Gayathri, N. ; Bagchi, Biman (1999) Computer simulation study of the subquadratic quantum number dependence of vibrational overtone dephasing: comparison with the modecoupling theory predictions Journal of Chemical Physics, 110 (1). pp. 539550. ISSN 00219606

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Official URL: http://link.aip.org/link/?JCPSA6/110/539/1
Related URL: http://dx.doi.org/10.1063/1.478111
Abstract
Experimental studies have demonstrated that the vibrational dephasing of overtones do not always follow the quadratic quantum number (n) dependence predicted by the KuboOxtoby theory of vibrational line shapes. While the reason for this failure of the theory is not quite clear yet, a recent theory suggested that the pronounced Gaussian time dependence of the frequencymodulation timecorrelation function (tcf) could be a possible reason [Gayathri et al., J. Chem. Phys., 107, 10381 (1997)]. The theoretical study was based on a mode coupling theory calculation of the forceforce timecorrelation function that is required in the calculation of the frequencymodulation tcf. In order to test this and other predictions of the above study, detailed computer simulations of two neat liquids have been carried out. The systems studied are NN stretch in liquid N_{2} and the CI stretch in CH_{3}I. It is found that although the frequencymodulation timecorrelation function is largely Gaussian in both the cases, the overtone dephasing remains largely quadratic in n for N_{2}. For methyl iodide, on the other hand, a pronounced subquadratic n dependence has been observed. Both the theory and the computer simulations suggest that this nonquadratic dependence can be expected when not only the decay of the frequency timecorrelation function is Gaussian but the time scale of decay of the frequencymodulation tcf is comparable to that of the normal coordinate. The latter can happen when the following conditions are satisfied. First, the frequency of the normal mode should not be too large. Second, the meansquare fluctuation of the frequencymodulation and the anharmonicity coefficient of vibration should be large. It is found that both for N_{2} and CH_{3}I, the resonant energy transfer between different molecules is significant. The effect of rotationalvibrational coupling, on the other hand, is found to be negligible for the systems studied.
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