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Ohio State University

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Coherent anti-Stokes Raman spectroscopy (CARS) is a new and exciting method for optically probing matter. If two input laser beams (one of which is tunable) are focused in a medium (gas, liquid or solid), a coherent, laser-like beam at the anti-Stokes frequency is generated. Conversion efficiencies can become quite high (as much as 1% of the tunable laser output) as the difference in frequency between the two exciting lasers approaches a Raman resonance in the material. A trace of the CARS output, with respect to the frequency difference in the two laser beams constitutes a CARS spectrum. The laser system used in these experiments consists of a double $TEM_{00}$-Nd:YAG laser operating at 532 nm with $0.03 cm^{-1}$ line width and 5 MW output in 18 ns pulse width at up to 10 Hz. A portion of this laser is split off to pump a tunable dye laser and amplifier. A telescope-grating-tuning-element combination yield a dye laser line width of $\sim 0.3$ $cm^{-1}$. An additional etalon may be used to reduce the line width to $\sim$ $cm^{-1}$. Because narrow line widths in laser may be achieved with only modest sacrifices in output power, measuring high resolution CARS Spectra of gases (0.03-$0.03 cm^{-1}$) at low pressures ($< 1$ Torr) are possible. In this presentation we show the CARS spectra of a number of different gases (e.g., $D_{2}$, $CH_{4}$, $NH_{3}$, various hydrocarbons, etc.). Some of these materials have been probed at the center of electrical discharge and/or in flames, where the anti-Stokes and coherent properties, together with the high intensity of CARS output, have enabled one to easily record Raman spectra under conditions very unfavorable for measurement by spontaneous Raman scattering. Vibrational and rotational temperatures of the species within the discharge or flame have been measured. For example, a vibrational temperature of $1050^\circ K$ and a rotational temperature of $400^\circ$ have been determined for $D_{2}$ (at $\sim50$ Torr) at the center of a dc electrical discharge.


Author Institution: Naval Research Laboratory