electron-hole pair excitations in the dissociative adsorption
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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 ). gas/ solid interfaces. desorption . dissociative - PowerPoint PPT PresentationTRANSCRIPT
Electron-hole pair excitations in the dissociative adsorption
of diatomic molecules on metal surfaces
Ricardo Díez MuiñoCentro de Física de MaterialesCentro Mixto CSIC-UPV/EHU
San Sebastián (Spain)
Liverpool, July 2008
gas/solid interfaces
an important goal is to understand how solid surfaces can be used to promote gas-phase chemical reactions
static properties (equilibrium)
- adsorption sites and energies- chemical bonding- induced reconstructions- self-assembling
experimental techniques: - LEED, STM, PE, etc.
dynamical properties - reaction rates (adsorption, recombination, …)- diffusion- induced desorption- energy and charge exchange
experimental techniques: - molecular beams, TPD, etc.
dissociative adsorption
molecularadsorption
desorption
theoretical model: two steps
dynamics of diatomic molecules on metal surfaces
adiabatic approximation frozen surface approximation 6D PES: V(X, Y, Z, r, q, j)
calculation of the Potential Energy Surface (PES)
• 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)]
6D PES construction
QEi
incidence conditions are fixed: (Ei, Q)
sampling on the internal degrees of freedom: (X, Y, q, j) and on (azimuthal angle of trajectory)
classical trajectory calculations: Monte Carlo sampling
why N2 abundantly dissociate on W(100) and not on W(110)
0.25 0.50 0.75 1.000.00
0.25
0.50
0.75
1.00
W(100)
T=800K
T=300K
T=100K
impact energy Ei (eV)
0.25 0.50 0.75 1.000.00
0.25
0.50
0.75
1.00
W(110)
T=800K
normal incidence
Rettner et al. (1988)Beutl et al. (1997)
Pfnür et al. (1986)Rettner et al. (1990)
stic
king
coe
ffici
ent S
0
0 25 50 75 1000.00
0.25
0.50
0.75
1.00
final
Z=2.0
Z=3.25
W(110)
final
Z=2.0
Z=3.25
W(100)
impact energy Ei (meV)
prob
abili
ity o
f rea
chin
g Z
the amount of N2 molecules thatare able to reach
Z=3.25 Å is much smaller
in the W(110) face
Z=2Å
Z=3.25Å
why N2 abundantly dissociate on W(100) and not on W(110)
Alducin et al., PRL 97, 056102 (2006); JCP 125, 144705 (2006)
why N2 abundantly dissociate on W(100) and not on W(110)
0.0 0.5 1.0 1.5 2.0 2.50.0
0.1
0.2
0.3
0.4
0.5 N2/W(110)Qi=0
0.5 1.0 1.5 2.0 2.5
N2/W(110)Qi=60
impact energy Ei (eV)
stic
king
coe
ffici
ent S
0
Alducin et al., PRL 97, 056102 (2006); JCP 125, 144705 (2006)
Pfnür et al. (1986)Rettner et al. (1990)
theory
theoretical model: two steps
dynamics of diatomic molecules on metal surfaces
adiabatic approximation frozen surface approximation 6D PES: V(X, Y, Z, r, q, j)
calculation of the Potential Energy Surface (PES)
• 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)]
6D PES construction
QEi
incidence conditions are fixed: (Ei, Q)
sampling on the internal degrees of freedom: (X, Y, q, j) and on (azimuthal angle of trajectory)
classical trajectory calculations: Monte Carlo sampling
We are assuming the validity of the Born-Oppenheimer approximation
non-adiabatic effects: electron-hole pair excitations
chemicurrents
Exposure (ML)
Gergen et al., Science 294, 2521 (2001).
vibrational promotion of electron transfer
Huang et al., Science 290, 111 (2000) White et al., Nature 433, 503 (2005)
NO on Cs/Au(111)electron emission asa function of initial
vibrational state
friction in N2/Ru(0001)
Luntz et al., JCP 123, 074704 (2005)Díaz et al., PRL 96, 096102 (2006)
adiabatic approximationfor H2 on Pt(111)
Nieto et al., Science 312, 86 (2006).
