Core! What a Scorcher!

Inner Shell Processes in Molecules

P.A. Hatherly

University of Reading

Outline

• Core ionisation phenomena in molecules– Electronic and fragmentation processes

• Techniques– Soft x-ray sources, data collection

• Example results– State-selective experiments– Electron dynamics

Core Ionisation Phenomena

• Electronic Processes

K

L

M

Post-Collision Interaction

K

L

M

Shake-Up

Ene

rgy

K

L

M

KLL Auger Decay

Auger Electron

Photo-Electron

Core Ionisation Phenomena

• Fragmentation Processes– Photon energies correspond to inner shell ionisation energies of

atoms– Energy localisation can occur

• e.g., OCS - Three edges

O C S

O 1s C 1s S 2p

Site Specific Ionisation - CO2

+ +

2+

+ +

Major Minor

C 1s

Photoelectron

Auger electron

?

Soft X-Ray Sources

• Synchrotron Radiation– SRS: 5U.1, MPW6.1– MAX II: I411

Synchrotron Radiation

• SRS, Daresbury

Soft X-Ray Sources

• Multipole wigglersand undulators

MPW 6.1 PHOENIX

• XUV/Soft x-ray beamline on multipole wiggler 6– Joint Reading/UMIST/Daresbury project– Large photon energy range

• (40 - 350 eV)

– High flux and resolving power• ~1013 - 1014 ph/s/100mA/0.1%BW • ~10,000 resolving power

– First results June 2001

MPW6.1 PHOENIX

• Performance - Wiggler Output

0.0E+00

2.0E+14

4.0E+14

6.0E+14

8.0E+14

1.0E+15

1.2E+15

0 50 100 150 200 250 300 350 400 450 500

Photon energy, eV

MPW6, 7.5 mrad fan

5U, full aperture, tuning envelope

SRS bending magnet, 7.5 mrad fan

MPW6.1 PHOENIX

• Performance - Beamline

1.0E+11

1.0E+12

1.0E+13

1.0E+14

0 50 100 150 200 250 300 350 400 450

photon energy, eV

phot

ons/

sec/

300m

A, 2

50 m

icro

n sl

its

MPW6

5D

MPW 6.1 - PHOENIX

• Carbon contamination

5x1011

1012

2x1012

5x1012

150 200 250 300 350 400

After

Before

Energy (eV)

Flu

x (

Ph

/s/.

1%

bp

at

20

0m

A)

Effect of Ozone Cleaning The Mirrors

Data Collection

• Single-Particle Detection– Ion drift tubes, threshold and energetic

electron analysers

• Multi-Particle Detection– Coincidence techniques

• Threshold Electron-Ion Coincidence• Auger Electron-Ion Coincidence

Threshold Electron Detection

• Detect electrons with < 10meV kinetic energy– Tune photons to exactly the energy

required– State selectivity

• Only one initial state is selected• In conventional PES, many states are excited

Auger Electron Detection

• Detect electrons with characteristic energies– For C 1s ionised molecules, typically ~250 eV– Auger spectrum independent of photon energy

• Intensities may vary

– Selects final state

Ion Detection

+40 V

-40 V

-97.5 V

1 cm

1 cm

5.5 cm

1 cm

1 cm

MCPs

• Wiley and McLaren configuration

where:A and B are constants dependent on the geometry,m and q are the mass and charge of the ion, V is the potential applied to the field-free region and;

where Pll is the component of momentum parallel to the drift tube axis

Aq

mVB

q

mV

mV

qPll 2

Coincidence Studies

+V1

-V1

-V2

Electron signal

Ion signal

Gatingelectronics

Start

Stop

Electron rate

Ion rate

TDC

Counter

Electron detector

SR

Ion drift tube

electrons

ionst1 t2 t 1

t2

Example Results

• Triatomic Molecules– OCS

• Threshold electron - ion studies

– CO2

• Auger electron - ion studies• Satellite threshold electron

structure

OCS - Site-Specific Ionisation

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

1.5 2.5 3.5 4.5 5.5

Ion TOF (s)

C 1s

O 1s

S 2p3/2

S 2p1/2

C2+

C+

O+CO+ S+

CS+

OCS2+

Strong OCS2+ at S 2p edges

Absent at C1s, but returning at O 1s

Variation in yields of C+

CO2 - Auger Electron - Ion

Low Auger energy, energetic O+

High Auger energy,low energy O+

Low Auger energy, high fragmentationHigh Auger energy,low fragmentation

CO2 - Auger Electron-Ion

CO2 Shake-up Satellites

• Shake-up satellites in carbon dioxide– Core ionisation above

threshold• sufficient energy to excite

a second electron

– Auger decay fills core hole• electron dynamics • timescales of decay

CO2 - C 1s Satellite TPES

0.000

0.005

0.010

0.015

C 1s * resonance

C 1s Threshold

Threshold electron yield

TP

E y

ield

(ar

b.un

its)

285 290 295 300 305 310 315 320

0.000

0.005

0.010

0.015

(b)

(a)

C 1s * resonance

Shake-up satellites

Doubleexcitation

Total ion yield

Tot

al io

n yi

eld

(arb

. uni

ts)

Photon Energy (eV)

CO2 - C 1s Satellites TPES

305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320

Photon Energy (eV)

S1

S4

S0

S3

S2

S’

CO2 - C 1s Satellites TPES

• S0 - S4 seen in photoelectron studies

• S’ is a new feature, only seen in TPES– Origin?

• Split from either S4 or S1?

• New transition?

• Absolute cross sections obtained– previously only down to 5 eV above

threshold

CO2 - C 1s Satellites TPES

• Electron dynamics– Identify satellites with known states of molecular ions – Infer fast versus slow core hole decays via “Frozen Core” approach

• relative timescales of threshold electron escape, shake-up and Auger decay

Electron Dynamics

305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320

Photon Energy (eV)

• Fast processes match CO2+

– photoelectron escape and shake-up on the sametimescale as Auger decay

– Inner core looks like C

Electron Dynamics

305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320

Photon Energy (eV)

– Slow processes match NO2+

• photoelectron escapes beforeAuger decay

• Inner core looks like N

Conclusions

• Core processes in molecules provide insight into energy localisation and transport– Site-specific processes– State-to-state studies via threshold and

Auger electron coincidence experiments– Study of electron dynamics via satellite

states

Conclusions

• Many new possibilities opening in the future– Study of transient species – Application to organic systems– New sources permitting new science

• FELs - multiphoton processes in the XUV/SXR• Attosecond (10-18s) lasers allowing direct

probes of electron dynamics

With Thanks to:• Research Students

– Dan Collins, Barry Fisher, Mark Thomas

• International Colleagues– Marek Stankiewicz (Poland), Jaume Rius i Riu (Spain, Sweden),

Peter Erman and Elisabeth Kallne (Sweden)

• Technical Support– Mick Millard

• CLRC staff at Daresbury– Especially Frances Quinn (MPW6.1 Project Manager)

• EPSRC– Funding for beamline construction and research programme