1. PHOTOELECTRON SPECTROSCOPY
CIRCE:
PHOTOELECTRON SPECTROSCOPY
Lucia Aballe
Michael Foerster
Virginia Perez
Carlos Escudero
Jordi Prat

2. overview basic principles some instrumentation
x-ray photoelectron spectroscopy - chemical analysis
photoelectron microscopy - chemical mapping
Near ambient pressure photoemission

3. basic principles PHOTOELECTRIC EFFECT: photon in – electron out
electrons absorb photon energy and “escape” K L M e- e- e-
Ekin= hn – Ebinding- F hn
monochromatic
discovery photoelectric effect:
Hertz 1886, Einstein 1905
development of XPS:
1960’s K. Siegbahn (1981 Physics Nobel Prize)
Auger decay: EKLM= EK-EL-EM

4. basic principles vacuum analyzer sample hn - fs fs fa
measured
spectrum
Ekin = hn - Eb - fa
valence
band EF hn
sample
analyzer
vacuum
core
level fs fa
surface
hn - fs

5. theory of photoemission
Fermi´s Golden Rule for N-particle states:
N-electron ground state of energy EN, 0
N-electron excited state of energy EN, s
(N-1 electrons in the solid and one free photoelectron of momentum k and energy  )
main term dipole transition operator
normally simplified one-electron approximation
dipole transition Δ l = ±1, Δs = 0 (allowed transitions: 1s → 2p, 2p → 3d, 3d → 4f,…)

6. relevant energies binding energies 0 eV (Fermi level)
to several keV (deep shells)
non interacting
chemical bonding

7. cross sections cross sections are energy and Z dependenth o t n e r g y ( V ) C u Z = 2 9 R a l i m - f c 1 3 4 5 6
barn/atom
atomic dipole calculation
( 1 barn = cm2 )
cross sections are energy and Z dependent
absorption more likely than scattering in soft x-ray region
photo-ionization cross section is core level dependent

8. to remember known photons energy, polarization, incidence angle +
analysis photo-emitted electrons
energy, angle, spin
conservation rules
energy, momentum ”
sample properties
composition, chemical state, depth profile,
electronic and magnetic properties,…

9. x-ray photoelectron spectroscopy
XPS wide scan
XPS wide scan
also
Electron Spectroscopy
for Chemical Analysis
identification of species
each core atomic orbital has a characteristic binding energy
each element/compound has a characteristic spectrum
Different elements can have similar energies. Some tricks
Check for different core levels
Check for Auger lines
Change photon energy to check cross section
- Spin orbital splitting and peak area ratios

10. (Ba,Sr)TiO3 thin film magnetron sputtering

11. (photon attenuation negligible within photoelectron escape depth l)
xps: surface sensitivity
surface sensitivity
short mean free electron path in solids
limited escape depth
information comes from surface layer (~3l)
I(z)=I0exp-z/l.cosq
~constant
(photon attenuation negligible within photoelectron escape depth l)
surface sensitivity can be varied
changing hn and thus Ekin
changing the geometry

12. quantification: Ii depends on
Ii = ni σi λi K
quantity of i atoms
differential cross-section of relevant level of species I
escape probability of electrons
instrumental (photon flux, detection efficiency, acceptance solid angle) hn
choice
Different types of backgrounds , the last one with a smooth step is the Shirley Background

13. other spectral contributions
electrons loose energy
on the way to the surface
’ background
’ plasmon loss peaks
electrons are emitted
via Auger decay (fixed Ekin!)
deeper = more collisions
Ekin = EK - EL1 - EL3
ion left in excited state
’ “shake-up” satellite peaks

14. xps: chemical shifts binding energy depends on: oxidation state3 20
305
reduced
RhCl
/Al 2 O
impregnated
XPS Rh
3d region 3d
5/2
3/2
315
binding energy depends on:
oxidation state
local chem-phys environment
qualitatively:
higher oxidation state
less core shielding
extra coulomb interaction between photo-electron & ion core
’ higher binding energy
more subtle final state effects also cause shifts
binding energy (eV)

