Laser-induced alignment dynamics of HCN: Roles of the permanent dipole moment and the polarizability

The alignment dynamics of HCN, in a linear configuration, interacting with linearly polarized infrared laser pulses are studied numerically by exact (nonperturbative) solutions of the time-dependent Schr\"odinger equation. The alignment, with respect to the laser field polarization vector, is measured from the angular distribution of the molecule using a defined half angle ₁/₂. It is shown that, at intensities I=10^13W/cm^2, alignment can be achieved on a subpicosecond time scale with a single laser frequency, in the presence of simultaneous dipole- and polarizability-field interactions. The results are compared to those of a laser-driven rigid-rotor analytical model that is thoroughly developed. The importance of the permanent and field-induced dipole moments on the alignment process is investigated, as well as the role of vibrational excitation of the two molecular bonds.