ELLIPSOMETRIC MEASUREMENT OF A SURFACE AMPLITUDE RATIO NEAR A CRITICAL END-POINT

We present the experimental determination of a universal surface ampli tude ratio near a critical end point. From ellipsometric measurements on five critical binary liquid mixtures, three with an upper critical point and two with a lower critical point, we obtain a value for the m odified critical adsorption amplitude ratio of R(ma) = 1.18 +/- 0. 13, where the error represents three standard deviations. Reasonable agre ement is obtained with the theoretical results of H. W. Diehl and A4. Smock [Phys. Rev. B 47, 5841 (1993)], from which we obtain R(ma) = 1.3 3.