Theoretical study of the collision-induced fundamental absorption spectra of O-2-O-2 pairs for temperatures between 193 and 273 K

A theoretical analysis of the collision-induced fundamental absorption spec tra of O-2-O-2 pairs is presented for temperatures between 193 and 273 K. M ost of the absorption arises from the long-range quadrupole and hexadecapol e-induced dipole mechanisms for which accurate matrix elements are availabl e from various other experimental measurements or from ab initio calculatio ns. The line shape used is that obtained from quantum computations of the f ar infrared collision-induced absorption of N-2, modified to include dips r esulting from the effects of intercollisional interference. Several refinem ents, including contributions from back-reaction and short-range induced-di pole moments, can improve the agreement in certain spectral regions but do not lead to significant improvement in the global fits to the experimental data. The small structural features superimposed on the smooth continuum ca n be modeled by the modified line shape but the half-widths of the dips hav e a density dependence inconsistent with that arising from interference. On the other hand, difference spectra obtained by subtracting the theoretical results from the experimental data appear quite similar to those obtained previously at low temperatures, leading to the conclusion that this structu re is due to dimers. Further experimental results at higher temperatures an d for O-2-N-2 pairs would be useful to validate this interpretation. (C) 19 99 Elsevier Science Ltd. All rights reserved.