Infrared spectrum of the continuum and dimer absorption in the vicinity of the O2 vibrational fundamental in O2/CO2 mixtures

The intensities of the collision-induced absorption (CIA) bands associated with the electric-dipole forbidden O-2 fundamental and the CO2 nu(1)/2nu(2) 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 O-2 sample, the intensity in the O-2 fundamental band region increases dramatically. At the lowest temperature stu0died, 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 O-2 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-O-2 complex is seen on the O-2 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-O-2 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. Published by Elsevier Inc.