electronic excitations in elementary reactive processes at metal surfaces
DESCRIPTION
Electronic excitations in elementary reactive processes at metal surfaces. Ricardo Díez Muiño Centro de Física de Materiales Centro Mixto CSIC-UPV/EHU San Sebastián (Spain). Contributors. Donostia - San Sebastián Maite Alducin (CSIC) Ricardo Díez Muiño (CSIC) - PowerPoint PPT PresentationTRANSCRIPT
Electronic excitations in elementary reactive processes at metal surfaces
Ricardo Díez MuiñoCentro de Física de MaterialesCentro Mixto CSIC-UPV/EHU
San Sebastián (Spain)
Contributors
Donostia - San SebastiánMaite Alducin (CSIC)Ricardo Díez Muiño (CSIC)Itziar Goikoetxea (PhD, CSIC)Iñaki Juaristi (UPV)Geetha Kanuvakkarai (Post-doc, CSIC)Ludovic Martin (Post-doc, DIPC)
OthersFabio Busnengo (Rosario, Argentina)Antoine Salin (Bordeaux, France)
outline
- brief introduction: adiabatic processes
- electronic excitations at the surface: friction
- electronic excitations at the molecule
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
Fe (111)
Fe (100)
Fe (110)
rates of ammonia synthesis over five iron single-crystal surfaces
surface face and reactivity
N2 + 3H2 2NH3
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
0dissociative adsorption of N2 on W(100) and 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
0dissociative adsorption of N2 on W(100) and on W(110)
why the difference in the N2 dissociation rate at low energies between
the (100) and (110) faces of W?
theoretical model: two stepstheoretical model: two steps
dynamics of diatomic molecules on metal surfaces: theory
adiabatic approximation frozen surface approximation 6D PES: V(X, Y, Z, r,, )
1.- calculation of the Potential Energy Surface (PES)
• extended set of DFT energy values, V(X, Y, Z, r,, )
• interpolation of the DFT data: corrugation reduction method
[Busnengo et al., JCP 112, 7641 (2000)]
6D PES construction
Ei
incidence conditions are fixed: (Ei,
sampling on the internal degrees of freedom: (X, Y, , ) and on (azimuthal angle of trajectory)
2.- classical trajectory calculations: Monte Carlo sampling
dissociation of N2 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)
i=0
0.5 1.0 1.5 2.0 2.5
N2/W(110)
i=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
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
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)
1 1 .2 1 .4 1 .6 1 .8 2 2 .2
1 .5
2
2 .5
3
3 .5
4
=0o
r(Å)
Z(Å
)
potential well
Difficult access to the precursor well
Once in the precursor well, molecules easily dissociate
in summary, dynamics matters
outline
- brief introduction: adiabatic processes
- electronic excitations at the surface: friction
- electronic excitations at the molecule
theoretical model: two stepstheoretical model: two steps
dynamics of diatomic molecules on metal surfaces
adiabatic approximation frozen surface approximation 6D PES: V(X, Y, Z, r,, )
calculation of the Potential Energy Surface (PES)
• extended set of DFT energy values, V(X, Y, Z, r,, )
• interpolation of the DFT data: corrugation reduction method
[Busnengo et al., JCP 112, 7641 (2000)]
6D PES construction
Ei
incidence conditions are fixed: (Ei,
sampling on the internal degrees of freedom: (X, Y, , ) 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
chemicurrentschemicurrents
Exposure (ML)
Gergen et al., Science 294, 2521 (2001).
vibrational promotion of electron transfer
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) 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)
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) – (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) – (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
=n0kFtr(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
kin
g co
effi
cie
nt
initial kinetic energy (eV)
polar angle of incidence
i=0
i=45
i=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
kin
g co
effi
cie
nt
initial kinetic energy (eV)
polar angle of incidence
i=0
i=45
i=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)
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
kin
g co
effi
cie
nt
initial kinetic energy (eV)
0.5 1.0 1.5 2.00.00
0.02
0.04
0.06
polar angle of incidence
i=0
i=45
i=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) – (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
energy loss of reflected molecules: N2 on W(110)
i=0i=45i=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)
Energy loss of reflected molecules: N2 on W(110)
• Ts=1200K
• Trot <5K (J=0)
• Normal incidence
and detection
Experimental conditions
Experimental data by Hanisco et al.
J.Vac.Sci.Technol. A 11 ,1907 (1993)
Energy loss of reflected molecules: N2 on W(110)
• Ts=1200K
• Trot <5K (J=0)
• Normal incidence
and detection
Experimental conditions
Experimental data by Hanisco et al.
J.Vac.Sci.Technol. A 11 ,1907 (1993)
adiabatic
electronic friction
Energy loss of reflected molecules: N2 on W(110)
• Ts=1200K
• Trot <5K (J=0)
• Normal incidence
and detection
Experimental conditions
Experimental data by Hanisco et al.
