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Electronic excitations in elementary reactive processes at metal surfaces

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elementary reactive processes at metal surfaces

Ricardo Díez Muiño

Centro de Física de Materiales

Centro Mixto CSIC-UPV/EHU

San Sebastián (Spain)

Donostia - San Sebastián

Maite Alducin (CSIC)

Ricardo Díez Muiño (CSIC)

Itziar Goikoetxea (PhD, CSIC)

Iñaki Juaristi (UPV)

Geetha Kanuvakkarai (Post-doc, CSIC)

Ludovic Martin (Post-doc, DIPC)

Others

Fabio Busnengo (Rosario, Argentina)

Antoine Salin (Bordeaux, France)

- brief introduction: adiabatic processes

- electronic excitations at the surface: friction

- electronic excitations at the molecule

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

N2 + 3H2 2NH3

surface face and reactivity

rates of ammonia synthesis

over five iron single-crystal surfaces

Fe (111)

Fe (110)

Fe (100)

dissociative adsorption of N2 on W(100) and on W(110)

W(100)

W(110)

Rettner et al. (1988)

Beutl et al. (1997)

Pfnür et al. (1986)

Rettner et al. (1990)

normal incidence

T=100K

sticking coefficient S0

T=800K

T=300K

T=800K

impact energy Ei (eV)

dissociative adsorption of N2 on W(100) and on W(110)

W(100)

W(110)

why the difference in

the N2 dissociation rate

at low energies between

the (100) and (110)

faces of W?

Rettner et al. (1988)

Beutl et al. (1997)

Pfnür et al. (1986)

Rettner et al. (1990)

normal incidence

T=100K

sticking coefficient S0

T=800K

T=300K

T=800K

impact energy Ei (eV)

dynamics of diatomic molecules on metal surfaces: theory

theoreticalmodel: twosteps

1.- 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)]

2.- 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)

Pfnür et al. (1986)

Rettner et al. (1990)

dissociation of N2 on W(110)

sticking coefficient S0

impact energy Ei (eV)

Alducin et al., PRL 97, 056102 (2006); JCP 125, 144705 (2006)

Z(Å)

q=0o

r(Å)

why N2 abundantly dissociate on W(100) and not on W(110)

Z=3.25Å

potential well

Z=2Å

probabiliity of reaching Z

impact energy Ei (meV)

Alducin et al., PRL 97, 056102 (2006); JCP 125, 144705 (2006)

to the precursor well

Once in the precursor well,

molecules easily dissociate

in summary, dynamics matters

- brief introduction: adiabatic processes

- electronic excitations at the surface: friction

- electronic excitations at the molecule

dynamics of diatomic molecules on metal surfaces

theoreticalmodel: twosteps

calculation of the Potential Energy Surface (PES)

- adiabatic approximation
- frozen surface approximation 6D PES: V(X, Y, Z, r,q, j)

6D PES construction

We are assuming the validity of the

Born-Oppenheimer approximation

• 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)

non-adiabatic effects: electron-hole pair excitations

chemicurrents

vibrationalpromotion

of electron transfer

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)

adiabaticapproximation

forH2on Pt(111)

Luntzet al., JCP 123, 074704 (2005)

Díaz et al., PRL 96, 096102 (2006)

Nieto et al., Science312, 86 (2006).

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, 074704 (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

description of electronic excitations by a friction coefficient

friction coefficient:

effective medium approximation

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, 074704 (2005)

bulk metal

classical equations of motion

for each atom “i” in the molecule

n(z)

mi(d2ri/dt2)=-dV(ri,rj)/d(ri) – h(ri)(dri/dt)

n0

z

adiabatic

force:

6D DFT PES

friction

coefficient

n0

h=n0kFstr(kF)

effective medium:

FEG with electronic density n0

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)

probability of dissociative adsorption: N2 on W(110)

non-adiabatic

polar angle

of incidence

Qi=0

adiabatic

sticking coefficient

Qi=45

butforthissystem,

dissociationis

roughlydecided at

Z=2.5A

(low energies)

