On chiral selectivity of enantiomers using a circularly polarized pulsed laser under resonant and off-resonant conditions

The rotating-wave approximation is employed to study the interaction of a pulsed circularly polarized laser with a chiral molecule with a view to controlling state probabilities. For a two-level model system, closed form solutions are available for ground and excited state populations for both resonant and off-resonant one-photon absorption transitions driven by lasers described by a wide class of pulse envelope functions, only one of which, the hyperbolic secant, is symmetric. By appropriate choice of the laser pulse width and envelope function, complete control is possible for molecules with fixed orientation with respect to the laser propagation direction when the optical frequency is equal to the difference in energy levels. In contrast, for off-resonant transitions, full control is lost for asymmetric pulse shapes. When the molecule assumes random orientations, the difference in the excited state relative populations of enantiomers is reduced significantly. Based on the study of off-resonant behavior, a new approach for chiral separation, which can dramatically improve the selectivity, is suggested.