Preparation Strategies

1. Preparation Strategies Preparation of heterogeneous catalysts

3. Energy ranges

4. Energy 0 Ef EB The Photoemission Process kinetic energy work function initial photon energy binding energy

5. Photemission - classical • The emitting atom remains unaffected by photoemision (photocation!). • The photoelectron is a single particle unaffected by core- and valence electrons (surface sensitive?). • There is infinite lifetime of the core hole, no coupling to other bound states, no relaxation and electrons behave as particles.

6. Photoemission – quantum mechanical • Photoemission as superposition of wave functions (photon-ground state; photoelectron-virtual bound state (Rydberg state); virtual bound state-continuum state (free electron in box): three phase model • Finite lifetime, relaxation by shrinking of bound states, by secondary emission (Auger, shake-off), by bond shortening and LMCT

7. Ry M O Photoemission – quantum mechanical

8. The QM picture of photoemission Free atom case and case of a solid with a band structure: note the difference in „Rydberg“ states.

9. ARUPS: Seeing orbitals photoemission is truely qm The angular resolution of the intensities allow to separate the s-states from the p-states in the sp3 bonding in Si

10. The Photoemission Process Sudden Approximation: well defined initial and final states (lines in scheme); no wave nature of photons and electrons. High energy spectroscopy without relaxation of system (atom plus first coordination). Ebind = Ephoton – Ekin – Ework function oversimplified: relaxation in energy and in geometry of system causes multiple complications (information)

11. Towards a picture of photoemission

12. Koopman´s Theorem, Relaxation The „binding energy“ form photemission equals the inverse of the dissociation energy from the ground state (theoretically accesible). Ekoop = Eadiab + Erelax In reality there is a relaxation process due to: geometry change (Frank Condon effect) nuclear charge change (photocation) vibrational excitation multiple exctitations inside (shakeup) or to the environement of the cation(shake off, LMCT)

13. Koopman´s Theorem, Relaxation Ekoop = Eadiab + Eshakeoff Example: Ne 1s: E adiab = 892 eV E koop = 870 eV (experimental XPS value) E shakeoff = 16 eV Difference: 6 eV various other relaxations plus various errors: much larger than „chemical shift“ that should be zero for a noble gas.

14. Time effects; natural widths Characteristic times for photoemission from bound states: 10-13s Born Oppenheimer approx breaks down: XPS „sees“ restructuring of electron shell and thus repositionning of nucleii (bond shortening) Additional relaxation effects by Auger processes shorten core hole lifetime and thus broaden lines: effect decreases for increasing secondary quantum numbers (width increases from f,d,p,s) Typical minimal linewidth in gases: 0.15 eV, in solids 0.55 eV Differntial charging cuases much additional linebroadening (up to several eV). Sample roughness enhances this effect.

15. Platinum, a practical example

16. Line profiles Two major causes for profiles above natural Lorentz shape: instrumental effects and final state coupling of the core hole. Unresolved differences in ground state (chemical heterogeniety) add Gaussian components as well as unresolved spin-orbit splittings with Gauss-Lorentz shapes. Differential charging (of sample and instrument parts along electron path) further adds Gaussian components. s-groundstates offer the simplest profiles unless they are multiplet split in paramagnetic samples.

17. A simple line: oxygen 1s

18. Solid structure and PE Thermal broadening of the Fermi energy due to the Boltzmann „tail“: UPS of Cu at 450 K blue and 623 K red. The peaks are the He I beta satellites of the light source representing the Cu 4d bad maximum as „ghost“.

19. Line profile modification by charging Mo oxide on silica model system is Si//SiO2 real catalyst is powder sample after impregnation and calcination.

20. Core hole functions The quantum mechanical nature of the PE process causes coupling of the core hole during relaxation with the valence DOS (Coster Kronig transitions). Different shapes of valence DOS lead to different peak asymmetries of naturally Lorentzian line profiles.

21. Lifetime broadening The contribution of core hole lifetime broadening in simple metals is small. Phonon broadening (vibrations of emitting atoms) is significant. Relaxation broadening determines parameter alpha and is unaffected by phonons (temperature).