J.Vac.Sci.Technol. A 11 ,1907 (1993)
GLO: phonons
Phonon excitationsare responsible for the
energy transfer observed experimentally
adiabatic
conclusions
• 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.
outline
- brief introduction: adiabatic processes
- electronic excitations at the surface: friction
- electronic excitations at the molecule
O2 on metal surfaces
Yourdshahyan et al., PRB 65, 075416 (2002)
Behler et al., PRL 94, 036104 (2005)Carbogno et al. PRL 101, 096104
(2008)
non-adiabatic effectsin the incoming O2 molecule
electronic excited statesmay play a role
excited state
triplet O2
singlet O2
3g
1 eVtriplet to singlet excitation energy(gas phase O2)
1g
we prepare the incoming O2 moleculein an excited state
: can we enhance dissociation?
adsorption of O2 on flat Ag surfaces Ei
Ts < 150K: O2 adsorbs only molecularly (Ei < 1eV)
• Ag (100)/Ag(110)
L. Vattuone et al., Surf. Sci. 408, L698
(1998).
A. Raukema et al., Surf. Sci. 347, 151 (1996).
70%
Low probability
• Ag (111)
molecular beam experiments
O2/Ag(100) - theoretical calculations
Ei
incidence conditions are fixed: (Ei, )
Monte-Carlo sampling on the internal degrees of freedom: (X, Y, , ) and on (parallel velocity)
Born-Oppenheimer approximation
frozen surface approximation 6D PES: V(X, Y, Z, r,, )
calculation of the Potential Energy Surface (PES)
classical trajectory calculations
XY
Z
x
y
z
surface unit cell
r
building the 6D PES
• about 2300 spin-polarized DFT values
• interpolation of the DFT data: Corrugation reducing procedure[Busnengo et al., JCP 112, 7641 (2000)]
numerical procedure
• 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
DFT energy data
z
top
hollow
bridge
x
y
top view front view
dissociation probability of spin-triplet O2/Ag(100)
Alducin, Busnengo, RDM, JCP 129, 224702 (2008)
dissociation probability of spin-triplet O2/Ag(100)
Alducin, Busnengo, RDM, JCP 129, 224702 (2008)
2D Potential energy surface
Energy barrier: E~ 1.1 eV
Position: Z≈1.5 Å
General features:
• Activation energy: ~1.1eV
• Low dissociation probability
Reason:
Only configurations around
bridge lead to dissociation
Alducin, Busnengo, RDM, JCP 129, 224702 (2008)
dissociation probability of spin-triplet O2/Ag(100)
the question
Gas phase O2
3g
1 eVtriplet to singlet excitation energy
1g
can we enhance O2 dissociation on clean Ag(100) ?
6D PES calculation: Non spin polarized DFT
differences between SP and NSP PESs
Non spin polarized
Spin polarized
1 eVtriplet to singlet excitation energy
Gas phase O2
3g
1g
XY
Z
x
y
z
surface unit cell
r
dissociation is enhanced for singlet O2
spin-triplet O2 spin-triplet O2 spin-triplet O2
spin-singlet O2 spin-singlet O2 spin-singlet O2
dissociation occurs for Ei < 1 eVdissociation can increase in one order of magnitude
Alducin, Busnengo, RDM, JCP 129, 224702 (2008)
dissociation occurs for Ei < 1 eVdissociation can increase in one order of magnitude
spin-triplet O2 spin-triplet O2 spin-triplet O2
spin-singlet O2 spin-singlet O2 spin-singlet O2
Gas phase O2
3g
1 eVtriplet to singlet excitation energy
1g
But there is a trick here!The total energy is 1 eV larger
for the singlet O2
dissociation is enhanced for singlet O2
Alducin, Busnengo, RDM, JCP 129, 224702 (2008)
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 eVdissociation can increase in one order of magnitude
for ≠ 0o, singlet-O2 is more efficient than triplet-O2 with the same total energy
dissociation is enhanced for singlet O2
why is that?
1 eV
1 eV
spin-triplet O2
spin-singlet O2
available paths to dissociation are different(and more!)
spin-triplet O2
spin-singlet O2
why is that?
it is not the same road
triplet O2 singlet O2
+ 1 eV
conclusions
• 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.
thank you for your attentionthank you for your attention
DIPC(2000)
CIC Nanogune (June 2007)CFM
(Febr. 2008)
chemistry faculty UPV/EHU (80’s)
Korta UPV/EHU (2007)
differences between SP and NSP PESs
Non spin polarized
Spin polarized
1 eVtriplet to singlet excitation energy
Gas phase O2
for Z < 2A, the SP and NSP PESs merge
3g
1g
XY
Z
x
y
z
surface unit cell
r
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
why N2 abundantly dissociate on W(100) and not on W(110)
Alducin et al., PRL 97, 056102 (2006); JCP 125, 144705 (2006)
it is not the same road
triplet O2 singlet O2
+ 1 eV
tripletO2
singletO2
Time-dependent density functional theory: Energy loss and friction
R
z axis
• TDDFT
-calculation from the force:-calculation from the energy:
time-dependent evolution of the electronic density
-25 -20 -15 -10 -5 0 5 10 15 20 25
20
15
10
5
0
-5
-10
-15
-20
z (a.u.)
x (a.u.)
