LIGHT SCATTERING FROM SURFACE WAVES ON CARBON DIOXIDE NEAR ITS CRITICAL POINT

The spectrum of light scattered from surface waves thermally excited on a liquid interface, has been computed theoretically using the approach of thermodynamic theory. Effects typical of a dissipation process into the bulk (previously unobserved) were predicted ; such effects should show up mainly near critical damping conditions, that is conditions for which surface waves become exponentially damped and no longer propagate. In low and high damping conditions the spectrum can be accurately identified to the fluctuation power spectrum of a one dimensional, harmonic oscillator (H. O. spectrum). We have measured this spectrum for scattering from the liquid-vapor interface of carbon dioxide near the critical point (Tc - T = 0.6 °C at scattering angles corresponding to q ≈ 3 000 cm-1). The agreement with the theoretical spectrum yields a significative evidence for the bulk dissipation process mentioned. Previous experiments were performed in the temperature range 30° ≥ Tc - T ≥ 3.5 X 10-2 °C and fitted with the help of the approximate H. O. spectrum. The temperature dependence of the surface tension deduced from the data was fitted by the empirical law with α = α0(1-T/Tc)µ> with µ = 1.253 ± 0.010 and α0 = 77 ± 5 dynes/cm. No anomaly was noticed in the temperature variation of the shear viscosity.