Stimulated Raman effect in ten liquids

Abstract

The nonlinear processes which occur in a liquid medium due to the presence of large amplitude laser radiation is investigated both theoretically and experimentally. In particular, the self-focussing mechanisms which precede the stimulated Raman scattering (SRS) effect and the SRS phenomena itself are reviewed. A new theoretical approach is presented which proposes a direct method of experimentally determining the self-focussing power of a medium due to the reorientational Kerr effect. It is shown that if one applies a static dc field across the medium such that the E-vector is directed perpendicular to the propagating E-vector of the ac field, then the change in the index of refraction of the medium in the direction parallel to the ac E-field vector varies as the difference of the squares of the amplitudes of the competing electric fields. Hence, when the fields are of equal magnitude the medium appears isotropic, thus significantly reducing the reorientational tendency of the anisotropic molecules and hence, reducing the corresponding self-focussing mechanism of the medium. Absolute threshold data and conversion efficiencies from the laser line to the Stokes lines for SRS together with spectroscopic data on the Raman-shifted frequencies are presented on ten liquids including benzene, carbon disulfide, toluene, ethlybenzene, isoprene, cyclohexane, methanol, iodobenzene, acetone, and doubly distilled deionized water. The absolute threshold data for water, ethylbenzene, isoprene, acetone, iodobenzene, methanol, and cyclohexane have not been previously reported. We report new Raman-shifted lines in cyclohexane at 801 .9 cm^-1 including the second (2 x 801.5 cm^-1) and third (3 x 801.5 cm^-1) Stokes emission. We also found other lines in cyclohexane at 2842.5 cm^-1 , 2846.7 cm^-1 and 2852.9 cm-1. All of the wavelength measurements reported are accurate to at least 1Å. We further found some anomalous doublets in the first Stokes emission of iodobenzene (separation= 2.0Å), in the second Stokes emission of toluene (separation= 2.3Å) and in the first Stokes emission of acetone (separation= 1.3Å). At the present we can not explain the appearance of these doublets.