dynamics of spin-triplet and spin-singlet o 2 on clean ag(100) surfaces
DESCRIPTION
Dynamics of spin-triplet and spin-singlet O 2 on clean Ag(100) surfaces. Maite Alducin Ricardo Díez Muiño Centro de Física de Materiales CSIC-UPV/EHU Donostia-San Sebastián ( Spain ). H. Fabio Busnengo Instituto de Física Rosario IFIR CONICET – UNR Rosario (Argentina). M. Alducin - PowerPoint PPT PresentationTRANSCRIPT
Dynamics of spin-triplet and spin-singlet O2
on clean Ag(100) surfaces
Maite AlducinRicardo Díez MuiñoCentro de Física de MaterialesCSIC-UPV/EHUDonostia-San Sebastián (Spain)
H. Fabio BusnengoInstituto de Física Rosario IFIR
CONICET – UNRRosario (Argentina)
motivation
Dynamics of spin-triplet and spin-singlet O2 on clean Ag(100) surfaces
M. AlducinH. F. BusnengoR. Díez Muiño
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
dynamical properties
- reaction rates (adsorption, recombination, …)- diffusion- induced desorption- energy and charge exchange
experimental techniques: - molecular beams, TPD, etc.
dissociative adsorption
motivation
Dynamics of spin-triplet and spin-singlet O2 on clean Ag(100) surfaces
M. AlducinH. F. BusnengoR. Díez Muiño
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.
molecularadsorption
desorption
QEi
adsorption probability depends on: • incidence kinetic energy• initial rovibrational state• incidence angle
dissociative adsorption
adiabatic approximation
Dynamics of spin-triplet and spin-singlet O2 on clean Ag(100) surfaces
M. AlducinH. F. BusnengoR. Díez Muiño
Surfac
e
We assume that the time-dependent potential is changing so slowly that the electronic wave function rearranges to
the new ground state at any instant of time:
The system remains in its instantaneous eigenstate.
excited electronic states are not relevant
electronic excitations
Dynamics of spin-triplet and spin-singlet O2 on clean Ag(100) surfaces
M. AlducinH. F. BusnengoR. Díez Muiño
Surfac
e
experimental evidence
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)
role of electronic friction
Trail et al., JCP 119, 4539 (2003) Luntz et al., JCP 123, 074704 (2005)Díaz et al., PRL 96, 096102 (2006)
Nieto et al., Science 312, 86 (2006).
stick
ing
coeffi
cien
tinitial 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)Luntz et al., PRL 102, 109601 (2009) (comment)
Juaristi et al., PRL 102, 109602 (2009) (reply)
electronic excitations at the surface can be considered as decoupled
O2 on metal surfaces
Dynamics of spin-triplet and spin-singlet O2 on clean Ag(100) surfaces
M. AlducinH. F. BusnengoR. Díez Muiño
Surfac
e
electronic excitations are created in the system
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
Yourdshahyan et al., PRB 65, 075416 (2002)Behler et al., PRL 94, 036104 (2005)
Carbogno et al. PRL 101, 096104 (2008)
O2 /Ag
Dynamics of spin-triplet and spin-singlet O2 on clean Ag(100) surfaces
M. AlducinH. F. BusnengoR. Díez Muiño
QEi
molecular beam experiments on flat Ag surfaces
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
dissociation of O2 on Ag(100)
possible ways to enhance dissociation:
role of excited electronic states?
• Ag (111)
O2/Ag(100) - theoretical calculations
Dynamics of spin-triplet and spin-singlet O2 on clean Ag(100) surfaces
M. AlducinH. F. BusnengoR. Díez Muiño
QEi
incidence conditions are fixed: (Ei, )Q
Monte-Carlo sampling on the internal degrees of freadom: (X, Y, q, j) and on (parallel velocity)
Born-Oppenheimer approximation
frozen surface approximation 6D PES: V(X, Y, Z, r, q, j)
calculation of the Potential Energy Surface (PES)
classical trajectory calculations
q
j
XY
Z
x
y
z
surface unit cell
r
building the 6D PES
Dynamics of spin-triplet and spin-singlet O2 on clean Ag(100) surfaces
M. AlducinH. F. BusnengoR. Díez Muiño
• 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
relevant configurations
Dynamics of spin-triplet and spin-singlet O2 on clean Ag(100) surfaces
M. AlducinH. F. BusnengoR. Díez Muiño
Molecular potential well
Energy depth: Ewell~ -0.25 eV
Position: Over hollow θ=90o
Z≈1.6 Å r ≈ 1.4 Å
Ag(100) surface unit cell
Dissociative configuration
Energy barrier: E~ 1.1 eV
Position: Over bridge θ=90º
Z≈1.5 Å
Ag(100) surface unit cell
Dynamics of spin-triplet and spin-singlet O2 on clean Ag(100) surfaces
M. AlducinH. F. BusnengoR. Díez Muiño
dissociation probability
=0Q o
Dynamics of spin-triplet and spin-singlet O2 on clean Ag(100) surfaces
M. AlducinH. F. BusnengoR. Díez Muiño
dissociation probability
=0Q o=30Q o
dissociation probability
Dynamics of spin-triplet and spin-singlet O2 on clean Ag(100) surfaces
M. AlducinH. F. BusnengoR. Díez Muiño
=45Q o =0Q o=30Q o
=60Q o
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
the question
Dynamics of spin-triplet and spin-singlet O2 on clean Ag(100) surfaces
M. AlducinH. F. BusnengoR. Díez Muiño
Gas phase O2
3Sg
1 eVsinglet to triplet excitation energy
1Dg
can we enhance O2 dissociation on clean Ag(100) ?
