Electron spectroscopy

James N. O’SheaSchool of Physics & AstronomyNottingham Nanoscience and Nanotechnology CentreUniversity of Nottingham

SMALL Kick-off meeting , Madrid| Jan 2011

SMALL Kick-off meeting , Madrid| Jan 2011

Outline

Something to study: Dye moleculesChemical information: XPSMolecular orbitals: PES & XASAdsorption geometry: XASCharge transfer dynamics: RPES

Dye molecules on surfaces

Charge injection interface

LUMO

HOMO

How is the molecule bonded to the surface?What can we say about the electronic coupling?

A model dye-sensitised system

LUMO

HOMO

Core-level photoemission

LUMO

HOMO

Core level

hv

Vacuum level

KE

BE

L. Patthey et al, J. Chem. Phys. 110, 5913 (1999)

Chemical information with x-ray photoelectron spectroscopy (XPS)

Inte

nsity

(arb

. Uni

ts)

Chemical shift (eV)

Ethyl-trifluoroacetateK. Siegbahn et al

Although the core-level electrons are not directly involved in the formation of bonds, a change in the valence electrons (chemical state) will change the potential a core-electron feels and therefore modify the binding energy.

Core-level photoemission

LUMO

HOMO

Core level

hv

Vacuum level

KE

BE

L. C Mayor et al, J. Chem. Phys. 129, 114701 (2008)

Electron spectroscopy is surface sensitive

We all know that x-rays themselves can penetrate deep into most matter…

…but the electrons coming out can only travel through a couple of layers without banging into something along the way.

Surface sensitivity of XPS

Occupied states: valence photoelectron spectroscopy

Vacuum level

hv BE

KE

Unoccupied states: x-ray absorption spectroscopy (XAS)

Vacuum level

hv

BEres

BEcore

Molecular orientation & XAS

J. Ben Taylor et al, J. Chem. Phys. 127, 134707 (2007)

RPES: Participator decay

LUMOHOMO

N1s

hv

Vacuum level

Participator electrons leave the atom in a final state just like valence photoemission.

They track with constant binding energy.

RPES: Spectator decay

LUMOHOMO

N1s

hv

Vacuum level

Spectator electrons leave the atom in a final state just like Auger decay with an extra electron in an unoccupied state.

They track with constant kinetic energy…like Auger electrons but shifted up.

RPES: Auger decay

LUMOHOMO

N1s

hv

Vacuum level

Charge transfer of the originally excited electron leaves Auger decay as the only non-radiative core-hole decay option.

They track with constant kinetic energy.

Bi-isonicotinic acid on rutile TiO2(110)

J. Schnadt et al, Nature 418, 621 (2002)

Participator

LUMO

LUMO+1

LUMO+2

J. Schnadt et al, Nature 418, 621 (2002)

The sensitivity of RPES compared to XAS intensity is assessed for the decoupled molecule (multilayer) where no charge transfer is expected for any unoccupied state.

A state-dependent ratio that we call C is generated for the case of an isolated molecule (in this case, C=⅓).

The benchmark

J. Schnadt et al, Nature 418, 621 (2002)

XASRPES

XASRPESCT IIC

II

fs

fs

LUMO

sN

T

C

3

6

2

1

Pulling out charge transfer timescales

SMALL Kick-off meeting , Madrid| Jan 2011

Conclusions

Chemical information: XPSMolecular orbitals: PES & XASAdsorption geometry: XASCharge transfer dynamics: RPES