Diapositiva 1

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Electronic excitations in elementary reactive
processes at metal surfaces
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Contributors
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)
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outline
- brief introduction adiabatic processes -
electronic excitations at the surface friction -
electronic excitations at the molecule
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gas/solid interfaces
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
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surface face and reactivity
rates of ammonia synthesis over five iron
single-crystal surfaces
Fe (111)
Fe (110)
Fe (100)
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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
T100K
sticking coefficient S0
T800K
T300K
T800K
impact energy Ei (eV)
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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
T100K
sticking coefficient S0
T800K
T300K
T800K
impact energy Ei (eV)
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dynamics of diatomic molecules on metal surfaces
theory
theoretical model two steps
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

• incidence conditions are fixed (Ei, Q)
• sampling on the internal degrees of freedom (X,
  Y, q, j) and on ? (azimuthal angle of trajectory)

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dissociation of N2 on W(110)
sticking coefficient S0
impact energy Ei (eV)
Alducin et al., PRL 97, 056102 (2006) JCP 125,
144705 (2006)
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why N2 abundantly dissociate on W(100) and not on
W(110)
Z3.25Å
potential well
Z2Å
probabiliity of reaching Z
impact energy Ei (meV)
Alducin et al., PRL 97, 056102 (2006) JCP 125,
144705 (2006)
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in summary, dynamics matters
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outline
- brief introduction adiabatic processes -
electronic excitations at the surface friction -
electronic excitations at the molecule
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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

• incidence conditions are fixed (Ei, Q)
• sampling on the internal degrees of freedom (X,
  Y, q, j) and on ? (azimuthal angle of trajectory)

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non-adiabatic effects electron-hole pair
excitations
chemicurrents
vibrational promotion of electron transfer
Gergen et al., Science 294, 2521 (2001).
Huang et al., Science 290, 111 (2000) White et
al., Nature 433, 503 (2005)
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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)
friction coefficient
adiabatic force 6D DFT PES
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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)

classical equations of motion
for each atom i in the molecule
mi(d2ri/dt2)-dV(ri,rj)/d(ri) h(ri)(dri/dt)
friction coefficient
adiabatic force 6D DFT PES
n0
hn0kFstr(kF)
effective medium FEG with electronic density n0
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probability of dissociative adsorption N2 on
W(110)
polar angle of incidence Qi0
adiabatic
sticking coefficient
Qi45
Qi60
initial kinetic energy (eV)
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probability of dissociative adsorption N2 on
W(110)
non-adiabatic
polar angle of incidence Qi0
adiabatic
sticking coefficient
Qi45
Qi60
initial kinetic energy (eV)
Juaristi et al., PRL 100, 116102 (2008)
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probability of dissociative adsorption H2 on
Cu(110)
Juaristi et al., PRL 100, 116102 (2008)
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why the excitation of electron-hole pairs is not
relevant
friction coefficient
velocity
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energy loss of reflected molecules N2 on W(110)
Qi0 Qi45 Qi60
Ei1.5 eV
energy losses in the reflected molecules due to
electronic excitations are lt 100 meV
Juaristi et al., PRL 100, 116102 (2008)
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Energy loss of reflected molecules N2 on W(110)
Experimental conditions

• Ts1200K
• Trot lt5K (J0)
• Normal incidence
• and detection

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Energy loss of reflected molecules N2 on W(110)
Experimental conditions

• Ts1200K
• Trot lt5K (J0)
• Normal incidence
• and detection

adiabatic
electronic friction
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Energy loss of reflected molecules N2 on W(110)
Experimental conditions

• Ts1200K
• Trot lt5K (J0)
• Normal incidence
• and detection

adiabatic
GLO phonons
Phonon excitations are responsible for the energy
transfer observed experimentally
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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.