Qi=60

initial kinetic energy (eV)

Juaristi et al., PRL 100, 116102 (2008)

of incidence

Qi=0

Qi=45

sticking coefficient

[Salin, JCP 124, 104704 (2006)]

Qi=60

non-adiabatic

adiabatic

initial kinetic energy (eV)

probability of dissociative adsorption: H2 on Cu(110)

Juaristi et al., PRL 100, 116102 (2008)

for each atom “i” in the molecule

mi(d2ri/dt2)=-dV(ri,rj)/d(ri) – h(ri)(dri/dt)

why the excitation of electron-hole pairs is not relevant

bulk metal

n(z)

n0

z

friction coefficient

velocity

energy loss of reflected molecules: N2 on W(110)

Qi=0

Qi=45

Qi=60

Ei=1.5 eV

energy losses in the reflected molecules

due to electronic excitations are < 100 meV

Juaristi et al., PRL 100, 116102 (2008)

Energy loss of reflected molecules: N2 on W(110)

Experimental conditions

Experimental data by

Hanisco et al.

J.Vac.Sci.Technol. A 11 ,1907 (1993)

- Ts=1200K
- Trot <5K (J=0)
- Normal incidence
- and detection

Energy loss of reflected molecules: N2 on W(110)

Experimental conditions

Experimental data by

Hanisco et al.

J.Vac.Sci.Technol. A 11 ,1907 (1993)

- Ts=1200K
- Trot <5K (J=0)
- Normal incidence
- and detection

adiabatic

electronic friction

Energy loss of reflected molecules: N2 on W(110)

Experimental conditions

Experimental data by

Hanisco et al.

J.Vac.Sci.Technol. A 11 ,1907 (1993)

- Ts=1200K
- Trot <5K (J=0)
- Normal incidence
- and detection

adiabatic

GLO: phonons

Phonon excitations

are responsible for the energy transfer observed experimentally

- A local description of the friction coefficient shows that electronic excitations play a minor role in the dissociation of N2/W and H2/Cu:
- The Born-Oppenheimer approximation remains valid in these systems.
- Open questions still remain about the role of electron-hole pair excitations in other situations, in which non-adiabatic effects are due to the crossing of two or more potential energy curves, with possible transfer of charge included.

- brief introduction: adiabatic processes

- electronic excitations at the surface: friction

- electronic excitations at the molecule

: can we enhance dissociation?

O2 on metal surfaces

non-adiabatic effects

in the incoming O2 molecule

singlet O2

Yourdshahyan et al., PRB 65, 075416 (2002)

Behler et al., PRL 94, 036104 (2005)

Carbogno et al. PRL 101, 096104 (2008)

triplet O2

we prepare the incoming O2 molecule

in an excited state

excited

state

Surface

1Dg

1 eV

triplet to singlet

excitation energy

(gas phase O2)

electronic excited states

may play a role

3Sg

adsorption of O2 on flat Ag surfaces

Q

Ei

- Ts < 150K: O2 adsorbs only molecularly (Ei < 1eV)

molecular beam experiments

- Ag (111)

- Ag (100)/Ag(110)

Low probability

70%

A. Raukema et al.,

Surf. Sci. 347, 151 (1996).

L. Vattuone et al.,

Surf. Sci. 408, L698 (1998).

