High-order harmonic generation as a probe of rotational dynamics

Abstract

High-order harmonic signals from aligned molecules subject to a dissipative medium is studied theoretically using a density matrix formalism. Rotations are taken into account exactly, through an extension of a recently developed theory [S. Ramakrishna and T. Seideman, Phys. Rev. Lett. 99, 113901 (2007)], leading to an expression for the harmonic signal in terms of a series of rotational expectation values that are determined by the geometry of the experiment and the symmetry of the molecular ground electronic state. A multilevel Bloch model is used to illustrate the imprint of rotational decoherence and relaxation in the harmonic emission. It is shown that the harmonic intensities can be decomposed into two components, each of which carries a different physical interpretation and information content. The usefulness of the decomposition scheme in unraveling the interplay between the different dipole moment components that make the signal, and in relating the observable time dependence of the signal to the underlying electronic-rotational dynamics, is delineated. Finally, we show that the phase of the harmonic signal contains unique information regarding the phases of the electronic dipole matrix elements and hence the underlying electronic continuum.