Collisional transfer of vibrational energy in vapors of polyatomic molecules

Abstract

Thc collisional transfer of vibrational energy has been the subject of much investigation l 1, 2 ], the results of which are of fundamental importance for the molecular physics of the gas phase and have numerous practical applications. However, the laws governing intermolecular vibrational relaxation in the quasi-continuum of vibrational states, which is characteristic of polyalomic molecules even at comparatively low levcls of vibrational excitation, have not rcceived sufficient study. This results from the limited number of direct expcrimenlal methods and from the absence of corresponding theoretical models. Experimental difficulties are responsiblc for the differences in the quantitative characteristics of collisional transfer as well as ira the laws governing their variation with an increase in the internal energy of the molecule. The diversity of the postulated means of deactivation of excited molecules and the complexity' of the energy spectrum and of the motion dynamics of polyatomic molecules make simulation of the process difficult and require new, original experimental works along these lines. As early as in the late fifties, B. S. Neporent and N. A. Borisevich [3 I demonstrated lhe high cfficiencv of luminescence methods as applied to the study of collisional transfer of vibrational energy. A new direct method of obsem'ing the process of collisional transfer and exchange of vibrational energy from the change in the rates of decay of the delayed fluorescence activated by the nonequilibrium vibrational laser excitation of triplet molecules was suggested in I4 1. In subsequent works 15-71 il was shown that the use of multiphoion excitalion (MPE) of triplet molecules by CO2-taser radiation permits onc to investigate the process at both low and high vibrational energies for molecules of differcnl degrees of complexity. In the present work the method is used Io investigatc the energy dependences of the characteristics of the collisional transfer of vibrational energy ira vapors of diacclvl and benzophenone and their mixtures with foreign gases. As noted earlier, the choice of objects for investigalion is based on the spectroscopic charactcristics of the molecules, which in the gas phase possess long-lived triplcl states, a low barrier to the activation of delayed fluorescence, resulting from efficicnt Tl----" SI interconversion, and the ability to absorb thc IR radiation of a CO2-1aser in the electron ground and tripleT, states. The characteristics of the collisional transfer of vibrational energy are estimated using dcpendences of {he intensities and rates of decay Kdc c of the fast, microsecond component of activated fluorescence on the pressures of the vapors and foreign gases. The pressure dependence of Kde c varies in the investigated range of pressures and energies of excitation. At a constant density of CO2-1ascr energy, the rate of decay Kde c increases linearly with the pressures of the vapors and foreign gases only in a limited range of pressures and then begins to decrease. It should be noted that such dependences in mixtures with foreign gases are obscrvcd when there is a noticeable increase in the range of the values of ECO 2 used to the higher side. Anomalous dependences manifesting themselves in a decrease of Kde c with pressure begin to dominate at low laser energy densities. The dependences of the instantaneous values of lhe relative intensity l(v, 0/lo (v, t), where l(v, 0 and lo(v, t) are the intensities of luminescence at the maximum of the pulse of the activated fluorescence and of the