Determination of orientation of the ground state using two-photon nonresonant excitation

Abstract
A method is presented for determining the population A{0}0+, the alignment factors A{2}q± and A{4}q ±, and the orientation factors A{1}q± and A{3}q± for a ground state distribution of diatomic (symmetric top) molecules probed by elliptically polarized two‐photon nonresonant excitation. General expressions are developed for the O, P, Q, R, and S branch transitions as a function of the rotational quantum number, J. This treatment assumes that the resonant state reached by the two‐photon transition is subsequently detected independent of its orientation and alignment. This can be achieved by 2+n multiphoton ionization in which the ionization steps are saturated, or by 2+1 laser induced fluorescence in which the fluorescence is collected independent of its polarization and spatial anisotropy. For the case where elliptically polarized light is created by passing linearly polarized light through a quarter‐wave plate, the alignment and orientation moments can be independently determined using a single experimental excitation–detection geometry.