Excitation of polyatomic molecules by radiation

An elementary procedure for calculating quantum mechanically the time-dependent rotation-vibration wave function for a collisionless model of a polyatomic molecule in a monochromatic radiation field is described and applied to some very simple cases. It is concluded that molecules can in principle be strongly and selectively excited at radiation intensities which are too low to produce appreciable excitation in classical calculations. The excitation process is a coherent multi-photon Rabi precession between two discrete levels followed by a transition to a quasi-continuum of vibration-rotation states.