Determination of alignment parameters for symmetric top molecules using nonresonant two-photon absorption

The polarization dependence of the two-photon absorption signal is described directly in terms of the matrix elements of the irreducible representation of the two-photon absorption tensor operator for an ensemble with cylindrical symmetry probed with identical photons of linear polarization. Non vanishing matrix elements are easily determined from the known tensor patterns of the specific two-photon transition. The formalism is applicable to the extraction of alignment parameters for symmetric top molecules as well as diatomics produced in collisions of unpolarized particles or in the photodissociation with a single photon of linear polarization.