COMBINATION FREQUENCY GENERATION IN RAMAN LASER EMISSION

Present address of L. J. Nugent: Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, $^{1}$ E. Garmire, F. Pandarese and C. H. Townes, Phys, Rev, Lett. 11, 160 (1963).Author Institution: Electro-Optical Systems, Incorporated, 300 N. Halstead Street; Chemistry Division, Oak Ridge National LaboratoryIn two experiments giant-pulse ruby-excited coherent Raman emission spectra were observed from high pressure $CH_{4}$ and $H_{2}$ gases. In the first experiment approximately equimolar mixtures of these gases were contained within one scattering cell. In the second experiment each gas was isolated in a separate cell and the cells were placed in series so the excitation could proceed first through one and then through the other. The spectra in both cases are essentially identical. Above an excitation threshold, characteristic stimulated Raman emission from each gas is observed at $v_{r}+m\nu (j)$, where $\nu_{r}$ is the excitation frequency, in is any positive or negative integer, and $\nu(j)$ is the frequency of a totally symmetric molecular vibration of molecule j. Of greater significance, however, was the presence in both cases of the combination frequencies $\nu_{r}+m \nu (H_{2}) + n\nu(CH_{4})$ in high intensity. In another series of experiments, Raman laser emission from the following isolated two-component series configurations also contained high intensity combination frequencies as well as the characteristic stimulated emission from each substance: $CH_{4}(g)-C_{5}H_{6}(1), H_{2}(g)-C_{5}H_{6}(1), CH_{4}(g)-CS_{2}(1)$, and $H_{2}(g)-CS_{2}(1)$, where the gas in each case directly followed the ruby laser, These results are examined in the light of the classical theory of stimulated Raman $emission.^{1}$. The effect of molecular interactions is discussed and a mechanism which can be independent of the interactions between different molecular species is proposed for the production of the combination frequencies. Work supported in part by USA Chemical Research and Development Laboratories, Edgewood Arsenal, Md. under Contract DA-04-195-AMC-1218(A)