The ability of PAES to determine the elemental content of the topmost atomic layer has been exploited in a number of investigations aimed at the study of the growth and ability of ultrathin metal layers on metal substrates including Pd on Cu, Au on Cu, Fe on Cu, Rh on Ag, Au on Si, Cs on Cu, and Si on Ge. These systems are of interest because of their unique chemical, electronic, and magnetic properties. Another application of the magnetic beam system was the positron work function measurement from an energy analysis of the re-emitted positrons when the surface was bombarded with keV positrons.

B-beam	System	Objects	Journal
Clean Surface	Ni(100), Cu(100) Au,Pd,Fe,Cu	PAES Mechanism Relative Auger Intensities	PRL 61 (1988) S. Sci. (1992)
Ultra-thin Film Growth	Pd/Cu(100) Cs/Cu(100) Au/Cu(100) Rh/Ag(100) Fe/Cu(100) Au/Si(100) Au/Si(111) Si/H/Ge(100)	Top layer selectivity/Intermixing Positron trapping at interface Stability of Au films on Cu Ag migration (Ag/Rh/Ag(100)) Cu migration (Cu/Fe/Cu(100)) Depth profile analysis Thin film growth characterization H effect on Ge segregation	PRB (1993) PRL (1992) J. Mater (1994) J. Vac. Sci (1994) - Surf. Sci (1996) - A. S. Sci. (1997)
Gas overlayer	S/Cu(100) H2,O2/Si(100)	Surface sensitivity Sticking coefficients/Surface sensitivity	PRB(1990) S. Sci (1997)
Work function	Fe,Mo,Ni,Pt,Ti,V 6H-SiC(0001) GaAs(100)	Polycrystalline materials Positron related characterizations Positron reemission	PRB(1991) Materials Sci. Forum Materials Sci. Forum
Catalysis	Au/O2/Si(100)	Identify the adsorption sites	A. S. Sci (1995)

Identification of Active Sites on Catalytic surfaces

Bimetallic surfaces, which are finding increased use as catalysts, the correct identification of prefered adsorption sites for gas molecules is a crucial starting point in the study of reactivity and selectivity. Preliminary measurements on a model system have demonstrated that PAES is able to identify sites of preferential adsorption.

(left) Schematic Diagram of "Blocking" of the positron wave function by Adsorbates

The preferential adsorption of molecules to a particular type of site (such as at only one of the elements of the alloy) will displace positrons from those sites leading to substantial and readily identifiable changes in PAES intensities.

(right) PAES spectra for a Si (100) surface covered with 0.3 ML of Au (open circles) before and (closed circles) after exposure to 300 Langmuir of 02.

The oxygen exposure caused the counts in the region of the Si peak to decrease by 60% while the Au peak maintained the same level. This indicates that the positrons are being pushed away from the Si but not the Au due to selective adsorption of oxygen at exposed Si sites. A schematic drawing indicating this mechanism is shown in this Figure.