Raimund FeifelUppsala University/Stockholm University

(raimund.feifel@fysik.uu.se)http://usxs.fysik.uu.se/projectweb/

portalproject/MEICSAM.html

A double time-of-flight multi-electron-ion coincidence spectroscopy technique

In memory of Fabian Österdahl

Historical overview

Historical overview

Photoionisation

Photoelectric law, A. Einstein 1905: h = Ekin+ EB IP

=

Photoelectron spectroscopy

+

-

h

Ae - (Eki )

FÖRE EFTER

A+

h + EA → EA+ + EkiEkin = h - EB

Before

After

Kai Siegbahn et al., Uppsala University

Photoelectron spectroscopy today

Bl I411 @ MAX-lab, Lund, Sweden

Photoelectron spectrum of Argon

Electron binding energy (eV)18 17 16 15 14 13

Inte

nsity

Ar3p

2P3/22P1/2 Spin-orbit

components

Dept. of Physics, course laboratory spectrometer

Intensity ratio 2 : 4

Ar+ - one electron lessElectron configuration1s2 2s2 2p6 3s2 3p5 (2P3/2, 2P1/2) Ar+

MJ = - J, -J+1, …, J-1, J

Photoexcitation and ionisation processes

Deexcitation processes

Single photon - multiple ionisation

Double (multiple) ionisation energy (DIP, …)

EB1 + EB2 (+ …) = h - Ekin1 - Ekin2 (- …)

TOF-PEPECO spectroscopy

J.H.D. Eland et al., Phys. Rev. Lett. 90, 053003, 2003

e-

e-

e-

e-

several m

€

t = DEKin

TOF-PEPECO instrument…

World’s longest electron time-of-flight spectrometer

… now operational at AlbaNova…

… and it’s ‘little brother’

Timing principle

Single photon - double ionisation of Argon (h

= 48.372 eV)

deadtime ~ 50 ns

Double ionisation spectrum of Ar

Electron configuration:1s2 2s2 2p6 3s2 3p4 (3P2,1,0, 1D2, 1S0) Ar2+

Ar2+, Kr2+ and Xe2+

Rule-of-thumbLowestDIP (eV)

LowestIP (eV)*

RatioDIP/IP

Ar 43.39 15.76 2.75

Kr 38.36 14.00 2.74

Xe 33.11 12.13 2.73

€

DIPlow ≈ 2.7 ⋅ IPlow

*PhysicsHandbook

empirically To appear in Kosmos 2007

Rule-of-thumb for molecules

€

DIPlow ≈ 2.0 ⋅ IPlow +11.5 ⋅ 1r12

empirically R.D. Molloy et al., Chem. Phys. 335, 49 (2007)P. Linusson et al., in manuscript 2008

Possible double ionisation processes

•A + h A2+ + 2e- (direct double ionisation; continuous energy sharing)

•A + h A+* + 1e- A2+ + 1e- (indirect double ionisation involving discrete intermediate states)

Formation of Xe2+ states

Xe+ intermediate states

A polyatomic example: SF6

Electron distribution of SF6

2+

Differently selected SF62+

spectra

‘complete’ e- e- spectrum

E1: 0 - 1 eV @ 38 eV

E1: 0 - 0.1 eV @ 38 eV; akin “TPEsCO”

Slice of 2 eV @ 38 eV; “direct” SF62+

R. Feifel, J.H.D. Eland, L. Storchi and F. Tarantelli, J. Chem. Phys. 122, 144309, 2005

Double ionisation energy (eV)

560 570 580 590 600

Core valence double ionisation of O2

E. Andersson, M. Stenrup et al. , in preparation

TOF-PEPEPIPICO spectroscopy

e-

e-

e-

e-

i++

i++

i+

i+

Simultaneously possible to conduct:1) Electron-ion coincidence (PEPICO)2) Electron-ion-ion coincidence (PEPIPICO)3) Electron-electron coincidence (PEPECO)4) Electron-electron-ion coincidence (PEPEPICO)5) Electron-electron-ion-ion coincidence (PEPEPIPICO)… “PE(n)PI(m)CO experiment”; n, m = 0, 1, 2…

fe ~ 40%

fi ~ 10%

2 m

J.H.D Eland and R. Feifel Chem. Phys. 327 85 (2006)R. Feifel et al., J. Chem. Phys. 125, 194318 (2006)

Mass selected TOF-PEPECO spectra of CS2

Collection efficiencies for ions taken into account

Threshold for CS+ + S+ formation ~ 29.3 eV

Breakdown diagrams

Two-dimensional mass spectra, e.g. CS+ + S+

KER as function of dication internal energy

Summary:•Time-Of-Flight PhotoElectron-PhotoElectron COincidence spectroscopy reveals electronic states of doubly-ionised systems in direct analogy to PES

•Rules-of-thumb for double ionisation

•Direct vs. indirect double ionisation mechanisms

•TOF-PEnPImCO spectroscopy

AcknowledgmentsStockholm:•Mats Larsson•Tony Hansson•Fabian Österdahl•Tomasz Kloda•Isak Bakken•Mohamed Elsakre•Kjell Schmidt & workshops•Meryll Colombet

Uppsala:•Leif Karlsson•Jan-Erik Rubensson•Egil Andersson•Lage Hedin•Martin Berglund•Pelle Linusson•Aila Gengelbach•Pernilla Andersson•Tommy Karlsson•Benedikt PfeifferOxford:

•John H.D. Eland

Swedish Research Council (VR), Göran Gustafsson (UU/KTH), Knut & Alice Wallenberg and Wenner Gren Foundations