Effect of Temperature on the Multiple Photon Absorption of Polyatomic Molecules.

The effect of temperature on the multiple photon absorption of many polyatomic molecules was investigated systematically for the first time. The purpose of this work was to gain a better understanding of the relation between the mechanisms of multiple photon absorption and the molecular parameters. The experimental technique used was a transmission measurement which gave the average number of photons absorbed per molecule in the laser beam path as functions of laser frequency, energy fluence, gas temperature, pressure and molecular species. A dramatic thermal enhancement of multiple photon absorption by SF(,6) was observed. This was manifested in an exponentially increasing integrated bandstrength as a function of temperature. The MPA integrated bandstrength approaches the linear absorption value, which is independent of temperature, with increasing temperature in the range of 200 K - 700 K. The hot bands absorb more efficiently at high temperatures than the fundamental band at low temperature. A method for determining the fraction of molecules participating in the multiple photon absorption process for SF(,6) at P(24) and P(28) laser frequencies was developed based on this new phenomenon. It was concluded that the fraction is a strong function of gas temperature but depends only weakly on the laser fluence in the region of hot band absorptions. The fraction determined by this method enabled a quantitative comparison of laser excitation with thermal heating. Laser excitation was found to be different from thermal heating at least up to an average absorption of 4 photons. Narrow multiple photon absorption spectra peaked at the Q branch of the linear absorption were observed for CH(,3)F with no evidence of overtone absorptions. Measurements of MPA by CH(,3)F were compared with a detailed calculation based on the accurately known spectroscopic data. Due to the large anharmonicity, direct n-photon transitions from the ground state to the v = n vibrational levels have negligible probability for n (GREATERTHEQ) 2 in the fluence range of the experiments. The temperature and frequency dependences of the calculated fraction of molecules absorbing are consistent with experimental observations. The levels of excitation for these calculated fractions of molecules inferred from experimental absorption cannot be explained by this calculation or any existing theories. Collisional enhancement of MPA and the high intensity spikes of the laser pulse were also insufficient to account for the levels of excitation. Therefore the multiple photon absorption of CH(,3)F must be caused by some mechanisms not yet considered. Eight molecules were chosen for a systematic investigation of the effect of temperature on the multiple photon absorption; SF(,6), OsO(,4), CCl(,3)F, CF(,3)I, CH(,3)Br, C(,2)H(,5)F, CD(,3)H and CH(,3)F. These molecules were chosen according to different categories of molecular parameters, which were defined by the degeneracies of the laser pumped mode and the vibrational modes with frequencies lower than that of the pumped mode. It was found that only molecules with the pumped mode and the lower frequency modes both degenerate have thermal enhancements of the multiple photon absorption. A first-order Fermi Golden Rule type leakage from the discrete states to background states and anharmonic splitting compensation of anharmonicity were both shown to be physical mechanisms that are consistent with this finding. A possible application of this phenomenon to enhancing the efficiency of deuterium isotope separation process was discussed.