Coefficients for oxygen absorption in the infrared induced by collisions with O2 and N2 are reported over the range 1400–1800 cm−1 and 225–356 K. These coefficients are used to calculate the absorption for O2 in air as a function of temperature and wavenumber, and comparisons are made with previous determinations. In addition, structured absorption features superimposed on the broad collision-induced absorption band, which were observed at all temperatures studied, are interpreted in terms of the presence of (O2)2 and O2-N2 van der Waals molecules.
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Gaseous oxygen was examined from 1480 to 1800 cm−1 at temperatures between 77 and 300°K at path lengths to 200 meters. At all temperatures a broad structureless band is observed which is attributed to induced absorption by colliding pairs of oxygen molecules, however near 90°K a doublet feature appears on top of the broad absorption. The doublet exhibits a quadratic pressure dependence and is assigned to bound state oxygen dimer (O2)2. The energy of formation of (O2)2, as determined from the temperature dependence of the doublet, is ΔEf = −530 cal/mole. Since the doublet resembles an unresolved P and R vibration-rotation band, models of the dimer are considered which could give the observed doublet separation. A rectangular geometry of (O2)2 indicates an intermolecular distance of 2.6 Å. Other geometries give similar distances which are considerably shorter than the Lennard-Jones value of 3.9 Å. A more realistic interpretation of the spectrum is that the doublet represents overlapping transitions from many bound levels of the intermolecular potential surface. There is then no simple relationship between the doublet separation and dimer geometry. However, a dimer in which each O2 unit is freely rotating would give a very broad absorption unlike the observed doublet feature. So while the system is too complicated to specify the dimer geometry, its structure must be somewhat rigid.
