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

embedding in a FEG

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