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

• Published in: Journal of Raman spectroscopy 2020-05, Vol.51 (5), p.807-828
• Main Authors: Satija, Aman, Chai, Ning, Arendt, Michael T., Lucht, Robert P.
• Format: Article
• Language: English
• Subjects: Broadband; coherent anti‐Stokes Raman scattering; Coherent scattering; Configurations; Dye lasers; Electron spin; Invariants; irreducible spherical tensor analysis; Laser beams; laser diagnostics; Mathematical analysis; Neodymium lasers; Nitric oxide; Polarizability; pure rotational Raman; Raman invariants; Raman spectra; Raman spectroscopy; Rotational spectra; Semiconductor lasers; Spectroscopy; Spectrum analysis; Tensor analysis; Tensors; Wave functions; YAG lasers
• ISSN: 0377-0486; 1097-4555
• DOI: 10.1002/jrs.5836

Coherent 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 ηv=0α^q=02ηv=0 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 ηv=0α^q=22ηv=0 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 ηv=0α^q=22ηv=0 polarizability tensor element are proposed. Coherent anti‐Stokes Raman scattering (CARS) measurements of the pure rotational Raman spectrum of nitric oxide were performed. The Raman spectrum of NO is of significant theoretical interest because of the spin splitting in the ground electronic level of NO. Pure rotational Raman tensor invariants were determined for NO from measurements of mixtures of NO and nitrogen. 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.