16. xps: spin-orbit splitting
initial state
binding energy (eV)
DESO = 2.18 eV
I ratio= 4:3
4f7/2
4f5/2
W 4f
5p3/2 25 or
final state without
spin-orbit interaction
spin-orbit interaction
lifts degeneracy
ΔESO
“spin-orbit doublet”
splitting and intensity ratio largely independent of compound
degeneracy = 2j+1
determines probability of each final state

17. xps: peak shape deconvolution
each core level peak might contain different components
non-equivalent atomic environments
fit
each component typically convolution of :
lorentzian
intrinsic width
(final state lifetime, temperature)
gaussian
experimental width
(incident radiation, analyzer resolution)
asymmetry
electron-electron interaction – metal-like compounds
C 1s
Nylon 6
—(CH2(CH2)3CH2—C—NH)n—
II O 1 4 3 2
Al2p

18. Surface core levels shifts: high resolution core level spectroscopy
Atoms at surfaces have lower atomic coordination than bulk atoms. This leads to
different core level binding energies T B
hν=70 eV
res. 80 meV
4f7/2
Lower atomic coordination: lower binding energy

19. Edge atoms in stepped surface may be identified

20. Surface core level shifts allow to identify chemisorption sites:
S. Lizzit et al. (Elettra SuperEsca)
Ru d5/2
Binding energy eV

21. basic instrumentation
analyzer
energy-filter electrons
detector
multiply &
count electrons
channeltron
pre-lenses
decelerate &
focus electrons
- resolution
- transmission 2D
micro-channel plate
sample environment
typically UHV
- electron path
- sample purity
delayline
high resolution: narrow slit, large R, low E0 (shielding, power supplies)
sometimes flood gun to avoid charging
other analyzers: cylindrical mirror analyzer, time of flight

22. CIRCE beamline T G
M3a and M3b KB M1 M2
monochromator grating illuminated by a V collimating beam: no E slit
M1 : deflecting mirror and collimating (sagittal cylinder)
M2: plane deflecting mirror in mono
G: Gratings : LE (700 l/mm) : eV, ME(900 l/mm): eV, HE(1200 l/mm) eV
Refocusing and branching mirrors
Exit slit
Refocusing to sample : KB and toroidal
Mono scan: rotate M2 and grating
Gratings water cooled

23. CIRCE optics performance

24. Stability of the microscope and optics mounted on a monolithic structure
Vibrations at the sample manipulator
of the microscope

25. CIRCE summary performances
PEEM
NAPP

26. BL24, CIRCE: Variable polarization soft X-ray beamline for Photoemission Electron Microscopy and
Near Ambient Pressure Photoemission spectroscopy
PEEM
NAPP

30. Photoemission Microscope and Low Energy Electron Microscope
LEEM: e- backscatered from sample form a
magnified image. Structural info. Res : 10 nm
XPEEM: several laterally resolved operation modes.
ΔE= 0.15 eV; Sample at keV and 2 mm : discharges.
Microchannel plate intensifier and phosphorus screen and CCD

31. PEEM sample environment
Four axes manipulator , sample hating (e- bombard), cooling (~), p~ mbar
Sample beam geometry:
Azimuthal rotation possible ; incidence angle : 16 deg (grazing) up to 74 deg.
Vertical pol Horiz. Pol Circular pol.

