Surface phase transitions in liquid Ga-Bi alloys studied by optical secondharmonic generation

Optical second harmonic generation (SHG) as a particularly surface sensitiv e technique was employed for the first time to investigate the surface phas e behaviour of a liquid alloy, BixGa1-x, in different heating and cooling c ycles up to 280 degrees C. The aim of these experiments is to establish a r elatively simple experimental access to the surface phase diagrams of liqui d alloys. Measurements of the characteristic changes of the SH signal have been performed on pure Pi and different Ga-rich alloys (x(Bi) less than or equal to 0.367). In pure bismuth the melting and freezing of the surface is indicated by a distinct polarization dependent Variation of the SH intensi ties. Of particular interest is the characterization of the wetting transit ion found in Ga-Bi recently at the monotectic phase transition (x(m) = 0.08 5, T-m = 222 degrees C). Surprisingly, on first heating of the Ga-rich allo ys up to 280 degrees C the SHG signals give no indication of the dramatic c ompositional change at the surface induced by the wetting transition. From these observations we conclude that the main source for the nonlinear polar ization is the outermost layer of the alloy which in the wet and the non-we t state consists of an adsorbed Pi-rich monolayer. It is shown that SHG is very sensitive to structural changes at the surface. Most interestingly, on cooling of the Ga-rich alloys a Bi-rich film crystallizes on top of the bu lk liquid alloy.