Dynamics of spin-triplet and spin-singlet O 2 on clean Ag(100) surfaces

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)

http://www.unr.edu.ar/

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.


motivation



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


adiabatic approximation



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



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



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



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



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



• 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



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




dissociation probability

=0Q o




=0Q o=30Q o




=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



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



Non spin polarized

Spin polarized


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



=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





=0Q o =30Q o =45Q o



Gas phase O2

3Sg


1Dg

But there is a trick here!The total energy is 1 eV larger

for the singlet O2



=0Q o =30Q o =45Q o



1 eV 1 eV 1 eV



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

why is that?



1 eV

1 eV

spin-triplet O2

spin-singlet O2

why is that?



available paths to dissociation are different(and more!)

spin-triplet O2

spin-singlet O2

it is not the same road



triplet O2 singlet O2

+ 1 eV

tripletO2

singletO2

conclusions



• 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



it is not the same road



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


Ei=1.5 eV Q=0o

3.5 Å


Z=2 Å

Z=3.5 Å

Ag(100) unit cell


Ei=1.5 eV Q=0o

3.5 Å

2.0 Å


Z=2 Å

Z=1.5 Å

Z=3.5 Å

Ag(100) unit cell


Ei=1.5 eV Q=0o

3.5 Å

2.0 Å1.5 Å


Z=2 Å

Z=1.5 Å

Z=3.5 Å

Ag(100) unit cell


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


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


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




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



1 eV 1 eV 1 eV


Dynamics of spin-triplet versus spin-singlet O2


Dissociation is a direct process

spin-triplet O2

num. rebounds >3num. rebounds <3

spin-singlet O2

Dynamic trapping also important

Surfac

edirect

trapping



1 eV

1 eV



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



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

Dynamics of spin-triplet and spin-singlet O 2 on clean Ag(100) surfaces

Documents

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instant of time