Knowledge Bank

Laser and combustor development relies heavily upon the characterization of the media in terms of species concentration, temperature, and velocity. In order to meet this need, CARS is presently being developed as a diagnostic toot for accomplishing real-time measurements of these parameters. Calibration of a CARS system for such measurements requires information on the functional dependence of CARS intensity upon pressure and temperature since real-time measurements have limited spectral resolution. In this paper, the results of a theoretical and experimental investigation of the pressure dependence of the integrated intensity are presented for $CH4,CO,N2$, and $H2$. Experimental data were obtained with a CARS system utilizing a single-mode ruby laser to generate the pump beam and a ruby-pumped dye laser to generate the Stokes beam which has a bandwidth sufficient for obtaining integrated intensities with a photomultiplier. Because of the pressure broadening of the Raman lines, the integrated intensity deviates from a simple square-law dependence of the number density, following an individualistic curve. In addition, ``collisional narrowing” is demonstrated for the $H2$ molecule, and the linewidth is determined through integrated-intensity measurements. Integrated-intensity data are required for real-time concentration measurements since the time resolution involved with spectral scanning is insufficient for characterization of combustion media.