Stimulated light scattering in gaseous Ar, 0₂, and Ar-0₂ mixtures

Hsu, Chimei James (1971) Stimulated light scattering in gaseous Ar, 0₂, and Ar-0₂ mixtures. Masters thesis, Memorial University of Newfoundland.

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Abstract

The backscattered light from pure argon and from argon-oxygen mixtures at ambient temperature was investigated using a giant-pulse ruby laser. The gas samples at pressures in the range of 400 psi to 5000 psi were irradiated by light at λ 6943Å, whose peak power was approximately 100 MW with temporal pulse width of 20 ns. When the backscattered light was analyzed by means of a Fabry-Perot interferometer, its frequency profile led to a nonlinear scattering phenomenon resembling stimulated thermal Rayleigh scattering (STRS) in absorbing gases. In the course of investigation, it was found that the presence of minute concentration of oxygen impurity in argon produced a pronounced effect on the intensity of this scattered light. The scattering is positively identified as STRS based on the following experimental features in conjunction with a modified Gray-Herman theory on STRS: -- (1) The frequency shift of the scattered light is typically 0.01 cm⁻¹ to the higher frequency side. -- (2) As the density of pure argon gas is increased, the threshold power, 0Pth, decreases following the relationship which is consistant with the modified theory, [special characters omitted], -- where ρ0 is the argon gas density and ρη is a constant. -- (3) When the threshold power, Pth, for an argon sample with a fixed oxygen impurity concentration is compared with that for pure argon at the same density, the following phenomenological relationship is derived: 0p2th/p2th = (1-Br), where B is a constant end r the percent concentration of oxygen in the sample. -- In the present modified theory, the existance of laser-induced plasma in the gas sample and the presence of oxygen molecules in the argon medium were taken into consideration. -- Stimulated Brillouin scattering (SBS) in pure argon and in pure oxygen gases was also investigated as a function of density. The experimental determination of Brillouin shifts in these gases revealed a remarkable fact that the values obtained from the "higher order" SBS lines seem to give sets of "smooth" hypersonic velocities while those obtained from the first SBS shifts show unusually large fluctuations outside the estimated experimental error. This fluctuation may be attributable to the initial laser-induced disturbance in the gas medium which has a short time constant and, consequently, this disturbance contributes little to the recurring SBS components.

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