Laser-induced molecular rotational dynamics: A high-frequency Floquet approach

The rotational dynamics of a molecule interacting with a linearly polarized intense laser field is investigated. A high-frequency Floquet approach proceeding to an adiabatic separation between ``fast'' field-oscillation modes and ``slow'' molecular rotational motion is adopted, which leads to an approximate effective Hamiltonian. A thorough analysis of the validity of the approximation is provided by examining the neglected terms. The HCN molecule, within a linear rigid-rotor description, is taken as an illustration of the model. The anisotropic interaction of the electric-field vector of the intense laser radiation with the permanent dipole moment and the polarizability of the molecule creates aligned pendular states. These states are eigenfunctions of an effective Hamiltonian governed by a cos²θ potential (with θ the angle between the molecular axis and the field vector) and serve as a basis for the interpretation and prediction of alignment dynamics. The results show, in particular, the difference in the alignment properties of linear HCN when excited by sudden versus adiabatic laser pulses, where adiabatic transport from field-free to pendular states can be evoked.