Transition spectra in the vibrational quasicontinuum of polyatomic molecules. IR multiple-photon absorption in SF6. 2. Theoretical simulation and comparison with experiment

The IR MP excitation spectra of the SF6 molecule in the vibrational quasico ntinuum (QC) measured experimentally in the vicinity of the nu(3) mode freq uency are compared with their theoretical counterparts with a view to revea ling the relative contribution from the statistical inhomogeneous broadenin g (SIB) and homogeneous broadening to the formation of the IR transition sp ectra in the QC. The IR MP spectra are found by solving rate equations, the cross sections of the successive transitions being calculated in the SIE a pproximation. The half-width gamma(L) of the Lorentzian profile associated with the homogeneous broadening is the only adjustment parameter of the mod el. A good agreement (within the accuracy of measurement) is obtained betwe en the experimental and theoretical IR MP spectra over a wide range of spec tral and energy parameters. The gamma(L) values are found which vary within the limits 0.53 less than or equal to gamma(L) less than or equal to 10.2 cm(-1) in the energy range 4500 less than or equal to E less than or equal to 30000 cm(-1). The conclusion is drawn that the main parameters of the IR transition spectra in the QC (the position of the maximum, width, and inte nsity) are governed largely by the SIE effect, though the Lorentzian wings may play a decisive role when excitation occurs at the edges of the spectru m. The IR transition cross sections found are used to compute the IR MP exc itation dynamics in the QC and determine the vibrational distribution funct ion being formed. It is found that excitation in the QC may also give rise to a nonequilibrium bimodal distribution.