CSIC and Nano-Bio Spectroscopy Group University of the Basque Country UPV/EHU Department of Experimental Physics, Freie Universität, Berlin Abt. Physikalische

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


Outline

• Motivations & Objectives

• Measurements & Experimental data

• Theoretical analysis

• Conclusions & ongoing work


Motivations

1.Water ice has plenty of sites for excess electrons from Cu(111) surface, thus promoting chemical reactions


Motivations


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


Motivations


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


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


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


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


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)


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


How to explain experimental findings


(Boltzmann-like) Population distributions

...........

2 2 2 2 2 2

333111

6 6

3 3

4

0 0 0 0

0

0

= 2



...........

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





s states wet states


(Boltzmann-like) Population distributions + configurations

s states wet states




Documents

CSIC and Nano-Bio Spectroscopy Group University of the Basque Country UPV/EHU Department of Experimental Physics, Freie Universität, Berlin Abt. Physikalische