Multiphoton excitation processes between vibrational states of molecules

An approximation method has been developed to evaluate the perturbation theory expressions for the probabilities of two- and three-photon transitions among vibrational-rotational levels of diatomic and polyatomic molecules. The sum over intermediate rotational states and the degenerate quantum number is evaluated for a general case, and the sum over intermediate vibrational states for particular cases. The molecule is taken to be a rotating anharmonic oscillator. The electrical anharmonicity and the effects of the electronic states of the molecule are considered. Two- and three-photon processes, with or without the presence of near-resonant intermediate states, should be observable with available infrared lasers. However, the intensities predicted for the observation of three-photon excitation would result in more dominant effects such as dissociation or gas breakdown.