This paper ends the series of reviews on multiple photon IR laser photophysics and photochemistry. The applications of IR MP processes in fundamental chemical studies and chemical technology are considered.

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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.

This paper begins our review paper on the multiple-photon excitation of molecules by intense infrared laser light. The whole review will be given in installments. In this first part we bring forward certain general concepts on spectroscopy, molecular physics and chemical kinetics essential for understanding the later material.

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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.

We studied in detail the IR multiple-photon (MP) absorption in the quasicontinuum of SF6 molecule in the vicinity of the frequency of the mode ν3. The molecules were preliminarily raised to the quasicontinuum by means of a pump laser pulse, and then we measured their IR MP absorption of a probe pulse, depending on its fluence and frequency ω2, as well as on the initial excitation level (vibrational temperature) of the molecules. The latter was varied from room value to T vib = 1700 K. The frequency ω2 was varied in the range 884.18−944.2 cm-1. The IR MP absorption spectra measured, as well as the fluence and T vib dependences of MP absorption for various ω2, are intended for making a comparison with the results of the theoretical modeling of the IR MP excitation dynamics and obtaining the spectral characteristics of the IR transitions of the SF6 molecule in the quasicontinuum.