22. Frank Condon principle- vib relaxation

23. Binding Energy Within Koopman´s theorem we discuss the contribution to a EB EBA = Enucl + Eval + E coord nucl: kinetic and potential energies of all core electrons (nb: groud state) val: potential of valence shells; chemical bonding coord: potential from environment, e.g. Madelung potential

24. Chemical shift difference in EB for the same initial state in two different compounds A, B Eshift = Ecoupl (chargeA-chargeB) + (EcoreA-EcoreB) Ecoupl: two electron coupling integral between core and valence state of an atom: ca. 14 eV Ecore: core potential of inital state in compounds A,B depends on: atom type (quantum numbers) aggregate state, crystal structure (external environment) In atoms and vdW crystals is term 2 small: shift works; in crystals and at surafces is term 2 large: shift concept breaks down

25. Chemical shift

26. Photoemission of metal oxides The metal d-states overlap differently with the relatively stable oxygen 2 p states of the „oxo-anions“ Strong effect on chemical shift as the anions are obviously differently „ionic“

27. Oxidation state by XPS conventional XPS after catalytic operation in attached prep chamber: data at 300 K

28. Chemical shift EBexp = Eadiab – ER – ET - EC Eshift = EBA - EBB Eshift = (EadiabA – EadiabB) – (ERA – ERB) – (ETA-ETB) – (ECA – ECB) only this is the chemical shift

29. ET ER EC ground state final state dynamic final state static phonon excitation valence state relaxation LDOS, oxidation state shake up shake off mean field change (n-1 approx) Madelung potential multiplet splitting extraatomic relaxation BE contributions

30. Magnitude of energies For nitrogen atoms on finds the following energies: Ionisation of the 1s state E adiab: 450 eV EB exp: 400 eV ER: 18 eV ET: 22 eV EC: 5 eV Great care with interpretation of EC in a classical picture Total errors amount to magnitude of EC

31. Chemical shift data

32. Referencing binding energies • Calibration of energy scale necessary due to work function uncertainities of sample-spectrometer array. • Au 4f 7/2 at 84.0 eV and Fermi edge of Au provide primary standards. Linearity check by looking at Cu 2p3/2 at 932.67 eV. • Samples calibrated by „adventitious carbon“ at 285.0 as referred to graphite at 284.6 eV.

33. Surface differential charging Practical samples are inhomogeneous in geometry and composition and create electrostatic field differences during photoemission across their surface.

34. Differential charging:practical

35. from: Tanuma at al., SIA 17, 911 (1991). Intensity I = f(instrument)+ f(electron-photon interaction)+f(electron-electron interaction)+f(atomic abundance) mean free path homogeneous sample, lateral and in depth

36. Practical data of mean free path

37. Surface sensitivity

38. Instrumental realisation

39. In situ XPS

40. X-rays enter the cell at 55° incidence through an SiNx window (thickness ~ 1000 Å) Hemisphercal electron analyzer (10-9p0) Analyzer input lens mass spectrometer and additional pumping Focal point of analyzer input lens Third differential pumping stage (10-8p0) Second differential pumping stage (10-6p0) Experimental cell supplied by gas lines (p0) First differential pumping stage (10-4p0)

41. 400 °C O 1s Cu2O O2 (g) CH2O (g) sub-surface oxygen surface oxygen CH3OH (g) H2O (g) CO2 (g) CH3OH:O2 1:2 3:1 6:1 Exp. localisation of species In-situ XPS at 0.5 mbar of Cu during methanol oxidation

42. Reducing conditions Oxidizing conditions CH3OH:O2 = 1:2 Non-destructive depth profile

43. Backgrounds More than 90% of all photoelectrons are scattered and change kinetic energy. The experimental spectrum is thus determined by background effects and not be characteristic lines. Operation modes of the analyser allow to supress the background but modulate the intensity by a variable transmission function Data analysis has to take care of background correction according to diffrent models of scattering. (Analyser in constant pass energy mode with stable transmission function.

44. The background

45. background functions and errors

46. background practical Ru metal on nanotube carbon. The modulation of the constant background is due to plasmon excitations of the carbon

47. XPS - UPS synchrotron radiation selection rules change: all spectra show Au 5d states Take care about physical conditions when comparing spectral shapes and when assigning „species“ to the spectral weight.

48. XPS core states atom specific quantitative complex final state effects (informative) chemical shift concept (caveats) theoretically difficult accessible UPS valence states non-atom specific not quantifiable complex selection rules similarity to DOS theoretically accessible Methodical comparison

49. Adsorption Increasing strength of chemisorption leads to d-band splitting

50. Adsorption experiment Good agreement between theory and experiment withoin the framework of „backbonding“ – strong interaction with weak charge transfer