Optimal control simulation of field-free molecular orientation: alignment-enhanced molecular orientation.

Nonresonant optimal control simulation is applied to a CO molecule to design two-color phase-locked laser pulses (800 nm + 400 nm) with the aim of orienting the molecule under the field-free condition. The optimal pulse consists of two subpulses: the first subpulse aligns the molecule and the second one orients it. The molecular alignment induced by the first subpulse considerably enhances the degree of orientation, the value of which is close to an ideal value at temperature T = 0 K. To confirm the effectiveness of this alignment-enhanced orientation mechanism, we adopt a set of model Gaussian pulses and calculate the maximum degrees of orientation as a function of the delay time and the intensity. In finite-temperature (T = 3.0 K and T = 5.0 K) cases, although the alignment subpulse can improve the degree of orientation, the control achievement decreases with temperature rapidly; this decrease can be attributed to the initial-state-dependent (phase-shifted) rotational wave packet motion.