O2/Ag(100) - theoretical calculations

Q

Ei

calculation of the Potential Energy Surface (PES)

z

- Born-Oppenheimer approximation
- frozen surface approximation 6D PES: V(X, Y, Z, r, q, j)

q

Z

r

Y

y

X

j

classical trajectory calculations

surface unit cell

x

- incidence conditions are fixed:
- (Ei, Q)
- Monte-Carlo sampling on the
- internal degrees of freedom:
- (X, Y, q, j) and on (parallel velocity)

numerical procedure

DFT energy data

• O2 in vacuum spin-triplet ground state:

• DFT - GGA (PW91) calculation with VASP

• plane-wave basis set and US pseudopotentials

• periodic supercell: (2 x 2) and 5-layer slab

y

hollow

top view

front view

x

top

bridge

z

• about 2300 spin-polarized DFTvalues

• interpolation of the DFT data:

Corrugation reducing procedure

[Busnengo et al., JCP 112, 7641 (2000)]

dissociation probability of spin-triplet O2/Ag(100)

Q=0o

Alducin, Busnengo, RDM, JCP 129, 224702 (2008)

dissociation probability of spin-triplet O2/Ag(100)

Q=30o

Q=0o

Alducin, Busnengo, RDM, JCP 129, 224702 (2008)

dissociation probability of spin-triplet O2/Ag(100)

Q=30o

Q=0o

Q=45o

- General features:
- Activation energy: ~1.1eV
- Low dissociation probability

Q=60o

2D Potential energy surface

Reason:

Only configurations around

bridge lead to dissociation

- Energy barrier:
- E~ 1.1 eV
- Position:
- Z≈1.5 Å

Alducin, Busnengo, RDM, JCP 129, 224702 (2008)

can we enhance O2 dissociation on clean Ag(100) ?

Gas phase O2

6D PES calculation: Non spin polarized DFT

1Dg

1 eV

triplet to singlet

excitation energy

3Sg

differences between SP and NSP PESs

z

q

Z

r

Gas phase O2

Y

1Dg

Non spin polarized

y

X

j

1 eV

triplet to singlet

excitation energy

surface unit cell

x

Spin polarized

3Sg

dissociation is enhanced for singlet O2

Q=0o

Q=30o

Q=45o

spin-triplet O2

spin-triplet O2

spin-triplet O2

spin-singlet O2

spin-singlet O2

spin-singlet O2

dissociation occurs for Ei < 1 eV

dissociation can increase in one order of magnitude

Alducin, Busnengo, RDM, JCP 129, 224702 (2008)

dissociation is enhanced for singlet O2

Q=0o

Q=30o

Q=45o

But there is a trick here!

The total energy is 1 eV larger

for the singlet O2

spin-triplet O2

spin-triplet O2

spin-triplet O2

spin-singlet O2

spin-singlet O2

spin-singlet O2

Gas phase O2

1Dg

1 eV

triplet to singlet

excitation energy

3Sg

dissociation occurs for Ei < 1 eV

dissociation can increase in one order of magnitude

Alducin, Busnengo, RDM, JCP 129, 224702 (2008)

dissociation is enhanced for singlet O2

Q=0o

Q=30o

Q=45o

spin-triplet O2

spin-triplet O2

spin-triplet O2

spin-singlet O2

spin-singlet O2

spin-singlet O2

1 eV

1 eV

1 eV

dissociation occurs for Ei < 1 eV

dissociation can increase in one order of magnitude

for Q ≠ 0o, singlet-O2 is more efficient than triplet-O2 with the same total energy

spin-triplet O2

spin-singlet O2

available paths to dissociation are different

(and more!)

- Dissociation increases in about one order of magnitude,
- if singlet–O2 molecular beams are used.
- The enhancement of the dissociation rate is not only due
- to the extra energy that we are adding to the system.
- A different spin state in the incoming O2 molecule opens
- new paths to dissociation.

(2007)

chemistry faculty

UPV/EHU (80’s)

DIPC

(2000)

CIC Nanogune

(June 2007)

CFM

(Febr. 2008)

differences between SP and NSP PESs

z

q

Z

r

Gas phase O2

Y

1Dg

Non spin polarized

y

X

j

1 eV

triplet to singlet

excitation energy

surface unit cell

x

Spin polarized

3Sg

for Z < 2A, the SP and NSP PESs merge

why N2 abundantly dissociate on W(100) and not on W(110)

probabiliity of reaching Z

impact energy Ei (meV)

Alducin et al., PRL 97, 056102 (2006); JCP 125, 144705 (2006)

z axis

-calculation from the energy:

-calculation from the force:

Time-dependent density functional theory: Energy loss and friction

- TDDFT

time-dependent evolution of the electronic density

TDDFT calculation of the

induced electronic density

cluster:

N=254

rs=2

Rcl=12,67 a.u.

antiproton velocity:

v=1,5 a.u.

