Quantum interference in the REMPI detection of aligned NO

Abstract

We study the detection of collision induced alignment in ensembles of NO molecules scattered from He through (2 + 1) resonance enhanced multiphoton ionization (REMPI) of NO via the Rydberg states: H2Σ,H′2Π. The two-photon spectroscopy of the H–X transition is complicated by the state mixing due to L-uncoupling and the simultaneous presence of zeroth and second rank tensor components for this strong transition. The creation of an aligned ensemble of NO molecules in a well-defined collision process enables us to study in detail the polarization effect for different branches of the H–X two-photon spectrum. The polarization effect results from the interference of the second rank tensor components with the zeroth rank tensor of the two-photon absorption tensor. Variations in the polarization effect for perturbed levels of the excited state reflect directly the changes in the expansion coefficients for the involved electronic wavefunctions. The experimentally observed polarization effects are consistent with the non-vanishing components (Z(0)0:Z(2)0:Z(2)2)=(1.0:−0.2:0.6) of the two-photon absorption tensors.