The design and development of a time-resolved electron diffractometer for the investigation of molecular dynamics

A time-resolved gas electron diffractometer has been designed and constructed to study the photoinduced dynamics of molecular systems. An ultrafast pulsed electron beam is created by the ionisation of a thin-film gold photocathode, using the third harmonic of a femtosecond Ti:Sapphire laser, and accelerated across a potential of up to 100 kV. Time-averaged diffraction from a polycrystalline platinum sample has been carried out in order to calibrate the apparatus, and the results have been shown to match well with theory. In addition to the design of the apparatus, novel experimental methods and techniques have been implemented, and software for analysing and extracting data has been developed.

Other calibration experiments have been carried out, including measuring the diameter of the pulsed electron beam produced, and how this varies as a solenoid magnetic lens acts to focus the beam. An optimal FWHM beam width of 1.2 mm has been observed at the detector for pulses containing 10^4 electrons. The time-zero position between a pump laser and probe electron beam has been found by studying the laser-induced plasma emitted from a copper mesh, and methodologies have been established for grating-enhanced ponderomotive experiments to be carried out to determine the duration of the pulsed electron beam.

Extensive electron pulse dynamics simulations, using SIMION and General Particle Tracer, accompany the experimental work. These have allowed for a full and thorough understanding of how both the duration and transverse size of the pulse changes as it propagates through the apparatus with and without the influence of the magnetic lens. It has also allowed for the ultimate time resolution of the apparatus to be determined as 416 fs.

Quantum chemical calculations have been carried out for dimethyl disulfide and diethyl disulfide, molecules that readily dissociate along the S–S bond upon excitation using a low-energy ultraviolet light. This has included a full mapping of the reaction potential-energy surface, and study of the molecular dynamics of the molecules in the ground and excited states. These studies have shown that the molecules are suitable candidates for early time-resolved gas electron diffraction studies using the new apparatus.