Transition Spectra in the Vibrational Quasicontinuum of Polyatomic Molecules. IR Multiple-Photon Absorption in SF₆. 1. Experimental Studies

Abstract

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.

Transition Spectra in the Vibrational Quasicontinuum of Polyatomic Molecules. IR Multiple-Photon Absorption in SF6. 2. Theoretical Simulation and Comparison with Experiment

“…Before proceeding to the description of the procedure of comparing between experimental data and model calculations, let us recall how the experiment was conducted (for details, see ref ). With a pump pulse at a frequency of ω 1 (the 10P(16) CO 2 laser line), the SF 6 molecules were excited into the quasicontinuum (see Figure in ref ).…”

“…It should be noted at the same time that according to this estimate the homogeneous broadening and the associated IVR rate are sufficiently great for the excitation incoherence condition 2γ d ≫ W to be satisfied, W being the characteristic radiative excitation rate. Simple estimates show that this condition is fulfilled by a wide margin, no matter what the experimental conditions …”

“…The main objective of the work is to reveal the contribution from the homogeneous broadening to the shape of QC f QC transition spectra. To this end, we study the IR multiple-photon (MP) absorption in SF 6 molecules preliminarily excited into the QC (see Figure 1 in ref 3). The idea of using such an approach is based on the possibility of accumulation of the relatively small contribution to the transition profiles from the Lorentzian wings associated with the homogeneous broadening as a result of a large number of transitions up the ladder of vibrational states in the course of IR MP excitation in the QC.…”

Intramolecular vibrational redistribution: from high-resolution spectra to real-time dynamics

“…We also add here that the authors of papers [313,323] interpreted a mixed situation: a small homogeneous broadening against the background of dominating SIB. Here, due to the multiplicity of the IR MPE process, the authors have managed to estimate the homogeneous component of the spectrum making the main contribution to the wings of the band profile.…”

“…The simplest approach consists in using IR MPE (see Section 1.3) and measuring the spectrum after a time delay ensuring the establishment of the Boltzmann distribution. This method was used for measuring the absorption spectrum in one of the bands of the C 6 F 12 molecule [312], the n 3 IR MPE spectrum of the SF 6 molecule [313], the n 1 Raman spectrum of the same molecule [314,315], and the n 1 Raman spectrum of the UF 6 molecule [316]. However, in some papers spectra were measured for narrow vibrational distributions.…”