Combination Lines in Stimulated Raman Emission from Gas Mixtures

Stimulated Raman emission (SRE) spectra were observed from high‐pressure CH₄ and H₂ in two different experiments. In the first experiment the gases were mixed together in one scattering cell, and a laser pulse excited the mixture; in the second experiment they were isolated from each other in separate cells placed in series so that one laser pulse excited both gases. The spectra observed in both experiments are essentially identical: The well‐known SRE lines from each gas were at ${\mathrm{\nu ̃}}_0{\mathrm{+n\nu ̃}}_1\mathrm{(i)}$ , where ${\mathrm{\nu ̃}}_0$ is the wavenumber associated with the laser excitation, ${\mathrm{\nu ̃}}_1\mathrm{(i)}$ is the wavenumber associated with the frequency of a totally symmetric vibration of molecule i, and n≐±1, ±2,···. In addition, strong lines at ${\mathrm{\nu ̃}}_0{\mathrm{+n\nu ̃}}_1({\mathrm{H}}_2{\mathrm{)+m\nu ̃}}_1({\mathrm{CH}}_4)$ were present in all combinations in all orders wherever the spectrograph was sensitive enough for detection. This is the first report of SRE combination lines from the gas phase. In general two thresholds were observed, the first for the appearance of SRE lines from one gas, the second for the appearance of the SRE lines from the other gas, and the threshold order depended on the particular experimental conditions. In every case, at the second threshold all the combination lines were present, and before the second threshold all the combination lines were absent. These results are examined in the light of the classical theory of SRE including the effects of molecular interactions, a mechanism for the generation of the SRE combination lines is presented, and it is conluded that molecular interactions play only a minor and unessential role in this process in the gas phase.