Pure rotational coherent anti‐Stokes Raman scattering spectroscopy of nitric oxide: Determination of Raman tensor invariants

Abstract

AbstractCoherent anti‐Stokes Raman scattering (CARS) measurements of the pure rotational Raman spectrum of nitric oxide were performed. Measurements were performed in a room‐temperature gas cell using a dual‐broadband pure rotational CARS configuration. In this configuration, a broadband dye laser was used to generate the pump and Stokes beams, and the 355‐nm third‐harmonic beam from a Nd:YAG laser was used as the probe beam. The Raman spectrum of NO is of significant theoretical interest because of the spin splitting in the ground electronic level of NO. A detailed model of the pure rotational Raman spectrum of NO was developed based on an irreducible tensor analysis using Hund's case (a) wave functions as basis states for the analysis. The pure rotational Raman polarizability tensor element in the molecular frame was determined for NO based on fitting pure rotational CARS spectra of mixtures of NO and nitrogen (N2). In addition to the pure rotational CARS spectrum featuring 2Π1/2→2Π1/2 and 2Π3/2→2Π3/2 transitions, there is an electronic Raman transition at 121 cm−1 between the spin split 2Π1/2 and 2Π3/2 ground electronic levels that has been the subject of previous theoretical interest, but the value of the tensor invariant that contributes to the intensity of the electronic Raman transitions has not been quantitatively determined. Analysis of our pure rotational CARS spectra indicates that the magnitude of this term is much smaller than indicated in previous work, but the analysis is complicated by the weakness of the electronic Raman transitions compared with pure rotational transitions in the same spectral region. Future experiments to more definitively determine the magnitude and sign of the polarizability tensor element are proposed.