Ning Chai

Perturbative theory and modeling of electronic-resonance-enhanced coherent anti-Stokes Raman scattering spectroscopy of nitric oxide.

A theory is developed for three-laser electronic-resonance-enhanced (ERE) coherent anti-Stokes Raman scattering (CARS) spectroscopy of nitric oxide (NO). A vibrational Q-branch Raman polarization is excited in the NO molecule by the frequency difference between visible Raman pump and Stokes beams. An ultraviolet probe beam is scattered from the induced Raman polarization to produce an ultraviolet ERE-CARS signal. The frequency of the ultraviolet probe beam is selected to be in electronic resonance with rotational transitions in the A (2)Sigma(+)<--X (2)Pi (1,0) band of NO. This choice results in a resonance between the frequency of the ERE-CARS signal and transitions in the (0,0) band. The theoretical model for ERE-CARS NO spectra has been developed in the perturbative limit. Comparisons to experimental spectra are presented where either the probe laser was scanned with fixed Stokes frequency or the Stokes laser was scanned with fixed probe frequency. At atmospheric pressure and an NO concentration of 100 ppm, good agreement is found between theoretical and experimental spectral peak locations and relative intensities for both types of spectra. Factors relating to saturation in the experiments are discussed, including implications for the theoretical predictions.

Single-laser-shot detection of nitric oxide in reacting flows using electronic resonance enhanced coherent anti-Stokes Raman scattering

Single-laser-shot electronic resonance enhanced coherent anti-Stokes Raman scattering (ERE-CARS) spectra of nitric oxide (NO) were generated using the 532 nm output of an injection-seeded Nd:YAG (yttrium aluminum garnet) laser as the pump beam, a broadband dye laser at approximately 591 nm as the Stokes beam, and a 236 nm narrowband ultraviolet probe beam. Single-laser-shot ERE-CARS spectra of NO were acquired in an atmospheric-pressure hydrogen/air counterflow diffusion flame. The single-shot detection limit in this flame was found to be approximately 30 ppm, and the standard deviation of the measured NO concentration was found to be approximately 20% of the mean.

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

[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).

Dual-pump CARS Temperature and Major Species Concentration Measurements in a Gas Turbine Combustor Facility

A gas turbine combustor facility (GTCF) has been built and operated for measuring temperature and major species concentrations using dual-pump CARS in combusting flows at above atmospheric pressures. The facility includes a stainless steel window assembly that allows optical access from three sides with a pair of thin and thick windows on each side. It is water-cooled and provides air film-cooling passages; thin windows are designed for thermal load while thick windows are designed for pressure loading. High-speed imaging of combusting flows is performed using the center injector of a 9-point lean direct injection (LDI) device developed at NASA Glenn Research Center. The combustor has been operated using Jet A fuel at inlet air temperatures up to 725 K and combustor pressures up to 10 atm. Dual-pump CARS temperature and major species concentration measurements have been performed in the GTCF at inlet air temperatures up to 725 K and combustor pressures up to 7 atm. Spatial maps of temperature and major species concentrations have been obtained by translating the CARS probe volume in axial and radial directions inside the combustor rig. These measurements will be used for validation of CFD codes under development at NASA Glenn Research Center.

Measurements of Nitric Oxide Using Single-Shot, Midband Electronic-Resonance-Enhanced Coherent Anti-Stokes Raman Scattering (ERE-CARS) Vibrational Spectroscopy

Development of a single-laser-shot technique for acquiring electronic-resonance-enhanced (ERE) coherent anti-Stokes Raman scattering (CARS) spectra of nitric oxide (NO) is reported. Single-shot vibrational spectra are measured in an atmospheric pressure, room-temperature jet of NO.