Two-photon spectroscopy of the low lying Rydberg states of NO. I. The 3p and 3d complexes

Abstract

The rotational structure and polarization dependence of two-photon spectra of aligned ensembles of open shell diatomics is investigated in terms of the spherical tensor components of the two-photon absorption operator. The formalism allows the straightforward incorporation of state interactions and perturbations. It is applied to the two-photon spectroscopy of NO, in particular to the excitation of the Rydberg states derived from the 3p and 3d complexes. All states investigated show a nearly quadratic power dependence indicating the saturation of the ionization step. Transitions dominated by a zeroth rank tensor component (e.g., C 2Π–X 2Π or H 2Σ, H′ 2Π–X 2Π) are insensitive to a possible angular momentum alignment in the ensemble. These transitions are ideally suited to determine degeneracy averaged observables, e.g., collision cross sections in a molecular beam scattering experiment or product velocity anisotropies in a single color photodissociation experiment. Rotational alignment data must be determined using two-photon transitions which are carried by a second rank tensor component (e.g., D 2Σ–X 2Π or F 2Δ–X 2Π).