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

Abstract

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.

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

“…The population distribution formed during the passage of the pump pulse may be of nonequilibrium character (see, e.g., refs and ), and so the delay time τ d was chosen so that the vibrational distribution established as the result of the V−V‘ relaxation of the molecules between the pulses was known to be of equilibrium character. This procedure was specially introduced in order that in the subsequent modeling of IR MP excitation within the quasicontinuum region in ref we can deal with a well-defined initial vibrational distribution function, namely, a Boltzmann distribution.

1 Illustrating the pump−probe technique for studying the IR MP absorption process in the quasicontinuum.

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“…It can be seen from Figure a that with the ratio a 01 / a 02 being as it is, the probe beam passes in the region where the pump beam intensity and hence the vibrational temperature of the preexcited molecules vary quite perceptibly in the radial direction. Therefore, this nonuniformity was allowed for in the subsequent theoretical interpretation of the measurements taken (see ref ). The more detailed probe beam profile measurements showed (Figure b) that at the edge of the spatial distribution there was an additional “ring” associated with the diffraction of radiation by the intracavity diaphragm (Figure ).…”

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.…”

Спектроскопия Квантовых Биений В Системе N Одинаковых, Близко Расположенных Атомов. I. Случай N=2

“…Вполне вероятна ситуация, когда эта большая ширина обусловлена в основном влиянием на частоту перехода распределением по числам заполнения в фононной подсистеме. Такое уширение (даже в одиночном сложном объекте) аналогично так называемому статистическому неоднородному уширению в колебательно-возбужденных многоатомных молекулах, исследованному теоретически и экспериментально в работах [12][13][14][15]. Если так, то можно рассчитывать на различные методы исключения спектральной неоднородности.…”