Simulations of the spectra of vibrational transitions in highly vibrationally excited XY6 molecules at certain energy E vib are performed. The infrared (IR) transitions in the mode ν3 of the molecules SF6 and WF6 as well as the Raman ones in the mode ν1 of SF6 are studied. The shapes and parameters of the spectral bands, such as the integral intensity, the mean frequency, and the width, are obtained in a wide range of E vib. The calculated widths prove to be much broader than the expected contributions to them because of intramolecular vibrational relaxation (IVR); this indicates the dominant role of statistical inhomogeneous broadening in the width formation for the investigated molecules.

The detailed studies of spontaneous Raman spectra are performed for highly excited SF6 molecules in the vicinity of the ν1 mode. Interpretation of the measured spectra is given within the framework of theoretical model, which assumes the dominating role of the statistical inhomogeneous broadening in the spectrum formation. A good agreement is achieved between the experimental and theoretical spectra throughout the whole temperature range used (T vib = 850−1660 K). A conclusion is drawn that the homogeneous broadening due to intramolecular vibrational relaxation gives a relatively small contribution to the observed spectra.

The IR MP excitation spectra of the SF 6 molecule in the vibrational quasicontinuum (QC) measured experimentally in the vicinity of the ν 3 mode frequency are compared with their theoretical counterparts with a view to revealing the relative contribution from the statistical inhomogeneous broadening (SIB) and homogeneous broadening to the formation of the IR transition spectra in the QC. The IR MP spectra are found by solving rate equations, the cross sections of the successive transitions being calculated in the SIB approximation. The half-width γ L of the Lorentzian profile associated with the homogeneous broadening is the only adjustment parameter of the model. A good agreement (within the accuracy of measurement) is obtained between the experimental and theoretical IR MP spectra over a wide range of spectral and energy parameters. The γ L values are found which vary within the limits 0.53 e γ L e 10.2 cm -1 in the energy range 4500 e E e 30 000 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 intensity) are governed largely by the SIB effect, though the Lorentzian wings may play a decisive role when excitation occurs at the edges of the spectrum. The IR transition cross sections found are used to compute the IR MP excitation dynamics in the QC and determine the vibrational distribution function being formed. It is found that excitation in the QC may also give rise to a nonequilibrium bimodal distribution.

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Spontaneous Raman spectra of highly excited UF 6 molecules in the vicinity of the mode ν 1 are studied. The spectra measured are interpreted within the framework of the model suggested, which presumes the dominant role of statistical inhomogeneous broadening in the formation of transition spectra in the vibrational quasicontinuum. A good agreement is obtained between the theoretical and experimental spectra of UF 6 molecules heated to T vib ) 1320 K. The relationships are found between the vibrational energy of the UF 6 molecule and the main parameters of its Raman transitions in the neighborhood of the frequency of the mode ν 1 , such as the intensity, width, and maximum positions of their profiles.

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