Fundamental Interactions on Surfaces
Core Hole Decay
XES one electron
picture
AES two electron
interaction; complex
Correlation effects
Sandell et. al. Phys. Rev. B48, 11347 (1993)
Core hole life time
Sum of all decay
channels
  aug  fluo
X-ray Spectroscopy
Nilsson and Pettersson, Surf. Sci. Reps. 55, 49 (2004).
The D-band Model
Vacuum
Coupling to s
Coupling to d
Energy
antibonding
d
bonding
Adsorbate projected
DOS
s
Metal projected
DOS
Hammer and Nørskov, Adv. Catal., 2000, 45, 71.
X-ray spectroscopy
X-ray spectroscopy
X-ray Photoelectron Spectroscopy
Additional probing of O and
metal core-level shifts with
XPS
Probing valence states
Photoemission and X-ray emission
Nitrogen 1s resonant x-ray emission
Photoemission
Cu
Nilsson and Pettersson, Surf. Sci. Reps. 55, 49 (2004).
Probing valence states
Photoemission and X-ray emission
Nitrogen 1s resonant x-ray emission
Photoemission
Cu
Nilsson and Pettersson, Surf. Sci. Reps. 55, 49 (2004).
Probing valence states
Photoemission and X-ray emission
Nitrogen 1s resonant x-ray emission
Photoemission
Cu
Nilsson and Pettersson, Surf. Sci. Reps. 55, 49 (2004).
Atomic Nitrogen on Ni and Cu
Occupation of antibonding states and bond strength
Ni
N-metal antibonding
Ni
Cu
d
N-metal bonding
s
Nitrogen 1s resonant x-ray spectroscopy
Occupied & unoccupied DOS:
Nilsson et. al, Catal. Lett. 100, 111 (2005)
Cu
Atomic Nitrogen on Ni and Cu
Bonding Strength
N-metal antibonding
Ni
Cu
d
N-metal bonding
s
Nitrogen 1s resonant x-ray spectroscopy
Occupied & unoccupied DOS:
Nilsson et. al, Catal. Lett. 100, 111 (2005)
Polymer Electrolyte Membrane Fuel Cells – Principle
Transforms chemical energy of fuel into electrical
energy
1
H 2  2 O2  H 2O
Membrane
H2
H+
e-
H+
O2
e-
Anode
H 2  2H   2e
Hydrogen Oxidation (HOR)
e-
O2
Cathode
H2O
1 O  2 H   2e   H O
2
2 2
Oxygen Reduction (ORR)
 Slow electrode kinetics
 Cost of catalyst
 Stability of catalyst
are most critical issues in fuel cell
research
1
Theoretical Modelling
Strong Pt–O bond
Weak Pt–O bond
Nørskov et al., J. Phys. Chem. B, 2004, 108, 46: Greeley et al., Nature Chemistry, 2009, 1, 7
Parameters to control the electronic structure
Coordination #
flat
step, kink, adatom
Alloy
Lattice strain
Ligand
Shift in D-band
Occupied Pt-DOS: Photoemission spectroscopy
Pt layers on Cu(111)
EF
d-band center
Anniyev, unpublished
Oxygen adsorption on Pt-3d-Pt(111) sandwich structure
Tuning Pt d-band DOS by controlling
3d metal in the second layer
Pt-3d-Pt sandwich structures are model
systems where second layer is
exchanged with that of
various 3d elements
Fe, Co, Ni
Pt
ligand effect
Due to a fixed substrate the lattice
parameter is same so ligand effect can be
isolated.
Valence band
hν = 620 eV
Oxygen/Pt-3d-Pt(111) – Oxygen 1s resonant x-ray spectroscopy results
O
Pt
The d-band center shifts….
Pt-O*
Binding Energy
Pt-O
Intensity of the antibonding
states in XES increases
Antibonding resonance in
XAS decreases
Probing the electronic structure of dealloyed nanoparticle catalysts
Anniyev et al, PCCP 2010, 12, 5694
support (carbon, Nafion)
nanoparticle catalysts are supported on
carbon
Core-shell structure determined from XPS.
