An introduction to An introduction to XX-- ray photoelectron ray photoelectron

spectroscopyspectroscopy

•X-ray photoelectron spectroscopy belongs to a broad class of spectroscopic

techniques, collectively called, electron spectroscopy.

•In general terms, electron spectroscopy can be defined as the energy

analysis of electrons ejected orreflected from materials.

•All of these spectroscopic techniquesyield information on the ELECTRONIC

STRUCTURE.

There are, generally five techniques

collectively called electron spectroscopy

X-ray photoelectron spectroscopy(XPS)

Ultraviolet photoelectron spectroscopy (UPS)

Aüger electron spectroscopy(AES)

Electron energy loss spectroscopy(EELS)

Inverse photoemission spectroscopy (IPS)

UPS Photon source variationHe I 21.2 eVHe II 40.8 eVNe ISynchrotron radiation……

There are a range of techniques in each of these

UPS Variations of the samebasic technique

One photon spectroscopy Gases

SolidsGas cellMolecular

beams

Multiphoton photoelectron spectroscopyPhotodetachment spectroscopy

Photoelectron-photoioncoincidence spectroscopy

Zero-kinetic energyphotoelectron spectroscopy

……..

Structure and Properties of Matter

SpectroscopyScattering

Physical Properties

Spectroscopy (pre-1965)

AbsorptionMagnetic

Mass

Spectroscopy using electrons

e + M+, M2+……

e +KE

M+

Cur

rent Detector

Electron KEI1

I2

I3

I4

M+

Ionization efficiency curves

+ M+

Detector

hν

M+

curr

ent

Photon EnergyI1

I2

I3

I4

hν + M M+ + e

Constant

(No M2+, generally)hν

Photoelectron Spectroscopy

Photoelectric effectEarly experiments in 1887hν = KE + φ 1905

M+ + e

KE

Detector

+

M + hν M+(Eint) + e

elec vib rot

hν - I - Eint KE of the electron

Photoion can be excited

Conservation of momentum requires that excess energy is partitioned in inverse proportion to the masses.

hνI2

I1

I1*e

M+

Ip

hν

Exc

ited

ioni

c st

ates

Eint

M

Electron and ion separateswith equal momenta.

mu = MUThe relative velocity,

V = u + U= U (1+ M/m)= u (1+ m/M)

The kinetic energies,

½ MU2 = 1 m MV 2

½ mu2 = 1 m MV 2

2M m+M

m+M2m

Eint ohν - KE Iphν - KE1 IP1

hν - KE2 IP2hν - KE3 IP3 …..

adiabaticVertical

E re

hν - (Ip + Eint) KE

hν - KE Ip + Eint

Depth of analysis depends on photon

He I 21.2 eV 21P → 1 1SHe II 40.8 eV 2 P → 1 S of He+

Al Kα1, 2 1486.6 eV 2 P → 1 SMg Kα1,2 1253.6 eV ”Na Kα1, 2 1041.0 eV ”Si K α1, 2 1739.5 eV ”

10000

0

10

100

1000 Core

Valenceuv

Soft

X r

ays

Har

dX

ray

s

3/2, 1/2

energy

Aügerelectron

e

hνehν

Photoemission X-ray fluorescence Aüger process

e

L2, 3 (Z)

Vac

K(Z)

L1(Z)

Neutral atom

Photoemission

Electronictransition

Coulombfieldredistribution

Aügurelectronemission

Finalstate

e

L2, 3 (z+∆z)

EK, L 1, L2 , 3 = Ek - EL1 - EL 2, 3

EABC(Z) = EA

(Z) - ½ [ EB(Z) + EB

(Z+1) ]- ½ [ EC(Z) + EC

(Z+1) ]

E’s are the binding energies.EABC K L1 L2, 3 , K L1 V , KVV

Intense Auger intensities if the valence electron density is high.

Fluorescence efficiency increases with transitionenergy. Fluorescence and Auger are comparable when ∆Ε ~ 10,000 eV.

VALENCE SHELL VALENCE SHELL PHOTOELECTRON PHOTOELECTRON

SPECTROSCOPYSPECTROSCOPY

( C )( 3 )

( 2 )( 1 )

M+

M

I1 I2 I3

( B )M+

M

( 1 ) ( 3 )( 2 )

Ionization Energy

( A )

Cou

nts /

sec

INTERNUCLEAR DISTANCE

2 P σg → non bonding 2345 to 2191 cm -1

2 Pπu → bonding 2345 to 1850 cm -1

2 Sσu → weakly antibonding 2345 to 2397 cm -1

Ev = Eo + ωe (v + ½) - ωe xe (v + ½)2

De = ω2/ 4 ωe xe

HeI UPS of H2 Vibrations and Rotations !

CORE LEVEL PHOTOELECTRONCORE LEVEL PHOTOELECTRONSPECTROSCOPYSPECTROSCOPY

XPS-spectra of the ls core levels of Li, Be, B, C, N, O, F (from S. Hüfner).

Binding energy eV

Cou

ntin

g R

ate

Cou

ntin

g R

ate

Chemical ShiftEB=291.2eV

INSTRUMENTATIONINSTRUMENTATION

Todiffusionpump

Gassampleµ

metalshields hν (584 Å)

Electroncountingsystem

Synchrotron Radiation and XPS

ELECTRON BEAM

Layout of the synchrotron radiation laboratory at DORIS.

References1. S. Hufner, Photoelectron Spectroscopy,Spinger-Verlag, Berlin, 1995.

2. H. Windawi and F. F. L. Ho, Ed. Applied Electron Spectroscopy for Chemical Analysis,John Wiley & Sons, New York, 1982. .

3. D. Briggs and M. P. Seah, Ed. PracticalSurface Analysis by Auger and X-ray

Photoelectron Spectroscopy, John Wiley& Sons, New York, 1983.

4. G. C. Smith, Surface Analysis by Electron Spectroscopy Measurement and Interpretation, Plenum Press, New York, 1994.5. C. R. Brundle and A. D. Baker, Ed.,Electron Spectroscopy: Theory, Techniques and Applications, Vol. 1-5, Academic press,London, 1978.

6. D. Briggs, Ed., Handbook of X- ray and Ultraviolet Photoelectron Spectroscopy, Heyden, London, 1977.

7. K. Siegbahn, C. Nordling, G. Johansson, J. Hedman, P. F. Heden, K. Hamrin, U. Gelius, T. Bergmark, L. O. Werme, R. Manne, andY. Baer, ESCA Applied to Free Molecules,North-Holland, Amsterdam, 1969.

8. D. A. Shirley, Ed., Electron Spectroscopy,North-Holland, Amsterdam, 1972.