In terms of optimal control, laser-induced molecular orientation is an optimisation problem involving a global minimum search on a multidimensional surface function of varying parameters characterising the laser pulse (frequency, peak intensity, temporal shape). Genetic algorithms, aiming at the optimisation of different possible targets, may temporarily be trapped in a local minimum, before reaching the global one. A careful study of such local (robust) minima provides a key for the thorough interpretation of the orientation dynamics, in terms of basic mechanisms. Two targets are retained: the first, simple, searching for an angle between molecular and laser polarisation axes as close as possible to zero (orientation) at a given time; the second, hybrid, combining the efficiency of orientation with its duration. Their respective roles are illustrated referring to two molecular systems, HCN and LiF, taken at a rigid rotor approximation level. A sudden and asymmetric laser pulse (provided by a frequency w superposed to its second harmonic 2w) leads to the kick mechanism. The result is a very fast (as compared to the rotational period) angular momentum transfer to the molecule, that turns out to be responsible for an efficient orientation after the laser pulse is off.

This article has been published in the Journal of Physics B 36, 4667 (2003) and may be found at http://www.iop.org/EJ/abstract/0953-4075/36/23/006/.

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