A NEAR-WING CORRECTION TO THE QUASI-STATIC FAR-WING LINE-SHAPE THEORY

A new representation is introduced in which the rapidly varying time-d ependent part of the time displacement operator can be factored out an d the remaining part, which varies with time more slowly, can be expan ded in the usual perturbational fashion. The lowest order approximatio n leads to the far-wing quasistatic line shape theory developed previo usly, whereas the next order approximation, related to the noncommutat ion of the Liouville operators describing the unperturbed absorber and bath molecules and the interaction between them, leads to a near-wing correction. Explicit expressions are derived for both the corrections to the spectral density and the statistical band-average line shape f unction assuming an anisotropic dipole-dipole interaction. Detailed co mputations for the case of self-broadened H2O are carried out for the line-shapes and the corresponding absorption coefficients for several temperatures and for frequencies to 10 000 cm(-1). From these results, we conclude that the near-wing corrections generally increase the lin e shape function between 10 and 200 cm(-1), and that this increase is more important for lower temperatures than for higher ones. This in tu rn leads to increased absorption-nearer the band centers, especially f or lower temperatures, and thus to improved agreement between theory a nd experiment.