Molecular alignment and orientation: From laser-induced mechanisms to optimal control

Abstract

Genetic algorithms, as implemented in optimal control strategies, are currently successfully exploited in a wide range of problems in molecular physics. In this context, laser control of molecular alignment and orienta- tion remains a very promising issue with challenging applications extending from chemical reactivity to nanoscale design. We emphasize the complemen- tarity between basic quantum mechanisms monitoring alignment/orientation processes and optimal control scenarios. More explicitly, if on one hand we can help the optimal control scheme to take advantage of such mechanisms by ap- propriately building the targets and delineating the parameter sampling space, on the other hand we expect to learn, from optimal control results, some robust and physically sound dynamical mechanisms. One of the basic mechanisms for alignment (i.e., molecular axis parallel to field polarization) is related to the pendular states accommodated by the molecule-plus-field effective potential. The laser control of alignment can be reached by an adiabatic transport of an initial isotropic rotational state on some pendular state trapping the molecule in well-aligned geometries. Symmetry breaking mechanisms are to be looked for when orientation (i.e., molecular axis in the same direction as field polar- ization) is the goal of laser control. Two mechanisms are considered. The first is based upon an asymmetric pulse combining a frequency ! and its second harmonic 2! resonant with a vibrational transition. A much more efficient mechanism is the so-called that a highly asymmetric sudden pulse can impart to the molecule. Half-cycle pulses, within the reach of current experi- mental technology, are among good candidates for producing such kicks. Very interestingly, an optimal control scheme for orientation, based on genetic algo- rithms, also leads to a sudden pulsed field bearing the characteristic features of the kick mechanism. Optimal pulse shaping for very efficient a long-lasting orientation, together with robustness with respect to temperature effects, are among our future prospects.