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Transcript of CSIC and Nano-Bio Spectroscopy Group University of the Basque Country UPV/EHU Department of...
CSIC and Nano-Bio Spectroscopy GroupUniversity of the Basque Country UPV/EHU
Department of Experimental Physics, Freie Universität, BerlinAbt. Physikalische Chemie, Fritz-Haber-Institut, Berlin
Michael Meyer, M. Bertin, U. Bovensiepen and M. Wolf
Properties of photoinduced states in water covered Alkali atoms on a Cu(111) surface
A. Iacomino, A. Perez Paz, A. Rubio
ETSF YRM 2013 Budapest - Hungary
ETSF YRM 2013 Budapest - Hungary
Outline
• Motivations & Objectives
• Measurements & Experimental data
• Theoretical analysis
• Conclusions & ongoing work
ETSF YRM 2013 Budapest - Hungary
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 Budapest - Hungary
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
Excited states couple with CB bands and quickly decay--> need gap above EF - Cu(111) gap at Γ
ETSF YRM 2013 Budapest - Hungary
Measurements
Time Programmed Desorption Work Function variation vs Exposure
-we know the number of water moleculesadsorbed on alkalis
No dissociation of water below critical alkali coverage (crit ~ 0.2 ML)
• We still do not know the configuration of adsorption
• How do polar molecules like water interact with the positively charged alkali ions?
preferential binding atAlkali ions
ETSF YRM 2013 Budapest - Hungary
Theoretical ApproachDFTcode: Quantum-ESPRESSO (PW representation, Ultrasoft PP)Slabs in supercell: 5x5 Cu(111), 6 layers (Cu fcc -> cu111 hcp)
Minimum size of solvation shellto 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 forwater dissociation is 0.2 ML
ETSF YRM 2013 Budapest - Hungary
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
Ice on Cu(111)
ETSF YRM 2013 Budapest - Hungary
How Geometry affects the charge redistribution
Δρ=ρtot-(ρsurf+ρcl)
Δρ=ρtot(3H2O)-ρtot(0)
- electron donation and redistribution is quite unaffected by water addition
ETSF YRM 2013 Budapest - Hungary
(Boltzmann-like) Population distributions
...........
2 2 2 2 2 2
333111
6 6
3 3
4
0 0 0 0
0
0
= 2
ETSF YRM 2013 Budapest - Hungary
(Boltzmann-like) Population distributions
...........
2 2 2 2 2 2
333111
6 6
3 3
4
0 0 0 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
ETSF YRM 2013 Budapest - Hungary
(Boltzmann-like) Population distributions + configurations
s states wet states