Two-frequency IR laser orientation of polar molecules. Numerical simulations for HCN

Using ab initio nuclear-coordinate-dependent dipole moments and polarizabilities, we study the orientation dynamics of HCN, by numerically solving the time-dependent Schrödinger equation, in the presence of a superposition of intense, linearly-polarized infrared laser pulses of frequency ω and 2ω. We show that polarizability acts in concert with permanent dipole moments to orient polar molecules, as opposed to alignment which occurs alone with a single laser frequency or one moment only (permanent or induced). Optimal orientation occurs for the field configuration E(t)=E0(t)cos ωt+0.5 cos 2ωt, where 2ω is resonant with a 0 → 1 vibrational transition and E0(t) is a picosecond pulse.