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 orientation remains a very promising issue with challenging applications extending from chemical reactivity to nanoscale design. We emphasize the complementarity 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 appropriately 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 polarization) is the goal of laser control. Two mechanisms are considered. The first is based upon an asymmetric pulse combining a frequency w and its second harmonic 2w resonant with a vibrational transition. A much more efficient mechanism is the so-called "kick" that a highly asymmetric sudden pulse can impart to the molecule. Half-cycle pulses, within the reach of current experimental technology, are among good candidates for producing such kicks. Very interestingly, an optimal control scheme for orientation, based on genetic algorithms, also leads to a sudden pulsed field bearing the characteristic features of the kick mechanism. Optimal pulse shaping for very efficient and long-lasting orientation, together with robustness with respect to temperature effects, are among our future prospects.

Published in Quantum Control: Mathematical and Numerical Challenges, A. D. Bandrauk, M. C. Delfour, and C. Le Bris, eds., CRM Proc. Lecture Notes 33, 1 (2003).

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