1. Spectroscopic and related techniques in surface science for unravelling heterogeneously catalyzed reaction mechanisms
Ludo Juurlink, Ph.D.
Leiden Institute of Chemistry
Leiden University, Leiden, the Netherlands
Office: Gorlaeus Laboratories DE0.01
phone
Course objectives: 
At the this short course students
can explain how surface science attempts to understand heterogeneous catalysis
can outline how common experimental (spectroscopic) techniques reveal information on surfaces, adsorbates, and chemical reactions
Understand why and how (supersonic) molecular beams are useful herein
are informed on some recent examples in the field of gas-surface dynamics

2. Surface Science and Gas-Surface Reaction Dynamics
Schedule
Date
Time
Topics
14-Dec-17
11:00 – 11:50
Introduction: Surface Science for Catalysis
Surface Crystallography
Low Energy Electron Diffraction
Scanning Tunneling Microscopy
12:00 – 12:50
Introduction to spectroscopic techniques
Auger Electron Spectroscopy
X-ray photoelectron spectroscopy
Reflection Absorption InfraRed Spectroscopy
Temperature Programmed Desorption
15-Dec-17
Controlling molecular impact: supersonic molecular beams
Examples of combined use of SMB and Surface Science
Examples from the recent literature on CH4
State-selected dissociation
Mode-selected dissociation
Bond-selected dissociation
Stereodynamical effects

3. Introduction to spectroscopic techniques

4. Electron spectroscopy
Probing of electronic structure of the surface through analysis of energy of secondary electrons emitted from sample.
Auger electron spectroscopy (AES)
Determines: surface composition
Uses: irradiation via electrons
Photoelectron spectroscopy (XPS, UPS)
Determines: surface composition and electronic structure
Uses: irradiation via photons

5. Spectrum of secondary electronsEp
Elastic peak
N(E)
electrons
photons
ions Ep
secondary electrons
N(E)
What is meant by secondary electrons?
Does this require a well defined, flat surface?
When would this be surface sensititive?
Ep = eV
with a narrow Ep

6. Spectrum of secondary electrons
photons
ions Ep
secondary electrons
N(E)
What is meant by secondary electrons?
Does this require a well defined, flat surface?
When would this be surface sensititive?
Ep = eV
with a narrow Ep 6

7. Electron energy analyzers
4-grid LEED optics: retarding field analyzer
electrons with E0<eV0 are repelled
applying a sine wave to the potential ramp with LIA detection enhances sensitivity
What is meant by secondary electrons?
Does this require a well defined, flat surface?
When would this be surface sensititive? 7

8. Electron energy analyzers
Cylindrical mirror analyzer (CMA): deflection analyzer
electrons in narrow energy window are detected
applying a sine wave to the potential ramp with LIA detection enhances sensitivity 8

9. Electron energy analyzers
Concentric hemispherical analyzer (CHA): deflection analyzer
electrons in narrow energy window are detected 9

10. Auger electron spectroscopy
Uses an electron beam to create an initial state of a hole in a (surface) atom. Through the Auger process, a second hole is created and a second electron emitted with a specific Ekin.
Lise Meitner
Pierre Auger
What type of electron energy analyzer would be used generally for AES?
How would one in pactice establish a coverage by AES?
Surface composition can be determined as every atom has unique atomic energy levels.

11. Auger electron spectroscopy
Ni(cyl) O S C
Dirty Cu sample

12. Auger electron spectroscopy
Quantitative analyses are possible for > 0.01 monolayer of adsorbate or alloy
A standard is required (e.g. an adsorbate yielding a known maximum coverage)

13. Auger electron spectroscopy
Uses an electron beam to create an initial state of a hole in a (surface) atom. Through the Auger process, a second hole is created and a second electron emitted with a specific Ekin.
Lise Meitner
Pierre Auger
What type of electron energy analyzer would be used generally for AES?
How would one in pactice establish a coverage by AES?

