Recently, Weiss et al. have demonstrated that it is possible to excite Auger transitions by annihilating core electrons using a low energy (less than 30eV) beam of positrons. This mechanism makes possible a new electron spectroscopy, Positron annihilation induced Auger Electron Spectroscopy (PAES). The probability of exciting an Auger transition is proportional to the overlap of the positron wavefunction with atomic core levels. Since the Auger electron energy provides a signature of the atomic species making the transition, PAES makes it possible to determine the overlap of the positron wavefunction with a particular element. PAES may therefore provide a means of detecting positron-atom complexes. Measurements of PAES intensities from clean and adsorbate covered Cu surfaces are presented which indicate that approx. 5 percent of positrons injected into CU at 25eV produce core annihilations that result in Auger transitions

Jensen, K. O.

Koymen, A. R.

Lee, K. H.

Lei, Chun

Mehl, David

Weiss, Alex

English

NASA Technical Reports Server

N90-18996
Positron Annihilation Induced Auger Electron Spectroscopy
Alex Weiss, A.R. Koymen, David Mehl, K.O. Jensen a, Chun Lei and K. H. Lee
Physics Department, The University of Texas at Arlington, Arlington Texas 76019
Recently, Weiss et al. have demonstrated that it is possible to excite Auger transitions by
annihilating core electrons using a low energy (less than 30eV) beam of positrons. This
mechanism makes possible a new electron spectroscopy, Positron annihilation induced
Auger Electron Spectroscopy (PAES). The probability of exciting an Auger transition is
proportional to the overlap of the positron wavefunction with atomic core levels. Since the
Auger electron energy provides a signature of the atomic species making the transition,
PAES makes it possible to determine the overlap of the positron wavefunction with a
particular element. PAES may therefore provide a means of detecting positron-atom
complexes. Measurements of PAES intensities from clean and adsorbate covered Cu
surfaces are presented which indicate that -5% of positrons injected into Cu at 25eV
produce core annihilations that result in Auger transitions.
I. Introduction
The Auger process is a nonradiative transition in
which an atom with a inner shell hole relaxes by
filling this hole with an less tightly bound electron
while simultaneously emitting another electron (the
Auger electron) which carries off the excess energy.
The energy of the Auger electron is given by the
equation, EXy Z = E X - Ey* - EZ* where E X is
the binding energy of the electron removed to form
the original inner shell hole, and Ey*, EZ* are the
binding energies associated with the two hole final
state. Because the energy levels of different
elements are in general unique, the elemental
identity of an atom may be deduced from the
energies of the Auger electrons emitted as a result of
core hole excitations. This fact along with the short
escape depth of low energy electrons has been
exploited in the widely used surface analysis tool,
Auger Electron Spectroscopy (AES).
Conventional Electron induced Auger Electron
Spectroscopy (EAES) makes use of high energy
electrons to collisionally ionized the atom.
However in many instances the utility of EAES is
limited by problems associated with the large
secondary electron background and the lack of
surface specificity inherent in the EAES excitation
process. Recently, Weiss et al.ihave demonstrated
that Auger electrons can be excited with high
efficiency by using low energy positrons to produce
the core hole excitations necessary for Auger
electron emission by matter - antimatter annihilation.
This process makes possible a new surface
spectroscopy, Positron annihilation induced Auger
Electron Spectroscopy (PAES) which has
significant advantages over conventional EAES in
some systems. In the remainder of the paper we
will describe experiments which demonstrate the
potential advantages of the PAES technique. We the
describe theoretical calculations from which we
estimate the efficiency with which positrons induce
Auger transitions. This estimate is then compared
to experimental values. The paper concludes with a
discussion of the possible use of PAES to detect
positron -atom or positron - molecule bound states
Elimination of Secondary Electron Back_ound
The PAES technique can be used to eliminate the
large secondary electron background that limits the
sensitivity and accuracy of conventional methods of
Auger Electron Spectroscopy (AES). 1-3 In PAES,
the core hole excitations necessary for AES are
generated by matter-antimatter annihilation and not
by collisional processes. It is therefore possible to
use an incident beam energy well below the Auger
electron energy thus precluding the creation of
secondary electrons in the energy range of the
Auger signal. In contrast, in conventional electron
289
PRECEDING PAGE BLANK NOT FILMED
https://ntrs.nasa.gov/search.jsp?R=19900009680 2020-03-19T22:57:15+00:00Z
stimulated Auger electron spectroscopy, (EAES),
the incident beam energy must be in excess of the
Auger electron energy which makes it impossible to
avoid creating a large secondary electron
background. The large improvement in signal to
background that can be obtained using PAES is
demonstrated in Figure 1. which compares Auger
spectra obtained using positron excitation to that
using conventional EAES. Both spectra were
obtained using the UTA positron Auger system.
Signal to background levels of greater than 40:1
were obtained (more than a factor of 80
improvement over conventional methods of AES).
The improved signal-to -background allows PAES
data to be taken with beam currents several orders
of magnitude less than in conventional electron
excited Auger (EAES). The low currents and low
beam energies used in PAES allow the energy dose
