Pressure dependence of vibrational Raman scattering of narrow-band, 248-nm, laser light by H2,N2,O2,CO2, CH4, C2H6, and C3H8 as high as 97 bar

We have previously investigated methods that image high-pressure processes such as combustion inside automobile cylinders and aircraft engines, or chemical phenomena in supercritical fluids. Here we show that vibrational Raman scattering can simply obtain, quantitatively, densities of some combustion-relevant molecules. We use narrowband KrF excimer-laser light. Measurements for H-2, N-2, O-2, CO2, and CH4 are in the pressure range from 1 to 60 bar, whereas those for C2H6 and C3H8 are up to their respective vapor pressures. All these species are at ambient temperature. Additional measurements are described for CO2 up to 96.8 bar and 318 K, where CO2 is a supercritical fluid. The O-2 measurements are complicated by a photochemical formation of O-3; those in supercritical CO2 by drastic bending of the laser beam within this medium. We show that, for each gas, the Raman signal is directly proportional to gas density, thereby making quantitative analysis particularly convenient. For each species, we present an estimate of its Raman cross-section relative to that of Nz. However we recommend that future diagnostics users calibrate their own systems for relative species sensitivity.