14. Photoelectron spectroscopy
Based on photoelectric effect: electron with binding energy Ei absorbs photon with energy ħω, and leaves solid with kinetic energy: Ekin = ħω - Ei – φ
where φ = Evacuum-EFermi is the work function of the material.
Conditions to detect escaping electron:
ħω > Ei + φ
Electron velocity is directed towards outer surface
Electron does not loose energy due to collisions with other electrons on its way to the surface

15. Laboratory photoelectron spectroscopy
Ekin = ħω - Ei – φ
X-ray photoelectron spectroscopy (XPS)
Mg K1,2 ħω = eV (9.891 Å)
Al K1,2 ħω = eV (8.341 Å)
Ultraviolet photoelectron spectroscopy (UPS)
discharge lamp He: ħω = eV (58.43 nm)
40.81 eV (30.38 nm)
X-ray source or UV lamp
Alternative: synchrotron facility

16. X-ray photoelectron spectroscopy
Ei = ħω - Ekin- φ
Why does the valence band give such a weak signal?

17. X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy: Core level shifts
Due to different environment of surface versus bulk atoms
Delta Ed: core level shift of d-band

18. X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy: Core level shifts
Chemical changes lead to large shifts.
C2H4/Co(0001) C
C2H2
Delta Ed: core level shift of d-band
C2H4
Courtesy of: C.J. Weststrate 18

19. Ultraviolet photoelectron spectroscopy
Ultraviolet photon spectroscopy (UPS): low photon energies (< 50 eV), so only valence levels become excited
Angle-integrated UPS
Angle-resolved UPS (ARUPS)

20. Vibrational spectroscopies
(High Resolution) Electron Energy Loss Spectroscopy (HREELS)
and
Reflection Absorption Infrared Spectroscopy (RAIRS)

21. Electron energy loss spectroscopy
Study of inelastically scattered electrons, which have lost well-defined energies during interaction with surface
Different scattering processes:
Core level excitation (CLEELS): 100 – 104 eV
Excitation of plasmons and electronic interband transitions (EELS): 1 – 100 eV
Excitation of vibrations of surface atoms and adsorbates (HREELS): 10-3 – 1 eV
Scattering mechanisms
Dipole scattering – long range
Very sharp features near specular angle
Impact scattering – short range
Wide angle scattering
Negative ion resonances
Strong dependence on impact energy
Excitation of electronic interband transitions: semiconductor/insulator. Excitation of e- from occupied state in valence band or occupied surface state to normally empty state in conduction band. e-
Ekin,out
Ekin,in 21

22. Electron energy loss spectroscopy
HREELS:
Identification of adsorbed species
Identification of adsorption sites
Identification of spatial orientation of adsorbed molecule
Source: e- gun; monochromator: first energy dispersive element; analyzer: second energy dispersive element; detector
ν(C-O)
CO (gas phase )
2143 cm-1
Terminal CO
cm-1
Bridging (2f site)
cm-1
Bridging (3f/4f site)
< 1800 cm-1 22

23. Electron energy loss spectroscopy
HREELS:
Identification of adsorbed species
Identification of adsorption sites
Identification of spatial orientation of adsorbed molecule
Source: e- gun; monochromator: first energy dispersive element; analyzer: second energy dispersive element; detector 23

24. Reflection Absorption InfraRed Spectroscopy (RAIRS)
Highest sensitivity for observing an absorption feature when
p-polarized light
grazing incidence
molecule with transition dipole arranged along surface normal
molecule with large transition moment
Selection rule:
Source: e- gun; monochromator: first energy dispersive element; analyzer: second energy dispersive element; detector 24

25. Reflection Absorption InfraRed Spectroscopy (RAIRS)
Source: e- gun; monochromator: first energy dispersive element; analyzer: second energy dispersive element; detector
Chen et al., Faraday Discuss. 157, 285 (2012) 25

