core! what a scorcher!
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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 - PowerPoint PPT PresentationTRANSCRIPT
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