Electronic-Resonance-Enhanced Coherent Anti-Stokes Raman Scattering of Nitric Oxide: Non-Perturbative Time- Dependent Modeling

Abstract

[Abstract] A theoretical analysis of electronic-resonance-enhanced (ERE) coherent anti-Stokes Raman scattering (CARS) of NO is discussed. The time-dependent density matrix equations for the nonlinear ERE-CARS process are derived and manipulated into a form suitable for direct numerical integration (DNI). In the ERE-CARS configuration considered in this paper, the pump and Stokes beams are far from electronic resonance. The visible 532 nm pump beam and the 591 nm Stokes beam are used to excite Qbranch Raman resonances in the vibrational bands of the X 2  electronic state of NO. An ultra-violet probe beam at 236 nm is tuned to P-, Q-, or R- branch transitions in the (v′ = 0, v′′ = 1) band of the A 2  + - X 2  electronic system of NO molecule. Experimental spectra are obtained either by scanning the ultraviolet probe beam while keeping the Stokes frequency fixed (probe scans) or by scanning the Stokes frequency while keeping the probe frequency fixed (Stokes scans). The calculated NO ERE-CARS spectra are compared with experimental spectra, and good agreement is observed between theory and experiment in terms of spectral peak locations and relative intensities. The saturation of the Raman transition is found to be dependent on the level of saturation of the electronic transition, and vice versa. The effect of Stark shifting of the upper level of the probe transition induced by high laser intensities in the pump and/or Stokes beams is discussed. In addition, we have developed a numerical code to simulate single-shot ERE CARS spectra acquired using a broadband Stokes beam. We have included the multimode structure of both the broadband Stokes beam (~50 cm -1 FWHM) and the ultraviolet Stokes beam (0.2 cm -1 FWHM).