userguide 35.en

8/12/2019 Userguide 35.En

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Electron Spectroscopy of Surfaces

1. General remarksElectrons are well suited probes for the investigation of electronic and geometrical properties

of clean and adsorbate covered surfaces. The use of electrons includes various techniques as:Low Energy Electron Diffraction(LEED) for the investigation of the surface geometry in

the reciprocal space.

Electron Microscopyfor structure investigations in real and (at the resolution limit) in the

reciprocal space.

Electron Energy Loss Spectroscopy(EELS) and High Resolution Electron Energy Loss

Spectroscopy(HREELS) for the investigation of electronic and vibrational excitations.

Electron Stimulated Desorption(ESD) for studies of the microscopic processes related with

irradiation damage

Photoelectron Spectroscopy(PES) and Auger Electron Spectroscopy(AES) for investiga-

tions of electronic properties and for surface analysis.

It is the purpose of this experiment to make you familiar with these last two techniques, PESand AES.

The main reason for the widespread use of electrons as probes in surface science is their short

inelastic mean free path (Fig.1). For electrons between 10 and 1000 eV kinetic energy it is

only a few atomic layers. In a typical photoemission experiment, only electrons from a narrow

region at the solid-vacuum interface will reach the detector without energy loss. This makes

electrons very surface sensitive probes.

Fig.1: Inelastic mean free path of electrons as a function of their kinetic energy, for various

elements.

2. Photoelectron Spectroscopy2.1. Basics

The basis of PES is the photoeffect, which was experimentally discovered by Hallwachs in

1887 and theoretically explained by Einstein in 1905. Although well known for a long time, it


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took more than 60 years of experimental development until Siegbahn and coworkers suc-

ceeded in establishing PES as a standard analytical method in atomic, molecular, and solid

state physics.

The basic process is simple: A photon of well known energy his absorbed by an electronicsystems, promoting an electron from an occupied electronic level into an unoccupied state. If

this unoccupied state lies beyond the vacuum level, the electron can escape into the vacuumand can be detected by an energy sensitive analyzer. The energy balance is as follows:

)()( sysEEsysE fkini +=+h

Ei(sys) and Ef(sys) are the total energies of the system (atom, molecule, cluster, or solid)

beforeand after photon absorption, respectively. Ekinis the kinetic energy of the photoelec-

tron. In a one electron picture(i.e. neglecting the energy of mutual electron interaction) the

difference of Ef(sys) - Ei(sys) can be understood as the binding energy of the ejected electron:

kinifB EsysEsysEE == h

)()(

Recording the number of photoelectrons as a function of their kinetic energy yields a spec-

trum of distinct lines, which in first approximation reflect the occupied orbitals of the elec-

tronic system under investigation. Because the number and binding energies of these occupied

orbitals are different for each chemical element, the PE spectra differ as well. They can serve

as "fingerprints" of the different elements, making PES a very useful analytical tool.

Fig.2: Schematic of the photoionization process (left). The total width of the photoelectron

spectrum is equal to h- sample(right side).

Depending on the photon energy we discriminate core electron (or: inner shell, Eb ~40 eV)and valence electron (outer shell) PES (0 Eb ~40 eV). The latter is termed UPS(ultraviolet photoelectron spectroscopy) because UV photons from gas discharge sources or

UV synchrotron beamlines are commonly used. PE spectroscopy with soft x-rays is labeledXPS (x-ray photoelectron spectroscopy). Common light sources are x-ray tubes with or

without an additional monochromator, and soft x-ray synchrotron beamlines. At this


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which correspond to parallel or anti parallel orientation of the residual 1s electron's spin with

respect to the unpaired electron in the molecular 2orbital (triplet and singlet final states,respectively). Even for photoemission from closed shell systems (e.g.: rare gases) multiplet

splitting is obtained for parallel or anti parallel orientation of spin and orbit (e.g.: 2p 3/2and

2p1/2). For open shell systems the situation is even more complicated.

Multi electron excitations and satellites.

