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

Abstract

Optical second harmonic generation (SHG) as a particularly surface sensitivetechnique was employed for the first time to investigate the surface phasebehaviour of a liquid alloy, BixGa1-x, in different heating and cooling cyclesup to 280 °C. The aim of these experiments is to establish a relatively simpleexperimental access to the surface phase diagrams of liquid alloys.Measurements of the characteristic changes of the SH signal have beenperformed on pure Bi and different Ga-rich alloys (xBi⩽0.367). In pure bismuththe melting and freezing of the surface is indicated by a distinctpolarization dependent variation of the SH intensities. Of particular interestis the characterization of the wetting transition found in Ga-Bi recently atthe monotectic phase transition (xm = 0.085, Tm = 222 °C). Surprisingly, on firstheating of the Ga-rich alloys up to 280 °C the SHG signals give no indicationof the dramatic compositional change at the surface induced by the wettingtransition. From these observations we conclude that the main source for thenonlinear polarization is the outermost layer of the alloy which in the wetand the non-wet state consists of an adsorbed Bi-rich monolayer. It is shownthat SHG is very sensitive to structural changes at the surface. Mostinterestingly, on cooling of the Ga-rich alloys a Bi-rich film crystallizes ontop of the bulk liquid alloy.