Properties of photoinduced states in water covered
Alkali atoms on a Cu(111) surface
A. Iacomino, A. Perez Paz, A. Rubio
CSIC and Nano-Bio Spectroscopy Group
University of the Basque Country UPV/EHU
Michael Meyer, M. Bertin, U. Bovensiepen and M. Wolf
Department of Experimental Physics, Freie Universität, Berlin
Abt. Physikalische Chemie, Fritz-Haber-Institut, Berlin
ETSF YRM 2013
Budapest - Hungary
Outline
• Motivations & Objectives
• Measurements & Experimental data
• Theoretical analysis
• Conclusions & ongoing work
ETSF YRM 2013
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Motivations
1.Water ice has plenty of sites for excess electrons from Cu(111) surface, thus
promoting chemical reactions
ETSF YRM 2013
Budapest - Hungary
Motivations
1.Water ice has plenty of sites for excess electrons from Cu(111) surface, thus
promoting chemical reactions
2.Alkali metals donate electrons to the Cu(111) and decrease the work function,
thus increasing electron excitation from the surface
control reactivity towards electronegative molecules
ETSF YRM 2013
Budapest - Hungary
Motivations
1.Water ice has plenty of sites for excess electrons from Cu(111) surface, thus
promoting chemical reactions
2.Alkali metals donate electrons to the Cu(111) and decrease the work function,
thus increasing electron excitation from the surface
control reactivity towards electronegative molecules
3. Low coverages regime under UHV conditions are easier to control than liquids
and gases solutions
atmospheric pollution reactions
stratospheric ionizing radiation and ozone depletion
geminal stage of heterogeneous photocatalysis
ETSF YRM 2013
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Objectives
- We need an electron reservoir
→ metal Cu(111)
- Excitations in the desired energy window
→ alkali atoms
- Long lifetime of excitations
→ wait and see
Alkali on Cu(111) reduce F
→ lower photon energy
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Excited states couple with CB bands and quickly deca
--> need gap above EF - Cu(111) gap at Γ
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Measurements
• How do polar molecules like water interact with the positively charged alkali ions?
Time Programmed Desorption
No dissociation of water below
critical alkali coverage
(crit ~ 0.2 ML)
preferential binding at
Alkali ions
Work Function variation vs Exposure
-we know the number of water molecules
adsorbed on alkalis
• We still do not know the configuration of adsorption
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Theoretical Approach
DFT
code: Quantum-ESPRESSO (PW representation, Ultrasoft PP)
Slabs in supercell: 5x5 Cu(111), 6 layers (Cu fcc -> cu111 hcp)
Minimum size of solvation shell
to stabilize an excess charge
Alkalis Coverage:
1/25=0.04
Na → 0.09 ML (1ML = 3/2 x 3/2)
K → 0.16 ML (1ML = 2x2)
Cs → 0.16 ML (1ML = 2x2)
Water Coverages:
1 H2O x Alk Atm → 0.046 BL water
6 H2O x Alk Atm → 0.28 BL water
WARNING: Critical alkalis Coverage for
water dissociation is 0.2 ML
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Optimized Structures
- H2O molecules are in plane
- H atoms toward surface Cu
- 1 H2O on top of alk less stable
- Alk lifted up
- 1st water shall: 5 H2O molecules
others through H bonds
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Ice on Cu(111)
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How Geometry affects the charge redistribution
Δρ=ρtot(3H2O)-ρtot(0)
Δρ=ρtot-(ρsurf+ρcl)
- electron donation and redistribution
is quite unaffected by water addition
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How to explain experimental findings
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(Boltzmann-like) Population distributions
2
2
2
1
1
1
6
6
0
3
3
0
4
0
2
2
2
3
3
3
0
0
0
=
2
...........
ETSF YRM 2013
Budapest - Hungary
(Boltzmann-like) Population distributions
2
2
2
1
1
1
6
6
0
3
3
0
4
0
...........
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2
2
2
3
3
3
0
0
0
=
2
1- Fix overall density
2- Consider only irreducible combinations
3- Discriminate in Energy (most/least stables)
4- Attribute fictitious T (T=293 K ~ 25 meV)
5- Attribute weight 1 to max stable
6- Total ΔΦ and final e- peaks from averages
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(Boltzmann-like) Population distributions
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(Boltzmann-like) Population distributions
s states
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wet states
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(Boltzmann-like) Population distributions + configurations
s states
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wet states
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ETSF YRM 2013
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ETSF YRM 2013
Budapest - Hungary
ETSF YRM 2013
Budapest - Hungary