The intensities of the collision-induced absorption (CIA) bands associated with the electric-dipole forbidden O2 fundamental and the CO2ν1/2ν2 Fermi dyad monomer vibrational bands have been studied over the temperature range 193–360 K and the frequency range 1100–2000 cm−1. As CO2 is added to a pure O2 sample, the intensity in the O2 fundamental band region increases dramatically. At the lowest temperature studied, 193 K, the band-integrated CIA coefficient for enhancement of the Fermi dyad absorption from CO2 to CO2 collisions, SCO2–CO2, is more than a factor of two larger than the band-integrated CIA coefficient for enhancement of the O2 vibrational fundamental by CO2 collisions, SO2–CO2. Moreover, the SCO2–CO2 coefficient shows a significantly larger temperature dependence, increasing by more than a factor of two from 345.6 to 193 K while SO2–CO2 increases by less than one third. The band shapes and their temperature dependence provide clear evidence for the formation of CO2–CO2 and CO2–O2 complexes. The CO2–CO2 dimer feature is most striking, contributing significantly to the infrared absorption near the expected CO2 monomer fundamentals. Evidence for the more weakly bound CO2–O2 complex is seen on the O2 CIA band, particularly at the lowest temperatures studied. The shapes for both dimer bands display sharp a-type Q branch central profiles and broad P and R branch like structure attributed to b-type Q branches for the CO2–CO2 complex and a-type P and R branch structure for the CO2–O2 complex. The present results stress the importance of including bound and metastable dimer absorption in any theoretical modeling of CIA, particularly when one of the collision partners has a large electrostatic moment, such as CO2 with its large electric quadrupole moment.
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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.
