Surface and molecular dynamics at gas-liquid interfaces probed by interface-sensitive forced light scattering in the time domain

The forced Brillouin and Rayleigh scattering techniques under the probe ref lected transient grating configuration were used to probe various dynamics at gas-liquid interfaces in the time domain over the wave-number range of 0 .2x10(6)similar to 2.0x10(6) m(-1). From the forced Brillouin scattering si gnal, the temporal behaviors of the capillary wave (CW) were investigated. The observed wave does not consist of one sine or cosine wave but shows dua l features. This feature was explained in terms of two mechanisms of the ca pillary wave creation by the photothermal effect; the thermal expansion and the temperature dependence of the surface tension. Besides the oscillatory behavior for nonviscous liquids, a heavily damped capillary wave was obser ved in a range of wave number above 1.6x10(6) m(-1) from a viscous liquid ( 1-hexanol) surface, which has never been detected before. The observed sign als were compared with the phenomenologically expressed wave forms and the theoretically calculated wave forms based on the hydrodynamic equations. Fo r CW on the organic liquid surface, the hydrodynamic equation predicts the observed temporal profile well, whereas the agreement is less satisfactory for CW on water surface. Using the theoretical equations, the surface motio n as well as liquid motion beneath the surface under the forced light scatt ering condition is presented. The temporal profile of the forced Rayleigh s cattering signal provides information an the thermal diffusivity and molecu lar movement at the surface. The thermal diffusivity at the surface region is found to be very close to that in the bulk phase, whereas the molecular motion at the surface is revealed to be faster than that in the bulk phase. The faster movement at the surface may be explained by the transverse moti on of the surface by CW.