description of electronic excitations by a friction coefficient
classical equations of motion
mi(d2ri/dt2)=-dV(ri,rj)/d(ri) – h(ri)(dri/dt)
for each atom “i” in the molecule
friction coefficient
adiabaticforce:
6D DFT PES
- 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, 074704 (2005)
previously used for:
description of electronic excitations by a friction coefficient
classical equations of motion
mi(d2ri/dt2)=-dV(ri,rj)/d(ri) – h(ri)(dri/dt)
for each atom “i” in the molecule
friction coefficient
adiabaticforce:
6D DFT PES
friction coefficient: effective medium approximation
n(z)
zn0
bulk metal
n0
h=n0kFstr(kF)
effective medium: FEG with electronic density n0
- 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, 074704 (2005)
previously used for:
probability of dissociative adsorption: N2 on W(110)
stic
king
coe
ffici
ent
initial kinetic energy (eV)
polar angle of incidence
Qi=0
Qi=45
Qi=60
0,2
0,4
0,2
0,4
0,5 1,0 1,5 2,0 2,50,0
0,1
0,2
0,3
adiabatic
probability of dissociative adsorption: N2 on W(110)
non-adiabatic
adiabatic
stic
king
coe
ffici
ent
initial kinetic energy (eV)
polar angle of incidence
Qi=0
Qi=45
Qi=60
0.2
0.4
0.2
0.4
0.5 1.0 1.5 2.0 2.50.0
0.1
0.2
0.3
Juaristi et al., PRL 100, 116102 (2008)
but for this system, dissociation is
roughly decided atZ=2.5A
(low energies)
1 1 .2 1 .4 1 .6 1 .8 2 2 .2
1 .5
2
2 .5
3
3 .5
4
probability of dissociative adsorption: H2 on Cu(110)
non-adiabatic
adiabatic
0.25
0.50
0.75
1.00
0.25
0.50
0.75
stic
king
coe
ffici
ent
initial kinetic energy (eV)0.5 1.0 1.5 2.0
0.00
0.02
0.04
0.06
polar angle of incidence
Qi=0
Qi=45
Qi=60
[Salin, JCP 124, 104704 (2006)]
Juaristi et al., PRL 100, 116102 (2008)
classical equations of motion
mi(d2ri/dt2)=-dV(ri,rj)/d(ri) – h(ri)(dri/dt)
for each atom “i” in the molecule
friction coefficient velocity
n(z)
zn0
bulk metal
why the excitation of electron-hole pairs is not relevant
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
gas/surface dynamics in San SebastianMaite 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)
CollaboratorsH. Fabio Busnengo (Universidad de Rosario)
Antoine Salin (Université de Bordeaux)
elbow plots show that the N atoms start to become
apart after dissociation
has been decided
dissociation path for N2/W(110)
1 1 .2 1 .4 1 .6 1 .8 2 2 .2
1 .5
2
2 .5
3
3 .5
4
1 1 .2 1 .4 1 .6 1 .8 2 2 .2
1 .5
2
2 .5
3
3 .5
4
1 1 .2 1 .4 1 .6 1 .8 2 2 .2
1 .5
2
2 .5
3
3 .5
4
1 1 .2 1 .4 1 .6 1 .8 2 2 .2
1 .5
2
2 .5
3
3 .5
4
q=45o
j=270o
q=90o
j 125oq=90o
j 54o
q=0o
r(Å) r(Å)
r(Å)r(Å)
Z(Å
)Z(
Å)
Z(Å
)Z(
Å)
Alducin et al., PRL 97, 056102 (2006)
energy loss of reflected molecules: N2 on W(110)
Qi=0Qi=45Qi=60
energy losses in the reflected molecules due to electronic excitations are < 100 meV
Ei=1.5 eV
Juaristi et al., PRL 100, 116102 (2008)
0.0 0.5 1.0 1.5 2.0 2.50.0
0.1
0.2
0.3
0.4
0.5 N2/W(110)Qi=0
0,5 1,0 1,5 2,0 2,5
N2/W(110)Q
i=60
impact energy Ei (eV)
stic
king
coe
ffici
ent S
0
Alducin et al., PRL 97, 056102 (2006); JCP 125, 144705 (2006)
adiabatic calculation of dissociative sticking
N2/W(110)Qi=60
Pfnür et al. (1986)Rettner et al. (1990)
theory
Díez Muiño and Salin, PRB 62, 5207 (2000)
two centers effects
centro de física de materialesCFM
friction coefficients multiplied by a factor of 10
centro de física de materialesCFM
Why are friction effects so minor?
Hydrogen Nitrogen
0 1 2 3 4
1
2
3
W (110)
Y(Å)
Z(Å
)
0 1 2 30
1
2
3
Cu (110)
Y(Å)
Z(Å
)
In the dissociation path, friction coeffient can take large values