Rotational and vibrational wave packet motion during the infrared multiphoton excitation of HF

The time dependent quantum dynamics of molecular rotations and vibrations during coherent infrared multiphoton excitation are investigated by calculation of the time evolution of the wave packet in the rotational and vibrational configuration space. Results are presented for the rovibrational motion of HF using recent potential energy and dipole moment functions developed elsewhere by ab initio calculations. Several initial conditions are studied, from the single rovibrational ground state to a superposition of states with different J quantum numbers corresponding to highly oriented molecules. A careful examination of the rovibrational wave packet motion allows for a simple interpretation of the rotational motion and the effects of different initial conditions on the intramolecular kinetics. It is found that, depending on the degree of orientation, the HF molecule undergoes very fast deorientation, faster than its "classical" rotational period, thus perturbing the generation of a semiclassical vibrational motion during the excitation process. (C) 2001 American Institute of Physics.