THERMAL AND STIMULATED MOLECULAR SCATTERING OF LIGHT IN LIQUIDS

Abstract

The temperature dependence of the spectrum of the light scattered by anisotropy fluctuations is studied in liquids (salol and benzophenone). Two branches are observed in the temperature dependence of the splitting between the components of the doublet induced by shear waves. There is a low temperature branch, in which the splitting between thecompoments decreases when temperature increases, and there is a high temperature branch, in which this splitting decreases when temperature decreases. In the Rayleigh line wing of the liquids that were studied one can distinguish two lorentzians the width of which differed by two orders. Two relaxation times and their temperature dependence are defined from these data. With the use of these measurements and data of the temperature dependence of an other parameter it is shown that Rytov's theory with two times of anisotropy relaxation describes very well the temperature behaviour of the splitting between components of Rayleigh wing fine structure. The gain of stimulated Rayleigh line wing light scattering is measured as well. The phenomenon of stimulated temperature light scattering due to an electrocalorical effect is observed in the liquids that were studied. Laser application in the investigations of physical optics led to the discovery of several new phenomena. Particularly, many good results have been obtained these recent years in the study of thermal and stimulated molecular scattering of light [I], [4]. The number of works in this field is ever growing. Some recent results of the authors and their colleagues will be under discussion in this paper concerning the studies of spectra of thermal depolarized light scattering, stimulated light scattering in Rayleigh line wing, and stimulated entropy or temperature scattering of light. Fine structure of Rayleigh line wing. The fine structure in the spectrum of thermal depolarized scattering of light (fine structure of Rayleigh line wing (FSW)) was first observed and interpreted by Tiganov and the authors [5]. The new phenomenon implies that a doublet can be observed for certain polarizations of exciting and scattered light in the spectrum of the Rayleigh line wing. This phenomenon occurs due to scattering of light by anisotropy fluctuations produced by the fluctuations of deformations (shear waves) [5]. This interpretation was proved by the studies of polarization, angular and light frequency dependence of FSW component shift [5] , [6], [7], [S], ~91. In the first temperature investigations it was found unexpectedly that when the temperature decreases the interval between the FSW components does not increase and even decreases [5], [6]. The further experiments made by Sabirov and the authors [8], [9] revealed the whole picture of somewhat extraordinary temperature dependence of the component shift produced by transverse hypersound. The diagram of the dependence of intervals between the components of the transverse doublet 2 Av, vs temperature in salol is given in figure 1 for the case when its viscosity changed from 10' to poise. In the temperature range of 500C to 3 OC the Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972137 CI-216 1. L. FABELINSIUI AND V. S. STARUNOV interval between the transverse components is decreasing when the temperature increases, as it was expected from the Maxwell scheme of viscosity and the Leontovich theory with one relaxation time [I]. FIG. 1. Temperature dependence of splittings between the FSW components in liquid salol. theory, 0 experiment. In the temperature range of 3 OC to 45 OC the doublet was not observed. In the temperature range of 45 OC to 120 OC FSW was observed, the distance between the FSW components was increasing when the temperature increases. The same dependence of Av, upon t was observed by Sabirov and the authors for the case of benzophenone. We showed in [8] that the described character of Av, dependence vs t could be qualitatively explained if one supposed the existence of two processes of fluctuation dissipation with different relaxation times 7, and z2 (2, > 2,). The 'theoretical calculation of light scattering anisotropy fluctuations with the account of two relaxation times has been recently performed by Volterra [lo], Rytov [Ill and Romanov and Solovyev [12]. If in the general Rytov's theory one assumes the same dispersion law for optoelastic coefficient X(o) as for shear modulus i (o) the following equation for the intensity distribution can be obtained [13] : Here X,, p, are the optoelastic coefficient and the shear modulus for o -+ co. 490, p are the shear viscosity and the density, z, = yo/p,, (2, 2 z, 2 z2), 0 is the angle of scattering. q = 2 K sin 912 where K is wave-number of the exciting light. T and R are represented as following : The first term in eq. (1) is responsible for arising of the shear components in the light scattering. Let us introduce the symbol y = (52; T:)-~, where