ELLIPSOMETRY OF THE LIQUID VAPOR INTERFACE CLOSE TO THE CRITICAL-POINT - A THEORETICAL-ANALYSIS

It is shown that the ellipsometric coefficient for a liquid-vapor interface may be written as the sum of three contributions. The first is given by Drude's formula. The second contribution is due to capillary wave fluctuations. Finally, the third contribution is due to fluctuations of the density profile around the Fisk-Widom profile with a wavelength up to roughly the bulk correlation length and thus short compared to the capillary length. Close to the critical point the first two contributions scale as (T - Tc)β-υ. The expression for the third contribution contains an integral over the excess density correlation function over wave vectors large compared to the inverse bulk correlation length. The scaling behavior of the third contribution is probably such that this term becomes unimportant close to the critical point. The formulae given in this paper only for the liquid-vapor interface may be used for a binary fluid if one makes the usual substitutions. An experimental analysis of the ellipsometric coefficient for binary fluids close to the critical point by Schmidt [1] indicates that the sum of the first two terms predicts a value which is somewhat to large but which has the correct scaling behavior. A discussion of this difference in amplitude is given. © 1990 Plenum Publishing Corporation.