Infrared multiphoton absorption and alignment of diatomic molecules in a continuous wave field

We compute multiphoton absorption spectra of HF and LiH in a continuous wave field, as generic examples of diatomic vib-rotors. Absorbed energy and other observables are determined by numerical integration of the time-dependent Schrodinger equation, taking into account the coupling of both permanent dipole and electronic polarizability with radiation. Three peculiar effects of strong laser fields are investigated: first, the frequency shift of the absorption peaks and its dependence on laser intensity, vibrational and rotational quantum numbers of the final state, and molecular parameters; second, the existence of subharmonic resonances, strongly influenced by the electronic polarizability; third, the considerable degree of alignment (with negligible orientation) along the laser polarization axis. (C) 2002 American Institute of Physics.