are mainly attributed to the probable breakdown of the impact and binary collision approximations and to a number of reasons including an incorrect ECS calculation of the interbranch coupling, the nonlinear density dependence due to the finite volume of the molecules, and the neglect of the unknown imaginary part of the off-diagonal elements in the calculated relaxation matrix.

Measurements of absorption coefficients in the 3v₃ band of CO₂ at 1.44 μm perturbed by Ar up to 146 bar have been analyzed by using two line-mixing theoretical calculations within the impact approximation. In the first approach, the relaxation operator is treated semi-classically with adiabatic corrections. In the second, the relaxation operator is modelled with the Energy Corrected Sudden (ECS) approximation associated with a power fitting law providing the basic rotational state-to-state rates. Although the line-coupling spectroscopic cross-sections of the two models are significantly different, they both lead to satisfactory agreement with bandshapes at moderate densities (< 100 Amagat). Significant deviations between experimental and calculated spectra appear at higher densities. They are mainly attributed to the probable breakdown of the impact and binary collision approximations and to a number of reasons including an incorrect ECS calculation of the interbranch coupling, the nonlinear density dependence due to the finite volume of the molecules, and the neglect of the unknown imaginary part of the off-diagonal elements in the calculated relaxation matrix.

We present high-density experimental and theoretical results on CO₂---Ar gas-phase absorption in the ν₃ and 3ν₃ infrared bands. Measurements have been made at room temperature for pressures up to 1000 bar in both the central and wing regions of the bands. A non-linear perturber density dependence of the absorption, clearly shown in the far wing, is attributed to the finite volume of the molecules. Furthermore, experiments show vibrational dephasing and narrowing effects. We have performed line-mixing computations based on the Energy Corrected Sudden approximation (ECS impact model). Significant discrepancies between experimental and calculated spectra appear when pressure increases. We then tested the influence of the finite duration of collision by using interpolations between ECS and quasi-static calculations, and we have evaluated the sensitivity of the band profiles to the interbranch mixing effects. Finally, an effective width is used in order to take other effects into account.

We present high-density experimental and theoretical results on CO₂---Ar gas-phase absorption in the ν₃ and 3ν₃ infrared bands. Measurements have been made at room temperature for pressures up to 1000 bar in both the central and wing regions of the bands. A non-linear perturber density dependence of the absorption, clearly shown in the far wing, is attributed to the finite volume of the molecules. Furthermore, experiments show vibrational dephasing and narrowing effects. We have performed line-mixing computations based on the Energy Corrected Sudden approximation (ECS impact model). Significant discrepancies between experimental and calculated spectra appear when pressure increases. We then tested the influence of the finite duration of collision by using interpolations between ECS and quasi-static calculations, and we have evaluated the sensitivity of the band profiles to the interbranch mixing effects. Finally, an effective width is used in order to take other effects into account.

We present high-density experimental and theoretical results on CO₂---Ar gas-phase absorption in the ν₃ and 3ν₃ infrared bands. Measurements have been made at room temperature for pressures up to 1000 bar in both the central and wing regions of the bands. A non-linear perturber density dependence of the absorption, clearly shown in the far wing, is attributed to the finite volume of the molecules. Furthermore, experiments show vibrational dephasing and narrowing effects. We have performed line-mixing computations based on the Energy Corrected Sudden approximation (ECS impact model). Significant discrepancies between experimental and calculated spectra appear when pressure increases. We then tested the influence of the finite duration of collision by using interpolations between ECS and quasi-static calculations, and we have evaluated the sensitivity of the band profiles to the interbranch mixing effects. Finally, an effective width is used in order to take other effects into account.

Absorption spectra of gas-phase molecular oxygen and zero air at temperatures of 223 and 283 K have been measured in the laboratory using a coolable multipass-optics gas cell and Fourier transform spectroscopy in the wavelength range 455 to 830 nm (12,000–22,000 cm⁻¹). Net absorption cross sections of the O₂A−, B−, and γ-bands at <0.002 nm spectral resolution, and pressures of 100 and 1000 hPa zero air have been determined. Binary absorption cross sections of the collision-induced O₄ bands at <0.18 nm spectral resolution and a pressure of 1000 hPa pure oxygen have been determined, with corrections for the O₂ γ-band absorption. Calculated integrated absorption intensities and, for the O₂A− and B−bands, “effective” Einstein A-coefficients are compared with previous literature values.

The infrared O–D stretching spectrum of fully deuterated jet-cooled water clusters is reported. Sequential red-shifts in the single donor O–D stretches, which characterize the cooperative effects in the hydrogen bond network, were accurately measured for clusters up to (D₂O)₈. Detailed comparisons with corresponding data obtained for (H₂O)_n clusters are presented. Additionally, rotational analyses of two D₂O dimer bands are presented. These measurements were made possible by the advent of infrared cavity ringdown laser absorption spectroscopy (IR-CRLAS) using Raman-shifted pulsed dye lasers, which creates many new opportunities for gas phase IR spectroscopy.

 The Energy Corrected Sudden approach is used in order to deduce collisional parameters and to model infrared quantities in Sigma->Sigma bands of CO₂-He and CO₂-Ar mixtures at room temperature. Measurements are first used for the determination (from a fit) of the rotational angular momentum relaxation time and of some parameters representative of the imaginary part of the relaxation operator. It is shown that line-broadening data as well as absorption in both the wing and central part of the v₃ and 3v₃ bands lead to consistent determinations. The model is then used for detailed analysis of line-mixing effects. The influences of pressure, of the band spectral structure, and of the collision partner are studied. Differences between the effects of collisions with He and Ar are pointed out and explained.