require to obtain data to be reduced four to six
6000000
4000000I.--
Z
LtJ
I 2000000
0
w
0
Z
w
0
z
0
(3
I
!
o)
CONVENTIONAL A.E.S.
@s=O
20O
150
100
5O
Cu M2,:_W
,'.
.,"
,, ,',.. "..
,.
@s=O
:. Cu MIW
1. ! _ _',,.._
25 50 75 1O0 125 150
ENERGY (eV)
Fig. 1. Comparison of Conventional
EAES and PAES spectra. Note the large
increase in signal to background for the
Cu M2,3VV peak obtained using PAES.
orders of magnitude as compared to EAES. This
will permit the use of PAES in fragile systems
where conventional methods of AES are severely
limited.
Surfac_ $_l_¢tivitv: Positron annihilation induced
Auger spectroscopy displays enhanced surface
selectivity. 3,4 This selectivity is due to the
restriction of the excitation volume to the top atomic
layer due to localization of the positron. This is in
contrast to conventional Auger in which the
excitation volume extends hundreds of atomic layer
below the surface. EAES acquires its surface
sensitivity solely from the 4-20A escape depth of
the Auger electron. The intensity of the Cu M2,3VV
PAES signal decrease by a factor of 4 with the
addition of a 1/2 monolayer of S on the surface (see
figure 2). This is contrasted with only a 25%
decrease in the EAES signal caused by the
overlayer. These results were accounted for by
theoretical calculations which show that the positron
wavefunction is pushed away from the Cu surface
causing the decrease in PAES intensity. These same
calculations demonstrate that as much as 97% of the
Auger signal will originate in the top atomic layer
using PAES as compared to about 50% using
conventional AES techniques.
ThcQretical Calculations: Theoretical calculations
were carried out 5 to determine the expected
magnitude of PAES intensities. In addition,
detailed surface calculations were carried out in
support of our experimental measurements of the
surface selectivity for PAES results to determine the
spatial extent of the positron wavefunction and the
degree of surface selectivity that could be attained
with PAES.3, 4 Calculations using a corrugated
mirror model for the positron surface potential were
performed on clean metal and overlayer on metal
surfaces producing good agreement with
experimental results. 3
Nearly all Cu 3p holes decay via emission of an
M2, 3 VV Auger electron (energy = 60eV) since the
radiative transition probability is extremely small. 5
For clean Cu(100) and Cu(110) the annihilation
probabilities for the 3s and 3p electrons are
calculated to be P3s = 0.83% and P3p = 3.0%,
respectively. Putting in the relevant Auger transiton
rates we estimate that, 6M23V V, the probability of
a positron trapped in a surface state causing the a
M2,3VV Auger transition is -3.6%. The
annihilation probabilities for the deeper lying 2s and
2p levels are two orders of magnitude lower. The
calculations also indicate that core annihilations take
290
placeprimarily in thetop surfacelayer,with about
5%and20%of the total rate arising from second or
deeper layers for Cu(100) and Cu(ll0),
respectively.
For the purpose of making a comparison with
theoretical calculations, we estimate erM23V V as
follows6: an integral over the energy spectrum of
the PAES Cu M2, 3 VV Auger peak was compared
0.0002
0.0001
co
1_ o.oooo
Z
LU
0.0002
ILl
#.,,
0.0001
LLI
o
z o.oooo
LIJ
¢q
0.0002
Z
O
0.0001
2",,
I
0.0000
I
0.0002
0.0001
I l } I I 7
Cu M_.3W (a)
::. @s=O
,, ",
...Cu MlW
J
I I I I I1' !
(b)
0s=0.27
- ..... : :
;:-'-"' S L2 _W
I I t I I I 1 --
(c)
@s=0.5
I I t I I T--
(d)
Oc,,_0.5
___J. .... J__.. 1__
0.0000 0 25 50 75 100 125 150 175 200
e- ENERGY (eV)
Figure 2. PAES spectra obtained from
clean and adsorbate covered Cu. The
large decrease in the Cu signal with
overlayer coverage demonstrates the top
layer selectivity of PALES.
to an integral over the positron induced secondary
electron peak. The secondary electron peak was
obtained with the sample at -60V, so the electrons
pass through the spectrometer at approximately the
same energy as the Auger electrons. The positrons
were incident with a kinetic energy of 80eV.
Measurements of positron induced secondary
electron emission from Ni at an angle of 50 ° to
normal incidence allowed an estimate of the ratio of
secondary electrons per incident positron, 6, in this
experiment. The measured ratio of the Auger yield
to the secondary yield was then substituted into a
formula 6 which takes into account detector solid
angle and efficiencies to give: Om2, 3 = 5.6%.
Part of the discrepancy between this value and the
theoretically calculated value of 3.6% may be due to
neglect of the many-body enhancement factor.
It is interesting to speculate on the possibility using
PAES as a means of signaling the existence of a
positron-atom or positron-molecule bound state•
Since the overlap of the positron wavefunction with
the core levels of an atom should be enhanced if the
positron were bound to that atom, the existence of a
bound state would be signaled by an increase in the
PAES intensity. It may be possible to test this
hypothesis by using a very low energy beam of
positrons incident on atoms or molecules
physisorbed on a metallic substrate.
This research was supported in part by the Texas
Advanced Research Program and the Robert A.
_Welch Foundation.
a. Permanent address: Physics Department, The
University East Anglia, Norwhich, UK
References
1. Weiss, Mayer, Mehl,Jibaly, Lei, and Lynn,
Phys. Rev. Lett. 61, 2245(1988).
2. Weiss, Mehl, Lei, Jibaly, Mayer and Lynn,
accepted for publication in Positron Annihilation,
World Scientific Press, Singapore (1989).
3. Mehl, Koymen, Jensen, Gotwald and Weiss,
submitted to Phys. Rev. Lett.
4. Lei, Mehl, Koymen, Jibaly, Gotwald and Weiss,
UTA preprint.
5. Jensen and Weiss, UTA preprint.
6. David Mehl, Thesis (unpublished)
291
ORIGINAL PAGE IS
OF POOR QUALITY


Positron annihilation induced Auger electron spectroscopy

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server