-0.015 -0.005 0.005
n-n0 (a.u.)
cluster: N=254 rs=2 Rcl=12,67 a.u.
antiproton velocity: v=1,5 a.u.
TDDFT calculation of the induced electronic density
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=18induced dissipative electronic density ndiss=nTDDFT-nadiab
Time-dependent density functional theory: Energy loss and friction
exchange correlation functionals in DFT: testing the full potential energy surface
Exp: Pfnür et al., JCP 85 7452 (1986)
PW91
the RPBE XC functional:
- fits better the experimental
chemisorption energies
- describes better the interaction
very near the metallic surface
- but worst in the long distances!
stic
king
coe
ffic
ient
initial kinetic energy (eV)
0,0 0,5 1,0 1,5 2,0 2,50,0
0,1
0,2
0,3
0,4
0,5
polar angle of incidence
i=0
0,0 0,5 1,0 1,5 2,0 2,50,0
0,1
0,2
0,3
0,4
0,5
RPBEBocan et al., JCP 128, 154704 (2008)
N2/W(110)
W(100) W(110)
adsorption energyDFT = 7.4 eV
Exp. = 6.6-7 eVadsorption distance
DFT = 0.63 Å
adsorption energyDFT = 6.8 eVExp. = 6.6 eV
adsorption distanceDFT = 1.15 Å
final state features: N adsorption on W
approach to surface: vertical over a surface atom
2.62Z(Å): 2.5
1091 meV
223 meV
868 meV
2.651.9Z(Å):
705 meV
395 meV
310 meV
W(100) W(110)
dynamic trapping in a well in both faces
Friction coefficient for a slow atomic particletraveling 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)
Z1
v
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:
j = n0v
Z1
S = n0v vF tr(vF)
vF: Fermi velocity
tr(vF): Transport cross section at the Fermi level
l(vF): scattering phase-shifts at the Fermi level = S / v
Description of electronic excitations by a friction coefficient Friction coefficient: effective medium approximation
n(x,y,z)
z
bulk metal
Dissociative adsorption: Role of electron-hole pair excitations
Non-Adiabatic Electronic Effects in dissociative adsorptionGRC-Dynamics at surfaces
2009
n0
n0
= n0 kFtr(kF)
Effective medium approximation: FEG with electronic density n0
Friction coefficient of an atom in a FEG
Transport cross section of the
electrons scattered by the atom FEG Fermi
momentum
FEG electron density
unidad de física de materialesUFM
energy & force along the particle trajectory
energy transfered to the cluster force on the antiproton
-0,4
-0,3
-0,2
-0,1
0
0,1
0,2
0,3
-150 -100 -50 0 50 100 150
For
ce (
atom
ic u
nits
)
antiproton position (atomic units)
-1
0
1
2
3
4
5
6
7
-150 -100 -50 0 50 100 150
Ene
rgy
(ato
mic
uni
ts)
antiproton position (atomic units)
E
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 materialesUFM
size dependence of the Energy loss
0,001
0,01
0,1
0,1 1 10
rs= 2
18 electrons 58 electrons138 electrons254 electronsFEG (linear theory)FEG (non-linear theory)
effe
ctiv
e st
oppi
ng p
ower
(a.
u.)
velocity (a.u.)
0,001
0,01
0,1
0,1 1 10
18 electrons 58 electrons132 electrons254 electronsFEG (linear theory)FEG (non-linear theory)
effe
ctiv
e st
opp
ing
pow
er (
a.u.
)
velocity (a.u.)
rs= 4
unidad de física de materialesUFM
size dependence of the Energy loss
0,001
0,01
0,1
0,1 1 10
rs= 2
18 electrons 58 electrons138 electrons254 electronsFEG (linear theory)FEG (non-linear theory)
effe
ctiv
e st
oppi
ng p
ower
(a.
u.)
velocity (a.u.)
•“Universal” behaviour
•Size dependence for small clusters and low velocities
Explanation:
-0,4
-0,3
-0,2
-0,1
0
0,1
0,2
0,3
-40 -20 0 20 40
For
ce (
atom
ic u
nits
)
antiproton position (atomic units)
-local process inside the cluster
-size effects due to interference with reflections of the travelling perturbation
unidad de física de materialesUFM
summary & conclusions
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
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
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 found only for velocities v~0.1 a.u. and small clusters