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The possibility of measuring the hydrogen impurity concentration in dense gas mixtures by coherent anti-Stokes Raman
scattering (CARS) is studied. In this technique, biharmonic laser pumping based on stimulated Raman scattering (SRS)
in compressed hydrogen is used. Because of the interference between the coherent scattering components from buffer
gas molecules and molecules of the impurity to be detected, the signal recorded may depend on the hydrogen
concentration by a parabolic law, which has a minimum and makes the results uncertain. It is shown that this uncertainty
can be removed if the frequency of the biharmonic laser pump, which is produced by the SRS oscillator, somewhat
differs from the frequency of molecular oscillations of hydrogen in the test mixture. A sensitivity of 5 ppm is obtained as
applied to the hydrogen-air mixture under normal pressure. The description of a set-up for the determination of the
coefficient of the hydrogen diffusion in gas mixtures is given. The main assembly units are a diffusion chamber and an
automated laser system for the selective hydrogen diagnostics in gas mixtures by the SRS-CARS method. The
determination of the diffusion coefficient is based on the approximation of the experimental data describing the hydrogen
concentration varying with time at a specified point in the diffusion chamber and the accurate solution of the diffusion
equation for the selected one-dimensional geometry of the experiment.
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