TESTING THE CHARGED ADATOM MODEL ONTO THE${\rm Sn}_{1-x}{\rm Si}_x/{\rm Si}(111) (\sqrt{3}\times \sqrt{3}){\rm R}30^\circ$ALLOY WITH HIGH RESOLUTION CORE LEVEL SPECTROSCOPY

Synchrotron radiation high resolution core level spectroscopy has been used to investigate the 1/3 monolayer (ML)${\rm Sn}_{1-x}{\rm Si}_x/{\rm Si}(111) (\sqrt{3}\times \sqrt{3}){\rm R}30^\circ$solid solution at well-defined values of the mutual surface concentration of the Sn and Si adatoms (x = 0.24andx = 0.40) as a function of temperature (100–300 K). For 24% Si adatoms at the surface there is low energy electron diffraction (LEED) evidence of a nascent 3 × 3 lowering of the surface symmetry at 100 K. Despite this evidence core level spectroscopy data do not show any signature of the nascent phase transition. Our evidence is in agreement with the analogous observation of no significant spectral changes at reduced temperature of the core level spectra of the isoelectronic and isostructural 1/3 ML Sn/Ge(111) surface.