Pt shell is compressively strained.
Strain induced lowering of the Pt 5d band results
in optimized Pt-O bond energy.
Probing the electronic structure of dealloyed nanoparticle catalysts
support (carbon, Nafion)
Valence band photoemission
8000 eV excitation, Spring-8 BL47XU
Sensitive to Pt
Valence band photoemission
1486 eV excitation, BL13-2
Core-shell structure determined from XPS.
Pt shell is compressively strained.
Strain induced lowering of the Pt 5d band results
in optimized Pt-O bond energy.
z
Dominated by support and Cu
Anniyev et al- PCCP 12, 5694 (2010)
Pt 5d DOS is obtainable!
Atom Selectivity
Selective excitation of inner and outer nitrogen atoms
Nilsson et.al. Phys. Rev. Lett. 78, 2847 (1997)
Bennich et. al. Phys. Rev. B57, 9275 (1998)
Nilsson et al., Surf. Sci. Reps. 55, 49 (2004).
LCLS pump-probe experiments
O 1s X-ray emission and X-ray absorption
spectroscopy
Electronic states
CO/Metal
Energy
CO/Metal CO gas
2π*
Spatially extended orbital
dπ
Ru-CO π-bond
1π
5σ
O1s
X-ray absorption spectroscopy
unoccupied valence state
Oxygen 2p component
Ru-CO σ-bond
X-ray emission spectroscopy
occupied valence state
Oxygen 2p component
Map valence electronic structure changes
by measuring x-ray emission spectra as a function of Laser–FEL delay & FEL energies.
Data set → Pump-probe XES & XAS
Nilsson et al., Surf. Sci. Reps. 55, 49 (2004).
Charge Density
O
Differences
O
C
C
loss of charge, repulsion
gain of charge, attraction
s looses charge and p gains charge, but not in a frontier orbital sense
All orbitals are modified and new orbitals appear
We will monitor these orbitals with time-resolved XES and XAS as the
CO/Ru bond weakens…
Nilsson and Pettersson, Surf. Sci. Reps. 55, 49 (2004).
Ultrafast Surface Chemistry at LCLS
This first work:
fs-laser (400nm) induced CO desorption from Ru(0001)
x-ray free electron laser at SLAC: LCLS
in operation from 2009
ultra short x-ray pulse: <100 fs – sub ps
Ultrafast electronic structure probe
LCLS
SSRL
Probing the Reactive State in Catalysis
Most important catalytic reactions
are driven by thermal processes
The number of turn-over events
at each active site at a given
time is extremely low
The Boltzmann energy
distribution gives only few
molecules to be in a reactive
state
Ultrafast laser-induced heating
leads to orders of magnitude
higher population of the
reactive state which can now
be probed with ultrafast
methods
Chemisorbed
state
Reactive
state
Pump-Probe
How to initiate the reaction?
Probing with adsorbate sensitivity the geometric and
electronic structure
What intermediate species do we have?
How intermediate species are bonding to the surface?
CO Desorption from Ru(0001):
Weakly Bound Precursor State
rigid quasi free
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chemisorbed
precursor
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Precursor state
LCLS pump-probe experiments
X-ray emission and X-ray absorption spectrscopy
Time Δt/ps
O1s
pump
heat transfer to CO: ~10ps
?
>50%
J. Electron Spectr. 187 (2013) 9
gradual desorption of CO
~30%
Times scales and temperature
<1ps frustrated rotations >3ps moving to presursor
Hot electron driven
Phonon driven
Phys. Rev. Lett. 110 (2013) 186101
New Era in Catalysis
• First surface chemical reaction with LCLS
• Proof of principle
Observation of two different excitations of CO
Strong coupling to motion parallel to the surface; early times
Precursor to desorption in a weakened surface chemical bond
• CO+O/Ru(0001)  CO2, H+CO  HCO, Fischer-Tropsch,…
• Higher pressure (~100 torr), solid-liquid interfaces, photocatalysis
• Shorter FEL pulses, THz radiation control (LCLS 2)
• “Chemist’s dream”