1. Electron-hole pair excitations in the dissociative adsorption
Liverpool, July 2008
Electron-hole pair excitations
in the dissociative adsorption
of diatomic molecules on metal surfaces
Ricardo Díez Muiño
Centro de Física de Materiales
Centro Mixto CSIC-UPV/EHU
San Sebastián (Spain)

2. gas/solid interfaces static properties (equilibrium)
desorption
dissociative
adsorption
molecular
adsorption
static properties (equilibrium)
dynamical properties
- adsorption sites and energies
chemical bonding
induced reconstructions
self-assembling
reaction rates
(adsorption, recombination, …)
- diffusion
- induced desorption
- energy and charge exchange
experimental techniques:
- LEED, STM, PE, etc.
experimental techniques:
- molecular beams, TPD, etc.
an important goal is to understand how solid surfaces can be used to promote gas-phase chemical reactions

3. dynamics of diatomic molecules on metal surfaces
theoretical model: two steps
calculation of the Potential Energy Surface (PES)
adiabatic approximation
frozen surface approximation  6D PES: V(X, Y, Z, r, q, j)
6D PES construction
• extended set of DFT energy values, V(X, Y, Z, r, q, j )
• interpolation of the DFT data: corrugation reduction method
[Busnengo et al., JCP 112, 7641 (2000)]
classical trajectory calculations: Monte Carlo sampling Q Ei 
incidence conditions are fixed: (Ei, Q)
sampling on the internal degrees of freedom: (X, Y, q, j) and on  (azimuthal angle of trajectory)

4. why N2 abundantly dissociate on W(100) and not on W(110)
T=800K
T=300K
T=100K
impact energy Ei (eV)
W(110)
normal incidence
Rettner et al. (1988)
Beutl et al. (1997)
Pfnür et al. (1986)
Rettner et al. (1990)
sticking coefficient S0

5. Alducin et al., PRL 97, 056102 (2006); JCP 125, 144705 (2006)
why N2 abundantly dissociate on W(100) and not on W(110)
Z=3.25Å
Z=2Å
probabiliity of reaching Z
the amount of
N2 molecules that
are able to reach
Z=3.25 Å is
much smaller
in the W(110) face
impact energy Ei (meV)
Alducin et al., PRL 97, (2006); JCP 125, (2006)

6. Alducin et al., PRL 97, 056102 (2006); JCP 125, 144705 (2006)
why N2 abundantly dissociate on W(100) and not on W(110)
Pfnür et al. (1986)
Rettner et al. (1990)
theory
sticking coefficient S0
impact energy Ei (eV)
Alducin et al., PRL 97, (2006); JCP 125, (2006)

7. We are assuming the validity of the Born-Oppenheimer approximation
dynamics of diatomic molecules on metal surfaces
theoretical model: two steps
calculation of the Potential Energy Surface (PES)
adiabatic approximation
frozen surface approximation  6D PES: V(X, Y, Z, r, q, j)
We are assuming the validity of the
Born-Oppenheimer approximation
6D PES construction
• extended set of DFT energy values, V(X, Y, Z, r, q, j )
• interpolation of the DFT data: corrugation reduction method
[Busnengo et al., JCP 112, 7641 (2000)]
classical trajectory calculations: Monte Carlo sampling Q Ei 
incidence conditions are fixed: (Ei, Q)
sampling on the internal degrees of freedom: (X, Y, q, j) and on  (azimuthal angle of trajectory)

8. vibrational promotion
non-adiabatic effects: electron-hole pair excitations
chemicurrents
vibrational promotion
of electron transfer
Exposure (ML)
NO on Cs/Au(111)
electron emission as
a function of initial
vibrational state
Gergen et al., Science 294, 2521 (2001).
Huang et al., Science 290, 111 (2000)
White et al., Nature 433, 503 (2005)
friction in N2/Ru(0001)
Luntz et al., JCP 123, (2005)
Díaz et al., PRL 96, (2006)
adiabatic approximation
for H2 on Pt(111)
Nieto et al., Science 312, 86 (2006).

