Surface freezing and wetting in Ga–Pb alloy: Second harmonic and plasma generation study

Abstract

The surface freezing and wetting behaviour at the liquid/vapour interface of Ga-rich alloys of Ga and Pb has been studied as a function of temperature at selected compositions both in the homogeneous liquid and the fluid two phase demixing regime. The interfacial phase transitions have been probed by second harmonic generation (SHG) in combination with plasma generation (PG) measurements at different cooling and heating cycles. In the composition range between the eutectic and monotectic points the occurrence of surface freezing is clearly indicated by distinct changes of the SHG and PG intensities, respectively. It is not observed below the eutectic composition for solution very dilute in Pb. Qualitatively, these results agree with the recent observations made in Ga–Bi alloys. A new wetting characteristic is found crossing the miscibility gap at constant composition with varying temperature. A Pb-rich macroscopic wetting film forms which must be concluded from the SHG intensities. This interfacial phase transition causes a clearly detectable drop of the sample temperature which is explained by the corresponding discontinuous change of the optical emissivity at the interface. These interfacial phenomena related with the miscibility gap have not been reported before.