Post on 16-Apr-2018
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.