1. Geok Mei CHONG Master Candidate of Advanced Spectroscopy in Chemistry University of Leipzig, ASC Network 4 th December 2009 1

2. Outline 1. Principle of XPS & ARXPS Instrumentation 2. Depth Profile by ARXPS 3. XPS and ARXPS applied to fluid analysis Experimental setup for fluid analysis 4. Application of in research Surfactant Water Biological molecules 2

3. Principle of XPS & ARXPS Photoelectric effect 3

4. Required spectrometer components 4

5. Electron energy analyzer & detector 5 1.Radius of curvature is dependent on kinetic energy of electron. 2.Channel electron multipliers

6. What information is learned from XPS? 1. Elemental Identification 2. Chemical State Identification 3. Quantification 4. Mapping 5. Depth profile ARXPS 6

7. Depth profile z = depth λ = mean free path θ = emission angle λ’ = observation depth 7 How is z related to I s ? λ’ = λ cos θ I o attenuated exponentially according to Beer Lambert law

8. the observed depth information varies with photoelectron detection angle θ 8 Angular resolved XPS z = depth λ = mean free path θ = emission angle (relative to surface normal) λ’ = observation depth λ’ = λ cos θ

9. 9 Angular resolved XPS λ’ = λ cos θ z = depth λ = mean free path θ = emission angle (relative to surface normal) λ’ = observation depth the observed depth information varies with photon energy

10. 10 Angular resolved XPS Quantification The Observed photoelectron intensity of element A: A fitting process I cal -> I obs

11. XPS & ARXPS applied to fluid First performed by H. Siegbahn, K. Siegbahn and colleagues. Complete separation between PE signals from liquid and vapour using a beam of liquid formamide. 11 H. Siegbahn, K. Siegbahn, J. Electron Spectrosc. Rel. Phenomena 2 1973, 319

12. XPS & ARXPS applied to fluid A challenging investigation Needs for producing “well-behaved” liquid beam in vacuum 12 1. Liquids of sufficient low vapour pressure (< 1 Torr). Cooled to -40 0 C Droplet formation for high vapour pressure Loss of PE when absorbed by the vapor 2. Surface smoothness 3. Sample charging effect

13. XPS & ARXPS applied to fluid How to produce “well behaved” liquid beam? 13 Rotating metal disc Allowed studies of liquids with low vapour pressure. Liquid lamella Produced flat liquid surface

14. XPS & ARXPS applied to fluid How to produce “well behaved” liquid beam? 14 Liquid microjet Vacuum jet consists of a smooth continuous region of liquid water, which decays into droplet at a distance of approximately 5mm. Allowed studies of liquids with higher vapour pressure, example: water. However, using HeI radiation, only the outer valence region could be probed. The size of the jet was reduced to the μ size range.

15. 15 Angular resolved XPS Quantification The Observed photoelectron intensity of element A: (Eq 1 ) Requires accurate knowledge of photoionization cross section and angular characteristics of emission direction A fitting process I cal -> I obs

16. Application of ARXPS in research Surfactant Concentration depth profile of TBAI in FA from C 1 s Used chemical shift to evaluate the relative intensities due to TBAI and FA. The contributions from TBAI, FA liq and FA gas are separated. The ratio of the peak area of TBAI to that of FA liq are determined for many combinations of photonenergies and observation angles. 16 F.Eschen, M. Heyerhoff, H. Morgner, J. Vogt, J. Phys. Condens. Matter 7 (1995) 1961

17. Application of ARXPS in research Surfactant Concentration depth profile of TBAI in FA from C 1 s Single molecular layer is assumed to be 1.5 Å thick. Large decrease in salt conc. after 3 rd layer. The thickness of the enhanced salt conc. was estimated to be about 12 Å. Given diameter of TBA + is 9.5 Å, the thickness of enhanced salt corresponds to 1 monolayer of salt. TBA + ions have preferred orientation near the surface 17 F.Eschen, M. Heyerhoff, H. Morgner, J. Vogt, J. Phys. Condens. Matter 7 (1995) 1961

18. Application of ARXPS in research Behaviour of hydroxide at the water interface O 1 s XPS (microjet) spectra of NaOH 0.2 – 2M aqueous solutions Spectral contributions from H2O(gas), H2O(aq), and OH - (aq) @ 600eV were assigned. Zoom into the OH - (aq) 2pπ. Fully quantitative of OH - intensity was not visible here as the intensity of O 1 s peak was small. 18 Bernd Winter et. al., Chemical Physics Letters 474 (2009) 241–247

19. Application of ARXPS in research Behaviour of hydroxide at the water/vapour interface Oxygen 1s XPS spectra of NaOH 0.2 – 2 M aqueous solutions OH - (aq) 2pπ and OH - (aq) O 1 s photoelectron signal as function of OH - conc. Linear dependence of the interfacial OH - density on bulk conc. MD results support PE experiments findings. 19 Bernd Winter et. al., Chemical Physics Letters 474 (2009) 241–247

20. Application of ARXPS in research Behaviour of hydroxide at the water/vapour interface Experimental and computational calculations suggest that: OH - do not have any special surface binding site. There is linear dependence of the interfacial OH - signal on its bulk concentration. 20 Bernd Winter et. al., Chemical Physics Letters 474 (2009) 241–247 Some earlier studies suggest that OH - strongly accumulates within the interfacial region (cluster?). The debates are still on going …

21. Applications in biological Molecules N 1 s PE spectral of 0.5m lysine at diff. pH Biological molecules in water environment is very challenging in monitoring local charge density. Microscopic structure of aa is sensitive to pH 21 D. Nolting, E.F. Aziz, N. Ottosson, M. Faubel, I.V. Hertel, B. Winter, J. Am. Chem. Soc. 129 (2007) 14068

22. Applications in biological Molecules N 1 s PE spectral of 2m imidazole aqueous at diff. pH Structural changes can be faster than time resolution of NMR (10- 5 s). At high pH, proton exchange between the 2 N site on time scale of 10- 12 s. The 2 chemically pseudo- equivalent N atoms resolved. 22 D. Nolting, N. Ottosson, M. Faubel, I.V. Hertel, B. Winter, J. Am. Chem. Soc. 130 (2008) 8150.

23. Conclusion ARXPS is highly surface sensitive. Possible to probe depth profile as small as 1.5 nm. ARXPS is very sensitive to study interfacial at various depths at microscopic scale. Still challenging to deal with fluid samples, especially high vapor pressure solution. 23 An interesting and challenging field …

24. Quiz ! 1. Which peak is caused by inelastic scattering? 2. Why XPS is surface sensitive? 3. What is the main factor that affect the spatial resolution of XPS? 24

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26. References 1. H. Siegbahn, K. Siegbahn, J. Electron Spectrosc. Rel. Phenomena, 2 (1973), 319 2. H. Siegbahn, S. Svensson and M. Lundholm, J. Electron Spectrosc. Rel. Phenomena 24 (1981), p. 205 3. Eschen F, Heyerhoff M, Morgner H and Vogt J (1995) J. Phys.: Condens. Matter 7 1961 4. Faubel M and Steiner B Ber. Bunsenges. Phys. Chem. 96 (1992)1167 5. Bernd Winter et. al., Chemical Physics Letters 474 (2009) 241– 247 6. B. Winter, M. Faubel, Chem. Rev. 106 (2006) 1176 7. D. Nolting, E.F. Aziz, N. Ottosson, M. Faubel, I.V. Hertel, B. Winter, J. Am. Chem. Soc. 129 (2007) 14068 26

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28. Just kidding 28