Parametric excitation of vibrations in polyatomic molecules by a subbarrier transition in energy space

An analysis is made of the excitation of a frequency-modulated quantum nonlinear oscillator when the modulation amplitude increases with the quantum number so that the classical condition of parametric excitation is only satisfied for fairly high levels. An estimate is given of the time taken to excite the oscillator from the ground state. Excitation becomes possible as a result of a subbarrier transition to higher levels. This model can describe the buildup of vibrations in polyatomic molecules in a laser radiation field.