Theoretical results for the far-wing line shapes and corresponding absorption coefficients in the high-frequency wing of the ν₃ fundamental band of self-broadened CO₂ are presented for a number of temperatures between 218 and 751 K. These first-principles calculations are made assuming binary collisions within the framework of a quasi-static theory with a more accurate interaction potential than in previous calculations. The theoretical results are compared with existing laboratory data and are in good agreement for all the temperatures considered.

Theoretical results for the far-wing line shapes and corresponding absorption coefficients in the high-frequency wing of the ν₃ fundamental band of self-broadened CO₂ are presented for a number of temperatures between 218 and 751 K. These first-principles calculations are made assuming binary collisions within the framework of a quasi-static theory with a more accurate interaction potential than in previous calculations. The theoretical results are compared with existing laboratory data and are in good agreement for all the temperatures considered.

Collision-induced absorption (CIA) by CO₂ is measured in the 1100–1600 cm⁻¹range using a Fourier-transform spectrometer with a resolution of 0.5 cm⁻¹. The current measurements, which agree well with previous ones but are more precise, reveal pronounced structures on top of both unresolved Fermi doublet bands consisting of P-, Q-,and R-like branches. Assignment of Q-branches at 1284.75 cm⁻¹and 1387.75 cm⁻¹to (CO₂)₂ dimers seems highly probable. The nature of other peaks observed in CIA and Raman spectra of the CO₂ Fermi doublet region is discussed.

New FTIR CO₂ high resolution spectra are recorded in argon and nitrogen matrices at 11 K at high dilution. Their evolution with CO₂ diffusion at higher temperature is also traced. New observations are discussed in regard to previous works. Even at high dilution (1/10 000), the temperature increase causes appearance of several bands both in argon and nitrogen. Identification of carbon dioxide dimer absorptions is tentatively proposed.

The carbon dioxide dimer spectroscopic patterns are retrieved from the analysis of collision-induced absorption (CIA) spectral bandshape at room temperature. It is shown that the use of the simplified model based on the symmetric-top approximation allows roughly consistent simulation of the observed (CO₂)₂ dimer spectrum. The rotational constants obtained can be considered as effective thermally averaged constants which characterize dimeric structure, strongly distorted from the ground state. The overall CIA bandshape and the integrated intensity of absorption are broken down into partial contributions from tightly bound and metastable dimers and free-pair states. This approach is shown to be in agreement with a wide range of independent spectroscopic and thermodynamic data.

This paper aims at examination of the (CO₂)₂ spectral absorption profile retrieved from the room temperature CO₂ collision-induced absorption (CIA) spectrum in the region of the ν₁, 2ν₂ Fermi doublet. The assignment of (CO₂)₂ vibrations is discussed, paying due attention to high-resolution CARS observations in the regions of ν_low and ν_up. A variational solution of the anharmonic vibrational problem for the (CO₂)₂ dimer was obtained using an extended basis set consisting of harmonic and Morse oscillator wave functions. This allowed for a satisfactory description of the vibrational dimer ν₁, 2ν₂ spectrum from first principles. In particular, we have succeeded in proving that the diffuse absorption band seen in the trough between the major Fermi-coupled CIA bands belongs to combinations of CO₂ bending vibrations in a dimer.

Cavity ring-down spectroscopy has been used to obtain the spectra of transient (O₂)₂ at 577 and 629 nm, at room temperature and pressures ranging from about 0.25 to 10.0 atm. A selection of these spectra are displayed showing the overlapping monomer and dimer features. Pressure-dependent cross sections have been obtained and Rayleigh extinction has been observed. The derived band parameters are compared to recent results from others.

Nous utilisons un spectroscomètre à cavité pour obtenir le spectre du fugace (O₂)₂ à 577 et 629 nm, à température de la pièce et à des pressions allant de 0,25 à 10,0 atm. Nous présentons une sélection de ces spectres qui montrent un chevauchement des caractéristiques du monomère et du dimère. Nous avons déterminé la dépendance en pression des sections efficaces et observé l’amortissement Rayleigh.

It is the purpose of this paper to present an infrared absorption spectroscopic study of the state of aggregation of water over a wide range of temperature (25–380°C) and pressure (1–250 bar) along the liquid-gas coexistence curve, and in the supercritical domain. The evolution of the spectral profiles asociated with the internal vibrational modes and with the librational motion has been investigated. In supercritical water, at T=380°C, low pressures (densities) in the range 25–50 bar (0.01–0.05 g.cm⁻³), only monomeric water is detected. Progressive increase of the pressure (density), from 50 to 250 bar (from 0.05 to 0.4 g.cm⁻³), shows clearly the appearance of water dimers and trimers. Finally, upon decreasing the temperature (250-25 °C) along the liquid-gas coexistence curve, one observes a continuous evolution of the shape of the infrared spectrum characteristic of the presence of oligomers of increasing size and for temperatures lower than 200°C, the progressive appearance of the hydrogen bond network.

It is the purpose of this paper to present an infrared absorption spectroscopic study of the state of aggregation of water over a wide range of temperature (25–380°C) and pressure (1–250 bar) along the liquid-gas coexistence curve, and in the supercritical domain. The evolution of the spectral profiles asociated with the internal vibrational modes and with the librational motion has been investigated. In supercritical water, at T=380°C, low pressures (densities) in the range 25–50 bar (0.01–0.05 g.cm⁻³), only monomeric water is detected. Progressive increase of the pressure (density), from 50 to 250 bar (from 0.05 to 0.4 g.cm⁻³), shows clearly the appearance of water dimers and trimers. Finally, upon decreasing the temperature (250-25 °C) along the liquid-gas coexistence curve, one observes a continuous evolution of the shape of the infrared spectrum characteristic of the presence of oligomers of increasing size and for temperatures lower than 200°C, the progressive appearance of the hydrogen bond network.