Ng. Fazleev et al., POSITRON-ANNIHILATION-INDUCED AUGER-ELECTRON-SPECTROSCOPY STUDIES OF PROPERTIES OF AN ALKALI-METAL OVERLAYER ON THE CU(100) SURFACE, Physical review. B, Condensed matter, 49(15), 1994, pp. 10577-10584
Positron-annihilation-induced Auger-electron spectroscopy (PAES) emplo
ys positrons trapped at the surface to create core-holes and to initia
te the Auger process in atoms in the topmost layer of the surface. The
results of experimental and theoretical investigations of the attenua
tion of the positron-annihilation-induced Cu M2,3 VV Auger signal with
Cs coverage on the Cu(100) surface at low and high temperatures are d
iscussed. They reveal that at 163 K the normalized intensity of the po
sitron-annihilation-induced Cu M2,3 VV Auger signal remains nearly con
stant at the clean-surface value until the Cs coverage reaches approxi
mately 0.7 physical monolayer, at which point the signal intensity dro
ps precipitously. We present an analysis of this unusual behavior usin
g a model that treats the positron as trapped in a double-well potenti
al in the direction perpendicular to the surface: one well is associat
ed with the Cu substrate and the other with the Cs adsorbate. The shar
p drop in the Cu PAES intensity which occurs over a small change in th
e Cs coverage is attributed to a migration of positrons trapped at low
Cs coverages at the Cs/Cu interface to the positron surface state on
the vacuum side of the alkali-metal overlayer at high Cs coverages. Th
is migration can be accounted for in terms of a structural phase trans
ition in the Cs overlayer from a disordered distribution of adsorbate
atoms to adsorbate metallic islands with an ordered hexagonal close-pa
cked structure.