9. description of electronic excitations by a friction coefficient
previously used for:
- damping of adsorbate vibrations:
Persson and Hellsing, PRL49, 662 (1982)
- dynamics of atomic adsorption
Trail, Bird, et al., JCP119, 4539 (2003)
dissociation dynamics (low dimensions)
Luntz et al., JCP 123, (2005)
classical equations of motion
for each atom “i” in the molecule
mi(d2ri/dt2)=-dV(ri,rj)/d(ri) – h(ri)(dri/dt)
adiabatic
force:
6D DFT PES
friction
coefficient

10. friction coefficient: effective medium approximation
description of electronic excitations by a friction coefficient
previously used for:
friction coefficient:
effective medium approximation
- damping of adsorbate vibrations:
Persson and Hellsing, PRL49, 662 (1982)
- dynamics of atomic adsorption
Trail, Bird, et al., JCP119, 4539 (2003)
dissociation dynamics (low dimensions)
Luntz et al., JCP 123, (2005)
n(z) z n0
bulk metal
classical equations of motion
for each atom “i” in the molecule
mi(d2ri/dt2)=-dV(ri,rj)/d(ri) – h(ri)(dri/dt)
adiabatic
force:
6D DFT PES
friction
coefficient n0
h=n0kFstr(kF)
effective medium:
FEG with electronic density n0

11. probability of dissociative adsorption: N2 on W(110)
polar angle
of incidence
Qi=0
adiabatic
sticking coefficient
Qi=45
Qi=60
initial kinetic energy (eV)

12. probability of dissociative adsorption: N2 on W(110)
non-adiabatic
polar angle
of incidence
Qi=0
adiabatic
but for this system,
dissociation is
roughly decided at
Z=2.5A
(low energies)
sticking coefficient
Qi=45
Qi=60
initial kinetic energy (eV)
Juaristi et al., PRL 100, (2008)

13. probability of dissociative adsorption: H2 on Cu(110)
non-adiabatic
adiabatic
sticking coefficient
initial kinetic energy (eV)
polar angle
of incidence
Qi=0
Qi=45
Qi=60
[Salin, JCP 124, (2006)]
Juaristi et al., PRL 100, (2008)

14. why the excitation of electron-hole pairs is not relevant
classical equations of motion
mi(d2ri/dt2)=-dV(ri,rj)/d(ri) – h(ri)(dri/dt)
for each atom “i” in the molecule
n(z) z n0
bulk metal
friction coefficient
velocity

15. the Born-Oppenheimer approximation remains valid
in summary
a local description of the friction coefficient shows that electronic excitations play a minor role in the dissociation of diatomic molecules on metal surfaces:
the Born-Oppenheimer approximation remains valid
open questions still remain about the role of electron-hole pair excitations in other situations, in which nonadiabatic effects are due to the crossing of two or more potential energy curves, with possible transfer of charge included

16. gas/surface dynamics in San Sebastian
Maite Alducin (CSIC-UPV/EHU)
Gisela A. Bocan (DIPC)
Ricardo Díez Muiño (CSIC-UPV/EHU)
Itziar Goikoetxea (CSIC-UPV/EHU)
J. Iñaki Juaristi (UPV/EHU)
Fernando Mingo (CSIC-UPV/EHU)
Collaborators
H. Fabio Busnengo (Universidad de Rosario)
Antoine Salin (Université de Bordeaux)

17. dissociation path for N2/W(110)
Z(Å)
q=0o
Z(Å)
q=45o
j=270o
elbow plots show that the N atoms start to become apart
after dissociation has been decided
r(Å)
r(Å)
Z(Å)
q=90o
j 54o
Z(Å)
q=90o
j 125o
r(Å)
r(Å)
Alducin et al., PRL 97, (2006)

18. Qi=0 Qi=45 Qi=60 energy loss of reflected molecules: N2 on W(110)
Ei=1.5 eV
energy losses in the reflected molecules
due to electronic excitations are < 100 meV
Juaristi et al., PRL 100, (2008)

19. Alducin et al., PRL 97, 056102 (2006); JCP 125, 144705 (2006)
adiabatic calculation of dissociative sticking
Pfnür et al. (1986)
Rettner et al. (1990)
theory
N2/W(110)
Qi=60
sticking coefficient S0
impact energy Ei (eV)
Alducin et al., PRL 97, (2006); JCP 125, (2006)

20. Díez Muiño and Salin, PRB 62, 5207 (2000)
two centers effects
Díez Muiño and Salin, PRB 62, 5207 (2000)

21. embedding in a FEG

22. friction coefficients multiplied by a factor of 10
centro de física de materiales
CFM
friction coefficients
multiplied by a factor of 10

23. In the dissociation path, friction coeffient can take large values
centro de física de materiales
CFM
Why are friction effects so minor?
Hydrogen
Nitrogen
Cu (110)
Y(Å)
Z(Å)
W (110)
Y(Å)
Z(Å)
In the dissociation path, friction coeffient can take large values