The reorganisation of the electrons upon the creation of the core hole can lead to electroni-

cally excited final states. Because of energy conservation this excitation energy is missing in

the kinetic energy of the photoelectron, i.e., we find a redshifted satellite line. We can dis-

criminate different processes. We encounter ashake-up satellite, if an additional electron is

promoted from an occupied into a boundunoccupied state (means: a state belowthe vacuum,

but abovethe Fermi level (for solids), or (for molecules) from theHOMO(highest occupied

molecular orbital) to theLUMO(lowest unoccupied MO, as far as this state lies below the

vacuum level). If the second electron is promoted into a continuumstate above the vacuum

level, we speak of ashake-off satellite. Other possible satellites areplasmon satellites(core

ionisation + plasmon excitation = collective oscillation of electrons with respect to the posi-tively charged ion cores), or inter band satellitesin solids corresponding to additional excita-

tions of electrons into unoccupied states above "Fermi". The latter may lead to discrete lines

in some cases, but may also show up as an asymmetric broadening of the low energy edge of

the photoemission peaks on the kinetic energy scale.

2.2.2. Intensity

The interaction of electromagnetic radiation with an atom is described by the Hamiltonian:

'

2

10

2

HHVeA

c

ep

m

H +=++

+= rr

with:

Vpm

H += 202

1 r

and:

( ) 22

2

22' A

mc

eepApA

mc

eH

rrrrr+++=

By assuming source free space and appropriate gauge transformations (Coulomb gauge:

0,0 == Adivr

) and neglecting higher orders of A we obtain:

pAmc

eH

rr

2'=

Ar

is the vector potential of the electromagnetic radiation defining the direction of polariza-

tion, and p is the momentum operator. With Fermi's golden rule we obtain in first order

perturbation theory the transition rate Pfifrom the initial state iwith Energy Ei to the finalstate fwith energy Ef:


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( );'22

hEEHP ififfi =h

And for the angle integrated cross section:

);(~2

hEEM

dE

diffi

f

with the matrix elementiffi pAM =

rr and the sum over all initial and final states which are

allowed by energy conservation (= for which the argument of the delta function is zero).

Because the matrix element in general does not depend on the chemical environment, the

intensity of the emission line of an atom can be used for quantitative analysis.

The absolute intensity I of a PE line from an element A per solid angle is equal to:

dVE

zzyxNzyxJdEyxT

d

d

d

dI

AB

AAAA )cos

)(exp(),,(),,(),,,(

=

with the transmission function of the electron analyzer T, the incident photon flux J, the den-

sity of atoms A NA, and the inelastic mean free path of electrons with the energy EAin

medium B B(EA). is the polar angle between analyzer axis and surface normal.In most cases, calibration of absolute intensities based on this formula is not possible. In

practical experiments, measurements of relative intensities are important, for instance for the

detection of relative surface coverages by adsorbates or contaminants.

Fig.3: Schematic of the detection geometry


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Fig.5: Different types of Auger decay processes: a) KLL decay; b) LMM decay; c) Coster-Kronig transition; d) Auger decay including valence levels supplies chemical information

Fig.6: Reduced KLL Auger energies from theory (lines) and experiment (points) as a function

of the atomic number Z. Low Z (left side): LS-coupling, high Z (right side): jj-coupling, in-termediate coupling case between.


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Satellite lines occur for AES as for PES. Multiplet features are even richer for AES than for

PES because of the doubly ionized final state, i.e. we have at least two holes which interact

(decay satellites, i.e. shake-up or shake-off processes upon decay make the situation even

more complicated). Depending on the atomic number Z, LS (for low Z) or jj coupling prevails

(for large Z). See Fig.6 for possible final states of KLL transitions.

Calculation of kinetic energies of Auger electrons according to the above formula can be dif-

ficult for electronic systems larger than isolated atoms. Approximations are then necessary.

One possibility is to take one electron binding energies for the initial and the two final holes

from PES with an additional term U which accounts for the interaction of the two holes in the

final state:

;,,,, UEEEEXPS

Zb

XPS

Yb

XPS

XbFkin =

Neglecting U we obtain the energy range of Ekinfor a distinct transition. U can then be ob-

tained experimentally and supplies important information about the interaction energy of thetwo holes (see Fig.6).

The intensity of an Auger line is governed by the local overlap of the final state wave func-

tions Yand Zwith the wave function Xof the initial hole. Because of this, AES enablesinvestigations of electron densities at the site of a specific core hole.

4. Adsorption4.1 PrefaceWe call the process when particles from the gas phase become bound to a surface "adsorp-

tion". The energy released upon this process is called energy of adorption Uador sometimes

also binding energy (not to mix up with the binding energy from PES).The reverse process, i.e. the release of gas particles from the surface is called desorption. De-

sorption may be stimulated by heat (thermal desorption), by electrons (electron stimulated

desorption), or by photons or ions (photon, respectively ion stimulated desorption).

4.2. Types of adsorption

4.2.1 Physisorption

The particles are bound by Van der Walls forces to the surface. The adsorption energy U adis

small, typically less then 40kJ/ mol. Formation of multilayers is possible (condensates).

