Stimulated thermal Rayleigh scattering in argon

We have observed stimulated light scattering from pure argon gas and argon gas with oxygen impurities (≤2%) in the gas density range of 30 to 350 amagat. By comparing our measured threshold laser power as a function of gas density with the corresponding theoretical values for stimulated thermal Rayleigh scattering (STRS), we identify the present scattering as STRS. We present arguments to show that the necessary absorption of laser light and subsequent thermalization which is essential for STRS can be provided by ${\mathrm{Ar}}_2^{*}$ molecules. These molecules are formed in the laser-produced plasma. The effect of small percentages of oxygen impurity in the argon gas seems to support this point of view.