Sensitivity of positron-annihilation-induced Auger-electron spectroscopy to the top surface layer

Abstract

We present the 6rst study of positron-annihilation-induced Auger-electron emission from a well defined overlayer-metal system. A 0.5 monolayer of S on Cu(110) produced a fourfold reduction in the positron induced Cu M2, 3VV Auger peak. The attenuation is accounted for by theoretical calculations indicating that the positron surface state is displaced away from the Cu surface by the S overlayer, decreasing the annihilation rate of positrons with Cu core electrons.Conventional methods of Auger-electron spectroscopy (AES) make use of energetic electron or photon beams to create the core-hole excitations that result in the Auger signal. Recently, Weiss and co-workers ' demonstrated that a low-energy positron beam could create core holes through matter-antimatter annihilation, generating Auger electrons with high efficiency and extremely low secondary electron background. They suggested that in addition to eliminating the secondary electron background, positron-annihilation-inducedAuger-electron spectroscopy (PAES) would have the advantage of increased surface selectivity in systems in which positrons are trapped in a surface state before annihilation. In such systems the localization of the positron at the surface should cause the excitation volume to be restricted almost exclusively to the top atomic layer. In addition, because calculated PAES intensities are very sensitive to the spatial extent of the positron wave function, PAES measurements provide an important test for models of the positron surface state.In this paper we describe experiments which provide the first quantitative demonstration of the enhanced surface selectivity of PAES by comparing Auger intensities obtained using electron-excited Auger-electron spectroscopy (EAES) and PAES as a function of sulfur coverage on Cu(110). The trends shown in the experimental data are in excellent agreement with predictions arising from calculations of the positron wave function based on the image-correlation potential on clean and S-covered surfaces. These results provide strong evidence that: (i) the PAES signal originates from annihilations with positrons trapped in an image-correlation well at the surface, (ii) the positron surface state is pushed away from the Cu surface by the adsorbed S overlayer, and (iii) the localization of the positron surface state near the surface allows lowenergy positrons to be used to selectively excite Auger transitions from the top atomic layer.The PAES and EAES measurements were performed using the magnetically guided positron beam surface system at the University of Texas at Arlington.A trochoidal energy spectrometer equipped with a 25-mmdiam microchannel plate was used to energy analyze and detect electrons emitted from the sample. A Sm-Co magnet was placed behind the Cu sample to reduce the angular spread of the Auger electrons at the analyzer, ' giving an effective energy resolution for the spectrometer of -7 eV full width at half maximum. NaI(T1) scintillators detected annihilation y rays emitted in coincidence with the Auger ...

Surface investigation of Si(100), Cu, Cu on Si(100), and Au on Cu with positron annihilation induced Auger-electron spectroscopy

“…PAES allows experiments with extremely high surface sensitivity, since most of the implanted positrons are trapped in surface bound states and annihilate with electrons in the topmost atomic layer [4,5]. It was demonstrated that 95% of the Auger-signal stemmed from the uppermost atomic layer [6].…”

Positron states and annihilation characteristics at the (100), (110), and (111) surfaces of alkali metals

“…[14][15][16][17] Recently the nature and location of positron bound states at metal surfaces, both clean and adsorbate-covered, have become the subject of experimental studies using positronannihilation-induced Auger-electron spectroscopy ͑PAES͒. [18][19][20][21][22][23][24][25] In PAES experiments most of the low-energy positrons, implanted into the sample under study, diffuse back to the vacuum-solid interface where on the order of half are trapped into a surface state. 18,25 A certain fraction of the surface trapped positrons annihilates with neighboring corelevel electrons, creating core-hole excitations that give rise to Auger-electron emission.…”

A novel time‐of‐flight spectrometer for PAES

“…Moreover, PAES is much more surface sensitive than AES, since most of the implanted positrons are trapped at the surface bound states and annihilate with electrons in the topmost atomic layer [2,3].…”