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outline
- brief introduction adiabatic processes -
electronic excitations at the surface friction -
electronic excitations at the molecule
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can we enhance dissociation?
O2 on metal surfaces
non-adiabatic effects in the incoming O2 molecule
Yourdshahyan et al., PRB 65, 075416 (2002) Behler
et al., PRL 94, 036104 (2005) Carbogno et al.
PRL 101, 096104 (2008)
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adsorption of O2 on flat Ag surfaces

• Ts lt 150K O2 adsorbs only molecularly (Ei lt 1eV)

molecular beam experiments

• Ag (111)

• Ag (100)/Ag(110)

Low probability
L. Vattuone et al., Surf. Sci. 408, L698 (1998).
A. Raukema et al., Surf. Sci. 347, 151 (1996).
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O2/Ag(100) - theoretical calculations
calculation of the Potential Energy Surface (PES)

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

classical trajectory calculations

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

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building the 6D PES
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dissociation probability of spin-triplet
O2/Ag(100)
Alducin, Busnengo, RDM, JCP 129, 224702 (2008)
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dissociation probability of spin-triplet
O2/Ag(100)
Alducin, Busnengo, RDM, JCP 129, 224702 (2008)
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dissociation probability of spin-triplet
O2/Ag(100)

• General features
• Activation energy 1.1eV
• Low dissociation probability

Reason Only configurations around bridge
lead to dissociation
Alducin, Busnengo, RDM, JCP 129, 224702 (2008)
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the question
can we enhance O2 dissociation on clean Ag(100) ?
Gas phase O2
3Sg
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differences between SP and NSP PESs
Gas phase O2
1Dg
?1 eV triplet to singlet excitation energy
3Sg
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dissociation is enhanced for singlet O2
dissociation occurs for Ei lt 1 eV dissociation
can increase in one order of magnitude
Alducin, Busnengo, RDM, JCP 129, 224702 (2008)
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dissociation is enhanced for singlet O2
dissociation occurs for Ei lt 1 eV dissociation
can increase in one order of magnitude
Alducin, Busnengo, RDM, JCP 129, 224702 (2008)
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dissociation is enhanced for singlet O2
Q0o
Q30o
Q45o
spin-triplet O2
spin-triplet O2
spin-triplet O2
spin-singlet O2
spin-singlet O2
spin-singlet O2
dissociation occurs for Ei lt 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
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why is that?
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why is that?
available paths to dissociation are
different (and more!)
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it is not the same road
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conclusions

• Dissociation increases in about one order of
  magnitude,
• if singletO2 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.

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thank you for your attention
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Korta UPV/EHU (2007)
chemistry faculty UPV/EHU (80s)
DIPC (2000)
CIC Nanogune (June 2007)
CFM (Febr. 2008)
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differences between SP and NSP PESs
Gas phase O2
1Dg
?1 eV triplet to singlet excitation energy
3Sg
for Z lt 2A, the SP and NSP PESs merge
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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)
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it is not the same road
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Time-dependent density functional theory Energy
loss and friction

• TDDFT

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time-dependent evolution of the electronic density
TDDFT calculation of the induced electronic
density
cluster N254 rs2 Rcl12,67
a.u. antiproton velocity v1,5 a.u.
Borisov et al., Chem. Phys. Lett. 387, 95
(2004) Quijada et al., Phys. Rev. A 75, 042902
(2007)
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Time-dependent density functional theory Energy
loss and friction
antiproton with v2 au interacting with a metal
cluster N18 induced dissipative electronic
density DndissnTDDFT-nadiab
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Time-dependent density functional theory Energy
loss and friction
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exchange correlation functionals in DFT testing
the full potential energy surface
N2/W(110)
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!

• Exp Pfnür et al., JCP 85 7452 (1986)?

PW91
RPBE
Bocan et al., JCP 128, 154704 (2008)
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final state features N adsorption on W
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
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dynamic trapping in a well in both faces
W(100)
W(110)
2.65
1.9
Z(Å)
2.62
Z(Å)
2.5
approach to surface vertical over a surface atom
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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 vF str(vF)
vF Fermi velocity
str(vF) Transport cross section at the Fermi
level
? S / v
dl(vF) scattering phase-shifts at the Fermi level
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Dissociative adsorption Role of electron-hole
pair excitations
Description 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
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UFM
energy force along the particle trajectory
energy transfered to the cluster
force on the antiproton
antiproton with velocity v1,5 a.u. and cluster
with rs2 and 254 electrons (Rcl12,67 a.u.)
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UFM
size dependence of the Energy loss
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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

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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 v0.1 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