Collisionless excitation of vibrational transitions of nearly symmetric top molecules in the field of a spectrally limited laser pulse was studied analytically. Within the framework of Shrodinger equation for probability amplitudes of vibrational-rotational states and statistical description of a complicated spectral structure of molecular absorption bands we derived formulas for the vibrational photoexcitation probability. It was shown that there existed three interaction regimes between the molecules and the field that differed in the dependence of the excitation probability on the radiant intensity. The excitation efficiency for O2 molecules under collision and collisionless conditions was compared.
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