Wann

Report
Workshop on Ultrafast
Electron Sources
UCLA
14th December 2012
Time-resolved gas electron
diffraction – building a new
apparatus in Edinburgh
Derek A. Wann
University of Edinburgh
Acknowledgements
Stuart
Young
Funding:
Matthew
Robinson
Paul
Lane
Overview
My background
• My background in solving inorganic chemistry problems using timeaveraged gas-phase ED data and quantum chemical methods
Apparatus development
• Adapting the existing continuous source ED apparatus (not for this
talk)
• Designing and building a time-resolved electron diffractometer
What to study?
• Pump-probe studies of photoinduced dissociations / conformational
changes / bond breaking or forming
Future developments – including using MeV electrons in the UK
Electron diffraction in a previous life
• Performed ED studies of many
novel and interesting inorganic
species
Electron
gun
CCD
camera
• Range of sizes and complexities
• Understanding function using
structures
J. Phys. Chem. A, 2007, 111, 6103.
Inorg. Chem., 2011, 50, 2988.
Inlet
system
Dalton Trans., 2008, 96. Inorg. Chem., 2012, 51, 3324.
Current projects
• Awarded an EPSRC Career Acceleration Fellowship to do work with
vibrationally cooled beams, and on time-resolved electron diffraction
• Two main areas of work
• developing the continuous beam apparatus
• building an entirely new apparatus in Edinburgh Chemistry’s
Ti:sapphire laser lab
• Goals: understanding molecular structures and reaction dynamics
based on direct diffraction measurements
• Computational chemistry is an important part of what we do
• can be very valuable in interpreting chemical phenomena
A 100 keV DC gun
New time-resolved electron diffraction
• Femtosecond laser used to
produce electron beam and to
pump molecules
• 100 keV electrons – pulsed up to kHz
• space-charge repulsion
• compact electron gun (simulated
best resolution 500 fs)
100 kV
electron gun
chamber with
vacuum pump
Diffraction
chamber with a
magnetic lens,
pulsed nozzle and
vacuum pump
Detector
chamber
Electron flight
chamber with
vacuum pump
Overall lab plan
Gases need nozzles
Even-Lavie valve
Some possible studies
• Helps if species of interest has some heavy atoms and
undergoes a significant structural change upon excitation
• Dissociations, such as H3CS-SCH3 →
H3CS·, would be very obvious
S–C
S–S
S–C
S···C
C–H
C–H
S···H
S···H
C···C
• Transition metal complexes known to
undergo photoinduced
isomerisations
• In an excited electronic state C60 has an enormous
breathing mode
• causes increase in diameter of 30%!
• requires ~100 fs time resolution to observe
Relativistic electrons
• The EBTF at Daresbury has a 6 MeV electron gun
• Jim Clarke and colleagues keen to find uses for the apparatus
• Currently simulating the beam characteristics to see if electron
diffraction can be possible
Conclusions
• Structure tells us a lot about function, but dynamics can tell us so
much more
• Build on expertise to perform sub-ps gas-phase ED using a
compact 100 keV DC electron gun
• 2013 should see the first electrons produced
• Working with central laboratory to investigate performing MeV
diffraction

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