6D PES calculation: Non spin polarized DFT
differences between SP and NSP PESs
Dynamics of spin-triplet and spin-singlet O2 on clean Ag(100) surfaces
M. AlducinH. F. BusnengoR. Díez Muiño
Non spin polarized
Spin polarized
1 eVsinglet to triplet excitation energy
Gas phase O2
for Z < 2A, the SP and NSP PESs merge
3Sg
1Dg
q
j
XY
Z
x
y
z
surface unit cell
r
dissociation is enhanced for singlet O2
Dynamics of spin-triplet and spin-singlet O2 on clean Ag(100) surfaces
M. AlducinH. F. BusnengoR. Díez Muiño
=0Q o =30Q o =45Q o
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
dissociation occurs for Ei < 1 eVdissociation can increase in one order of magnitude
dissociation is enhanced for singlet O2
Dynamics of spin-triplet and spin-singlet O2 on clean Ag(100) surfaces
M. AlducinH. F. BusnengoR. Díez Muiño
=0Q o =30Q o =45Q o
spin-triplet O2 spin-triplet O2 spin-triplet O2
spin-singlet O2 spin-singlet O2 spin-singlet O2
Gas phase O2
3Sg
1 eVsinglet to triplet excitation energy
1Dg
But there is a trick here!The total energy is 1 eV larger
for the singlet O2
Dynamics of spin-triplet and spin-singlet O2 on clean Ag(100) surfaces
M. AlducinH. F. BusnengoR. Díez Muiño
=0Q o =30Q o =45Q o
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 is enhanced for singlet O2
dissociation occurs for Ei < 1 eVdissociation can increase in one order of magnitude
for Q ≠ 0o, singlet-O2 is more efficient than triplet-O2 with the same total energy
why is that?
Dynamics of spin-triplet and spin-singlet O2 on clean Ag(100) surfaces
M. AlducinH. F. BusnengoR. Díez Muiño
1 eV
1 eV
spin-triplet O2
spin-singlet O2
why is that?
Dynamics of spin-triplet and spin-singlet O2 on clean Ag(100) surfaces
M. AlducinH. F. BusnengoR. Díez Muiño
available paths to dissociation are different(and more!)
spin-triplet O2
spin-singlet O2
it is not the same road
Dynamics of spin-triplet and spin-singlet O2 on clean Ag(100) surfaces
M. AlducinH. F. BusnengoR. Díez Muiño
triplet O2 singlet O2
+ 1 eV
tripletO2
singletO2
conclusions
Dynamics of spin-triplet and spin-singlet O2 on clean Ag(100) surfaces
M. AlducinH. F. BusnengoR. Díez Muiño
• The low dissociation of O2 on Ag(100) is due to two main factors:- The existence of large activation energy barriers of about 1eV.- Only a small number of configurations in phase space lead to
dissociation.
• Dissociation increases in about one order of magnitud, if singlet–O2 molecular beams are used.
•Under off-normal incidence angles, the efficiency of singlet-O2 to dissociation is remarkable: it exceeds the reactivity of triplet-O2 with an extra kinetic energy of 1eV.
thank you for your attention
Dynamics of spin-triplet and spin-singlet O2 on clean Ag(100) surfaces
M. AlducinH. F. BusnengoR. Díez Muiño
it is not the same road
Dynamics of spin-triplet and spin-singlet O2 on clean Ag(100) surfaces
M. AlducinH. F. BusnengoR. Díez Muiño
triplet O2singlet O2
+ 1 eV
tripletO2
singletO2
Why O2 on Ag(100) ?
QEi
Molecular beam experiments on flat Ag surfaces
Ts < 150K: O2 adsorbs only molecularly (Ei < 1eV, )
Dynamics of spin-triplet and spin-singlet O2 on Ag(100) UAM, December 2008
• 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
Dissociation of O2 on Ag(100)
• reasons for the lack of dissociation
• possible ways to enhance dissociation:
role of excited electronic states?