Borisov et al., Chem. Phys. Lett. 387, 95 (2004)

Quijada et al., Phys. Rev. A 75, 042902 (2007)

Time-dependent density functional theory: Energy loss and friction

antiproton with v=2 au interacting with a metal cluster N=18

induced dissipative electronic density Dndiss=nTDDFT-nadiab

exchange correlation functionals in DFT: friction

testing the full potential energy surface

polar angle

of incidence

Qi=0

N2/W(110)

the RPBE XC functional:

- fits better the experimental

chemisorption energies

sticking coefficient

- describes better the interaction

very near the metallic surface

- but worst in the long distances!

initial kinetic energy (eV)

- Exp: Pfnür et al., JCP 85 7452 (1986)

PW91

RPBE

Bocan et al., JCP 128, 154704 (2008)

final state features: N adsorption on W friction

W(100)

W(110)

adsorption energy

DFT = 7.4 eV

Exp. = 6.6-7 eV

adsorption distance

DFT = 0.63 Å

adsorption energy

DFT = 6.8 eV

Exp. = 6.6 eV

adsorption distance

DFT = 1.15 Å

310 meV friction

868 meV

705 meV

395 meV

1091 meV

223 meV

dynamic trapping in a well in both faces

W(100)

W(110)

Z(Å):

1.9

2.65

Z(Å):

2.5

2.62

approach to surface:

vertical over a surface atom

Z friction1

j = n0v

Z1

v

Friction coefficient for a slow atomic particle

traveling through a free electron gas ()

- The potential created by the projectile in the electron gas of density n0 is calculated using density functional theory for a static atomic particle of atomic number Z1 embedded in a free electron gas (non-linear theory of screeening)

The stopping power S (the energy loss per unit path length) is calculated in terms of the momentum transferred

per unit time to a uniform

current (n0v) of electrons

scattered by the screened

static impurity:

S = n0v vFstr(vF)

vF: Fermi velocity

str(vF): Transport cross section at the Fermi level

= S / v

dl(vF): scattering phase-shifts at the Fermi level

n friction0

Dissociative adsorption: Role of electron-hole pair excitationsDescription of electronic excitations by a friction coefficient

Friction coefficient: effective medium approximation

Effective medium approximation:

FEG with electronic density n0

bulk metal

n(x,y,z)

n0

Friction coefficient of an atom in a FEG

h = n0 kF str(kF)

z

Transport cross section of the electrons scattered

by the atom

FEG electron density

FEG Fermi momentum

unidad de física de materiales friction

DE

UFM

energy & force along the particle trajectory

energy transfered to the cluster

force on the antiproton

antiproton with velocity v=1,5 a.u. and

cluster with rs=2 and 254 electrons (Rcl=12,67 a.u.)

unidad de física de materiales friction

UFM

size dependence of the Energy loss

Explanation:

-local process

inside the cluster

-size effects due to interference with

reflections of the travelling perturbation

- “Universal” behaviour
- Size dependence for
- small clusters and low
- velocities

unidad de física de materiales friction

UFM

summary & conclusions

Universal behaviour of the energy loss as a function of projectile velocity, independent of size and in agreement with the bulk curves, for nanoparticle sizes as small as ~1 nm

Size effects found only for velocities v~0.1 a.u. and small clusters

Most of the contribution to the stopping of the antiproton takes place inside the cluster. The process is very local, which explains the universal behaviour.

Size effects appear only when the reflection of the electronic excitation at the cluster surface interfers with the screening hole that accompanies the antiproton along its trajectory

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