32. In the last years, several developments of non-standard sample environments:
Magnets for XMCD
M. Foerster, L. Aballe

33. (1/2, 0) diffracted beam from Si (100) 2x1 reconstruction.
Examples of LEEM:
(1/2, 0) diffracted beam from Si (100) 2x1 reconstruction.
Dark field image showing monoatomic steps.
In the red profile hat shows the steps, two lines
indicate 10 nm separation.
Electron energy 4.6 eV
Anneal SiC to form graphene layers
Reflectivity curves at selected
positions show characteristic features
of graphene single layer, two
and three layers

34. PEEM examples Silicon wafer covered with Au
nanoparticles after a special
thermal treatment. Only below the Au
Si is oxidized

35. PEEM applications
LEM
“Moons” :
SrO termination

37. XMCD laterally resolved . Permalloy nanostructures on Si. Saturate the
sample and measure in remanence
Fe L3 elemental image
Fe XMCD contrast
Xrays m

39. Near Ambient Pressure Photoemission
Electrons need vacuum to travel far away (UHV photoemission and storage rings).
In air or gases , the mean free path is quite small
200
400
600
800
1000 2 4 6
Elastic Mean Free Path in Oxygen Gas
Electron Kinetic Energy [eV]
Mean Free Path [Torr-mm]
For 500 eV electrons:
P ~ 4 Torr for a 1 mm travel
P ~ 45 Torr for a 0.1 mm travel
In spite of this in 1985 someone already overcome this difficulty

41. Solution:
Capture electrons before they collide with gas molecules by means of differential pumping h e-
gas
previous designs:
conventional X-ray source
new design:
X-rays from synchrotron
Use electrostatic focusing !

42. “ambient” pressure
photoemission

44. The usage of NAPP is increasing a lot in the last years :
Gas-solid interfaces extensively
Investigated
H. Bluhm et al.

45. near ambient pressure PES
Dissolution of KBr salts
Ghosal et al. Science (2005)
ALS, BL 9.3.2
relative humidity (RH) increased from 5% to deliquescence point
an abrupt raise of the Br/K XPS peaks areas takes place

46. αPtO2 has also been suggested to exist after oxygen exposure on Pt110
based on ambient photoemission and calculations :
O chemisorbed
bulk Pt
Oxide (αPtO2)
0.5 Torr
473K
(DFT calculated shift)
10 -6 Torr CO on O precovered
Pt110 (0.5 Torr O2 exposure)
Bhlum et al ALS
Exposure at room T of a clean Pt110 surface to O2 reveals that at pressures of 50 torr the photoemission 4f peak has an extra component attributed to chemisorbed oxygen. Additional
Exposure to 0.5 Torr causes a.nother peak (red) assigned to alpha PtO2 on the basis of calculations.
The oxide peak was also bserved on Pt nanoparticles synthetized from solution chemistry . Particles of 1.5 nm shoed an oxide peak as major component of the Pt 4f.
The pre-covered oxygen surface was prepared with X ray beam assisted.
CO exposure at T= K causes decrease of the chemisorbed and oxide concentration and increase of the on top CO.
top CO

47. CH3CH2OH + 3H2O H2 + 2CO2 Pd Rh
CeO2
RhPd ?
RhPd
CeO2

48. Model catalyst RhPd/CeO2:Δ

49. Oxidized metal atoms in supported metal particles have been seen many cases.
Example:
CH3CH2OH + 3H2O H2 + 2CO2
RhPd particles, supported on CeO2 and unsupported
Photoemission.
T: 550 ⁰C P: 0.05 mbar
The reactivity is higher for the supported catalyst.
Pd is oxidized under reaction conditions
Here are Pd, Rh and O spectra taken at 550C under reaction conditions (ethanol hydrogenation). The first group at the top part are results for the unsupprted RhPd particles (unsupported means that they are lying on a W foil) , at the lower part are the results for particles supported on CeO2.
Different photon energies were used to explore the composition of the particles. It was found that the particles have an excess of Pd atoms at their surface.
The spectra of Pd show that the supported particles are partially oxidized (blue peaks). This is not the case for the unsupported particles. The Rh spectra also show an oxide peak
For the supported particles the oxygen spectra shows the presence of OH groups in the supported particles.
The oxidation of Pd and Rh shows that the CeO2 is an active part of the catalysis providing oxygen to the metals. The oxidized metals are likely the active particles for the reaction

50. reaction 550ºC red ox O-Ce O-H O-M Pd 3d Rh 3d O 1s CeO2 RhPd 875 eV

51. Thanks for listening !