• Ag (111)
Dissociative dynamics of O2/Ag(100): Classical trajectory calculations
Z=3.5 Å
Ag(100) unit cell
Dynamics of spin-triplet and spin-singlet O2 on Ag(100) UAM, December 2008
Ei=1.5 eV Q=0o
3.5 Å
Dissociative dynamics of O2/Ag(100): Classical trajectory calculations
Z=2 Å
Z=3.5 Å
Ag(100) unit cell
Dynamics of spin-triplet and spin-singlet O2 on Ag(100) UAM, December 2008
Ei=1.5 eV Q=0o
3.5 Å
2.0 Å
Dissociative dynamics of O2/Ag(100): Classical trajectory calculations
Z=2 Å
Z=1.5 Å
Z=3.5 Å
Ag(100) unit cell
Dynamics of spin-triplet and spin-singlet O2 on Ag(100) UAM, December 2008
Ei=1.5 eV Q=0o
3.5 Å
2.0 Å1.5 Å
Dissociative dynamics of O2/Ag(100): Classical trajectory calculations
Z=2 Å
Z=1.5 Å
Z=3.5 Å
Ag(100) unit cell
Dynamics of spin-triplet and spin-singlet O2 on Ag(100) UAM, December 2008
Ei=1.5 eV Q=0o Ei=2 eV Q=0o
3.5 Å
2.0 Å1.5 Å
Ag(100) unit cell
Non spin polarized
Spin polarized
Reactivity of spin-singlet O2on the Ag(100) surface
1 eVsinglet to triplet excitation energy
Gas phase O2
PES calculation: Non spin polarized DFT
• Z < 2A: SP and NSP PESs merging
Classical trajectory calculations
NSP PESSurfac
e
SP PES
3Sg
1Dg
Dynamics of spin-triplet and spin-singlet O2 on Ag(100) UAM, December 2008
Dissociative dynamics of spin-singlet O2 on Ag(100)
• Dissociation occurs for Ei < 1 eV• Dissociation can increase in one order of magnitud
=0Q o =30Q o =45Q o
spin-triplet O2 spin-triplet O2 spin-triplet O2
spin-singlet O2 spin-singlet O2 spin-singlet O2
Dynamics of spin-triplet and spin-singlet O2 on Ag(100) UAM, December 2008
Dissociative dynamics of spin-singlet O2 on Ag(100)
• Dissociation occurs for Ei < 1 eV• Dissociation can increase in one order of magnitud• For Q ≠ 0o, singlet-O2 is more efficient than triplet-O2 with higher Ei
=0Q o =30Q o =45Q o
spin-triplet O2 spin-triplet O2 spin-triplet O2
spin-singlet O2 spin-singlet O2 spin-singlet O2
1 eV 1 eV 1 eV
Dynamics of spin-triplet and spin-singlet O2 on Ag(100) UAM, December 2008
Dynamics of spin-triplet versus spin-singlet O2
Dynamics of spin-triplet and spin-singlet O2 on Ag(100) UAM, December 2008
Dissociation is a direct process
spin-triplet O2
num. rebounds >3num. rebounds <3
spin-singlet O2
Dynamic trapping also important
Surfac
edirect
trapping
Dynamics of spin-triplet versus spin-singlet O2
Dynamics of spin-triplet and spin-singlet O2 on Ag(100) UAM, December 2008
1 eV
1 eV
Dynamics of spin-triplet versus spin-singlet O2
Dynamics of spin-triplet and spin-singlet O2 on Ag(100) UAM, December 2008
1 eV
1 eV
Under off-normal incidence angles, the efficiency of singlet-O2 to dissociation is due to the existence of more paths leading to
dissociation
Thank you for your attention !
Centro de Física de MaterialesCFM
Gas/solid interfaces Group(San Sebastián)
Donostia International Physics Center
Work in progress and open questions
Experimental data from L. Vattuone et al.,
Surf. Sci. 408, L698 (1998)
Molecular trapping vs
Molecular sticking
Molecular potential wellNo energy barriers in the entrance channel !!
Energy depth: Ewell~ -0.25 eV
Position: Over hollow θ=90o
Z≈1.6 Å r ≈ 1.4 Å
Ag(100) surface unit cell
Dynamics of spin-triplet and spin-singlet O2 on Ag(100) UAM, December 2008
Dynamics on NSP PES
Dynamics on NSP PES
Note: (Different scales in Y-axis)
NSP vs ‘adiabatic’ singlet O2
Qi=0
Direct vs indirect in ‘adiabatic singlet-PES’
NSP PES: RPBE vs PW91
Technical details: Ab initio SP PES
Dependence of the difference between NSP and SP energies on the distance from the surface Z
Filled symbols: DFT values Open symbols: Interpolated
values
Thank you for your attention !
CFMCentro de Física de Materiales,
Centro Mixto CSIC-UPV/EHU