Electronic-Resonance-Enhanced (ERE) Coherent Anti-Stokes Raman Scattering (CARS) Spectroscopy of Nitric Oxide

A dual-pump, electronic-resonance-enhanced (ERE) coherent anti-Stokes Raman scattering (CARS) technique for the measurement of minor species concentrations has been demonstrated. The frequency difference between a visible Raman pump beam and Stokes beam is tuned to a vibrational Q-branch Raman resonance of the nitric oxide (NO) molecule to create a Raman polarization in the medium. The second pump beam is tuned into resonance with rotational transitions in the (1,0) band of the AΣ XΠ electronic transition at 236 nm, and the CARS signal is thus resonant with transitions in the (0,0) band. We observe significant resonant enhancement of the NO CARS signal and have obtained good agreement between calculated and experimental spectra. Corresponding author: [email protected] Associated Web site: http://ME.www.ecn.purdue.edu/ME/Research/Combustion/ Proceedings of the Third Joint Meeting of the U.S. Sections of The Combustion Institute Introduction Electronic-resonance-enhanced (ERE) coherent antiStokes Raman scattering (CARS) measurements of nitric oxide (NO) were performed using a three-color CARS technique. In this dual-pump technique, the second pump beam is an ultraviolet laser beam with a frequency tuned into electronic resonance with specific transitions in the NO molecule. The technique that we have demonstrated is a variant of the dual-pump CARS technique developed for the simultaneous detection of two species [1,2]. The first pump and the Stokes beam are visible laser beams with frequencies far from resonance with the AΣ XΠ electronic transition. The second pump beam at frequency ω3 is at or near electronic resonance. This wide separation of the frequencies ω1 and ω3 of the two pump beams distinguishes our technique from previous ERE CARS experiments [3-5], which have been performed with the same laser frequency for both pump beams, and with both the pump and Stokes beams at or near electronic resonance. In some cases, three laser frequencies, all in or near electronic resonance, have been used in ERE CARS experiments [6,7]. Experimental System The experimental system for the ERE CARS measurements of NO is shown in Fig. 1. The pump source for the ω1 pump beam was a Continuum Model Powerlite 9010 injection-seeded, Q-switched Nd:YAG laser with a repetition rate of 10 Hz, pulse length of approximately 7 ns, and pulse energy for the 532-nm output of approximately 750 mJ. The 532-nm output was also used to pump a Continuum Model ND6000 narrowband, tunable dye laser to produce the Stokes beam (ω2) at a wavelength near 590 nm with a frequency bandwidth of approximately 0.08 cm. The 355-nm third-harmonic output of a Continuum Model Powerlite 8010 Nd:YAG laser was used to pump a second Continuum Model ND6000 dye laser to produce tunable laser radiation at a wavelength of 472 nm. The 472-nm output of the dye laser was frequency-doubled to 236 nm using a beta barium borate (β-BBO) crystal to produce the ultraviolet pump beam at frequency ω3 with an estimated frequency bandwidth of 0.2-0.3 cm. The CARS signal was generated using a threedimensionally phase-matched arrangement as shown in Fig. 2. The pulse energies for the 532-nm, 590-nm, and 236-nm beams at the CARS probe volume were typically 30 mJ, 20 mJ, and 1 mJ, respectively. The CARS