4.2.2 Chemisorption

The particles are bound by chemical forces, e.g. transfer of electrons and/or formation of co-valent bonds (common molecular orbitals). In most cases the adsorption energy is much larger

than 40 kJ/mole. Chemisorbates form typically only one layer (= direct neighborhood of sur-

face atoms and chemisorbate is required).

4.3. Adsorption order

First order adsorption: One particle from the gas phase yields one particle of the adsorbate,

i.e., molecules, which do not dissociate upon adsorption. Reversed process: First oder desorp-

tion.

Examples: CO/nickel CO chemisorbes on Ni surfaces molecularly with ~ 120 kJ/mole

Ar/Ruthenium Ar physisorbes on Ru with ~10 kJ/mole


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Second order adsorption(dissociativeadsorption):

A molecule dissociates upon adsorption. One particle in the gas phase yields two particles on

the surface. The reverse process is associativedesorption or second order desorption

Example: H2/Nickel: H2dissociates into 2 H atoms on Ni (~ 90 kJ/mole).

Fig.7 shows typical potential curves of second order adsorption of H2on a transition metal

like Ni . The full line corresponds to the interaction of the intact molecule with the surface,

the broken line belongs to the chemisorptive bonds of the H atoms. At the intersection of both

curves the molecule dissociates. If this intersection lies at positive energy values, the disso-

ciation is activated. The activation energy EAhas to be supplied by the particle approaching

the surface, e.g. by its kinetic energy. If the intersection of the two curves lies below zero en-

ergy, we have the case of non-activated adsorption.

Fig.7: Energy potential curves for 2nd order adsorption, e.g. for H2/Ni. EDISS: Energy of

dissociation; EA: Activation energy.

5. Nomenclature (common in surface science):

(Relative) coverage : Number of adsorbate particles Nad/ number of substrate partic-

les Nsub

Collision rate : Number of particles hitting the surface / time x area;

= p/(2mkBT)1/2; T: Temperature; m: Particle mass; p: Pres-

sure

Sticking coefficient s: Probability that a particle colliding with the surface becomes

adsorbed.

s = (Nad/dt)/(Ncoll/dt); 0 s 1;


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Lifetime of an adsorbate : Time, an adsorbed particle stays at the surface before desorp-tion.

= (k0)-1exp(Uad/kBTsurface). k0: typically 10

13/s

6. Experimental (details during the experiment):

Vacuum system: Ultra high vacuum (UHV) system with a base pressure in the 10 -8 range.

Pumps: Ion getter pump + titanium sublimation pump.

Sample: Tungsten ribbon; it can be heated by direct current up to 2500K.

X-ray source: Characteristic Al Kradiation from an Al x-ray anode. Typical operation con-ditions: Uanode= 12 kV; Iemission = 20 mA.

X-ray lines: Al K1,2: 1486.6 eV + K3,4 : 1496 eV (10% intensity of K1,2). Linewidth of the

K1,2line: HWFM = 0.78 eV (0.43 eV multiplet splitting + 0.47 eV lifetime broadening).

Electron analyzer: Electrostatic 1800hemisperical analyzer. An electrostatic lens between the

sample and the entrance slit of the analyzer focuses the photoelectrons from the sample onto

the entrance slit. The 1800analyzer is doubly focusing: A point source at its entrance is

imaged into a point image at its exit. Its energy resolution is:

E/Ep= d/2r + ; E: Energy window; Ep: Pass energy; d: geometrical average of entranceand exit slit widts; r: average radius of hemispheres; : 1/2 of angle of acceptance.

Fig.8: Schematic of potentials and voltages


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8.4 Main Auger lines (see also: http://xdb.lbl.gov/Section1/Sec_1-4.html):


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U. Is the value you got as expected by you? Explain differences seen (if any) for the two

different exit angles.

7. Compare 9.8 and 9.9. Explanation?

8. Compare results from 9.7 and 9.10. Interpretation?

9. After which time at a pressure of 10-5 Pa of oxygen will an originally clean metal sur-

face be covered with one monolayer of O atoms (assume s = 1 and 2 ndorder adsorp-

tion; make reasonable assumption about the density of the substrate atoms).

10. Assume a hemisherical analyzer with radii ri(inner) and ro(outer). Which potential

difference has to be applied to the hemispheres in order to obtain a trajectory with

r = (ri+ ro)/2 for electrons with the energy Ep?

userguide 35.en

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