26. Reflection Absorption InfraRed Spectroscopy (RAIRS)
50 ML Amorphous Solid Water
Crystalline ice
1 ML ASW on H/Pt(533)
Source: e- gun; monochromator: first energy dispersive element; analyzer: second energy dispersive element; detector
free OH group 26

27. RAIRS vs EELS
Source: e- gun; monochromator: first energy dispersive element; analyzer: second energy dispersive element; detector 27

28. Temperature Programmed Desorption (TPD)
Thermal Desorption Spectroscopy (TDS)
Temperature Programmed Reaction Spectroscopy (TPRS)

29. Potential energy diagram
O2/M(hkl)
The depth of the invidual well depends on
material
surface structure
adsorption site
molecular orientation
Dissociation energy in the gas phase
O2 (g)  2 O(g)
physisorption
V(r)
molecular
chemisorption
O2, chem
The crossings of the adsorption curves determine whether a process is activated or not.
O2, phys
O2 (g)
dissociative chemisorption
adiabatic behavior
Oads + Oads r

30. Potential energy surface
O2/Pt(111)
from: Groß, Eichler, Hafner, Mehl, and Papaconstantopoulos, Surf. Sci. 539, L542 (2003)

31. occupied fraction of the surface
Desorption
Desorption is the reverse process of adsorption
Molecular or atomic desorption
Recombinative desorption (opposite of dissociative adsorption)
V(r)
Rate of adsorption:
occupied fraction of the surface
Rate of desorption:
A2 (g)
A2, phys
A2,chem
2 Achem r

32. Desorption Desorption is the reverse process of adsorption
Molecular or atomic desorption
Recombinative desorption (opposite of dissociative adsorption)
V(r)
Rate of adsorption:
Rate of desorption:
A2 (g)
A2, phys
A2,chem
2 Achem r

33. Langmuir isotherm Langmuir adsorption model
Only one type of adsorption site
All these sites are equivalent
Only one adsorbate per site/no interactions
At equilibrium rates add up to zero.
non-dissociative
dissociative

34. Temperature Programmed Desorption
The experiment
cool sample under UHV conditions
expose it to the relevant gas
heat the sample while monitoring desorption
Quadrupole Mass Spectrometer
track Aads spectroscopically …
Polanyi-Wigner equation
Note:
and

35. H2O interaction with Pt[n(111)x(100)]
Example
H2O interaction with Pt[n(111)x(100)]

36. Temperature Programmed Desorption
H2O/Pt(111)
From: Hay et al., Surf. Sci. 505, 171 (2002)

37. Temperature Programmed Desorption
H2O/D/Pt(S)
H2O/Pt(S)
Pt(111)
Pt(533)
Pt(755)
Pt(977)
From: Den Dunnen et al., PCCP 17, 8530 (2015)

38. ‘complete analysis’

39. Temperature Programmed Desorption
D2/Cu(211) vs D2/Cu(111)
From: Kao, Kleyn and Juurlink, in preparation

40. Temperature Programmed Desorption
D2/Cu(211) vs D2/Cu(111)
From: Kao, Kleyn and Juurlink, in preparation

41. Temperature Programmed Desorption
D2/Cu(211) vs D2/Cu(111)
From: Kao, Kleyn and Juurlink, in preparation

42. Surface Science and Gas-Surface Reaction Dynamics
Schedule
Date
Time
Topics
14-Dec-17
11:00 – 11:50
Introduction: Surface Science for Catalysis
Surface Crystallography
Low Energy Electron Diffraction
Scanning Tunneling Microscopy
12:00 – 12:50
Introduction to spectroscopic techniques
Auger Electron Spectroscopy
X-ray photoelectron spectroscopy
Reflection Absorption InfraRed Spectroscopy
Temperature Programmed Desorption
15-Dec-17
Controlling molecular impact: supersonic molecular beams
Examples of combined use of SMB and Surface Science
Examples from the recent literature on CH4
State-selected dissociation
Mode-selected dissociation
Bond-selected dissociation
Stereodynamical effects