An introduction to X- ray photoelectron spectroscopy

Transcription

1 An introduction to X- ray photoelectron spectroscopy

2 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 or reflected from materials. All of these spectroscopic techniques yield information on the ELECTRONIC STRUCTURE.

3 There are, generally five techniques collectively called electron spectroscopy

4 X-ray photoelectron spectroscopy (XPS) Ultraviolet photoelectron spectroscopy (UPS) Aüger electron spectroscopy (AES) Electron energy loss spectroscopy (EELS) Inverse photoemission spectroscopy (IPS)

5 There are a range of techniques in each of these UPS Photon source variation He I 21.2 ev He II 40.8 ev Ne I Synchrotron radiation

6 UPS Variations of the same basic technique One photon spectroscopy Solids Gases Gas cell Molecular beams

7 Photoelectron-photoion coincidence spectroscopy Zero-kinetic energy photoelectron spectroscopy Multiphoton photoelectron spectroscopy Photodetachment spectroscopy..

8 Structure and Properties of Matter Spectroscopy Scattering Physical Properties Spectroscopy (pre-1965) Absorption Magnetic Mass

9 Spectroscopy using electrons e + M +, M 2+ E e + M + M + Current I 4 I 3 I 2 I 1 Electron KE Ionization efficiency curves Detector

10 hν + M + Detecto M + current I 1 I 2 I 3 I 4 Photon Energy

11 hν + M M + + e hν Constant + (No M 2+, generally) M + + e Photoelectron Spectroscopy Photoelectric effect Early experiments in 1887 Detector KE hν = KE + φ 1905

12 Photoion can be excited M + hν M + (E int ) + e elec vib rot hν -I -E int KE of the electron

13 Conservation of momentum requires that excess energy is partitioned in inverse proportion to the masses. Excited ionic states e I 1 * hν M + I 1 M I 2 E int I p hν

14 Electron and ion separates with equal momenta. mu = MU The relative velocity, V = u + U = U (1+ M/m) = u (1+ m/m) The kinetic energies, ½ MU 2 = 1 m MV 2 m+m 2M ½ mu 2 = 1 m MV 2 2m m+m

15 hν - (I p + E int ) KE hν - KE I p + E int hν -KE 1 IP 1 hν -KE 2 IP 2 hν -KE 3 IP 3.. E int o hν - KE I p Vertical adiabatic E r e

16 Depth of analysis depends on photon energy He I 21.2 ev 2 1 P 1 1 S He II 40.8 ev 2 P 1 S of He + 3 / Al K α1, ev 2 P 2, 1 / 2 1 S Mg K α1, ev Na K α1, ev Si K α1, ev 1000 Core Valence uv Hard X rays Soft X rays

17 Aüger electron e hν e hν Photoemission X-ray fluorescence Aüger process

18 e e Vac 2, 3 (Z) L 1 (Z) L 2, 3 (z+ z) K(Z) Neutral atom Photo emission Electronic transition Coulomb field redistribution Aügur electron emission Final state

19 E K, L 1, L2, 3 = E k -E L1 - E L 2, 3 E ABC (Z) = E A (Z) - ½ [ E B (Z) + E B (Z+1) ]- ½ [ E C (Z) + E C (Z+1) E s are the binding energies. E ABC K L 1 L 2, 3, K L 1 V, KVV Intense Auger intensities if the valence electron density is high. Fluorescence efficiency increases with transition energy. Fluorescence and Auger are comparable when Ε ~ 10,000 ev.

20 VALENCE SHELL PHOTOELECTRON SPECTROSCOPY

21 Counts / sec ( A ) ( B ) M + Ionization Energy ( 1 ) ( 2 ) ( 3 ) M ( C ) M + ( 3 ) ( 2 ) ( 1 ) I 1 I 2 I 3 M

22 INTERNUCLEAR DISTANCE

23 P σ g non bonding 2345 to 2191 cm -1 Pπ u bonding 2345 to 1850 cm -1 Sσ u weakly antibonding 2345 to 2397 cm -1 E v = E o + ω e (v + ½) - ω e x e (v + ½) 2 D e = ω 2 / 4 ω e x e

24 HeI UPS of H 2 Vibrations and Rotations!

25 ORE LEVEL PHOTOELECTRON SPECTROSCOPY

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

27 Counting Rate Binding energy ev

28 Counting Rate Chemical Shift E B =291.2eV

29 INSTRUMENTATION

30 To diffusion pump µ metal shields hν (584 Å) Gas sample Electro countin system

31 Synchrotron Radiation and XPS ELECTRON BEAM Layout of the synchrotron radiation laboratory at DORIS.

32 References 1. S. Hufner, Photoelectron Spectroscopy, Spinger-Verlag, Berlin, H. Windawi and F. F. L. Ho, Ed. Applied Electron Spectroscopy for Chemical Analysis John Wiley & Sons, New York, D. Briggs and M. P. Seah, Ed. Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, John Wiley & Sons, New York, 1983.

33 4. G. C. Smith, Surface Analysis by Electron Spectroscopy Measurement and Interpretation Plenum Press, New York, C. R. Brundle and A. D. Baker, Ed., Electron Spectroscopy: Theory, Techniques and Applications, Vol. 1-5, Academic press, London, D. Briggs, Ed., Handbook of X- ray and Ultraviolet Photoelectron Spectroscopy, Heyden, London, 1977.

34 7. K. Siegbahn, C. Nordling, G. Johansson, J. Hedman, P. F. Heden, K. Hamrin, U. Gelius, T Bergmark, L. O. Werme, R. Manne, and Y. Baer, ESCA Applied to Free Molecules, North-Holland, Amsterdam, D. A. Shirley, Ed., Electron Spectroscopy, North-Holland, Amsterdam, 1972.