Time-resolved gas electron diffraction – building a new

apparatus in Edinburgh

Derek A. WannUniversity of Edinburgh

Workshop on Ultrafast Electron Sources

UCLA14th December 2012

Acknowledgements

Funding:

Stuart Young

MatthewRobinson

PaulLane

Overview

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

My background• My background in solving inorganic chemistry problems using time-

averaged gas-phase ED data and quantum chemical methods

Electron diffraction in a previous life

• Performed ED studies of many novel and interesting inorganic species

• Range of sizes and complexities• Understanding function using

structures

J. Phys. Chem. A, 2007, 111, 6103.Inorg. Chem., 2011, 50, 2988. Dalton Trans., 2008, 96. Inorg. Chem., 2012, 51, 3324.

Electrongun

Inletsystem

CCDcamera

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

• Femtosecond laser used to produce electron beam and to pump molecules

New time-resolved electron diffraction

• 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

Electron flight chamber with vacuum pump

Detector chamber

Overall lab plan

Gases need nozzles

Even-Lavie valve

Some possible studies

• In an excited electronic state C60 has an enormous breathing mode

• causes increase in diameter of 30%!• requires ~100 fs time resolution to observe

• Transition metal complexes known to undergo photoinduced isomerisations

• Helps if species of interest has some heavy atoms and undergoes a significant structural change upon excitation

M

A

M

B

AB

B

CA C

BAh

S–S

C···C

S–C

C–HS···H

S–C

C–HS···C

S···H

• Dissociations, such as H3CS-SCH3 → H3CS·, would be very obvious

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