Ultrafast electron diffraction (UED) has the potential to capture changes in the structure of isolated molecules on the natural spatial and temporal scale of chemical reactions, that is, sub-Angstrom changes in the atomic positions that happen on femtosecond time scales. UED has the advantage that electron sources can easily reach sub-Angstrom spatial resolution, but so far femtosecond resolution had not been available for gas phase experiments due to the challenges in delivering short enough electron pulses on a gas target and the velocity mismatch between laser and electron pulses. Recently, we have used relativistic electron pulses at MeV energy to solve these challenges and reach femtosecond resolution. We have, for the first time, imaged coherent nuclear motion in a molecule with UED. In a proof-of-principle experiment, we captured the motion of a laser-excited vibrational wavepacket in iodine molecules. We are currently performing experiments in more complex molecules to capture laser-induced dissociation and conformational changes. We have also developed a table top 100 keV source that relies on a pulse compressor to deliver femtosecond electron pulses on a target and uses a tilted laser pulse to compensate for the velocity mismatch between the laser and the electrons. This source has a high repetition rate that will complement the high temporal resolution of the relativistic source.
We have constructed an electron gun that delivers highly charged femtosecond electron pulses to a target with kHz
repetition rate. Electron pulses are generated by femtosecond laser pulses in a photoemission process and are accelerated
up to 100 kV and compressed to sub-picosecond duration. Compression is essential to compensate for the space charge
effect that increases the size of electron pulses in all directions significantly. The pulses are compressed transversely by
magnetic lenses and longitudinally by the longitudinal electric field of a radio-frequency cavity. The longitudinal
compression is achieved by decelerating the electrons in the leading edge of the pulse, and accelerating the electrons in
the trailing edge of the pulse. This results in the pulse compressing and reaching the minimum pulse duration at a known
distance from the compression cavity. The short pulse duration and high repetition rate will be essential to observe subpicosecond
dynamic processes in molecules in gas phase with a good signal to noise ratio. A streak camera, consisting of
a millimeter-sized parallel plate capacitor, was used to measure the pulse duration in situ.
A two-step algorithm is developed that can reconstruct the full 3-D molecular structure from diffraction patterns of
partially aligned molecules in gas phase. This method is applicable to asymmetric-top molecules that do not need to have
any specific symmetry. This method will be important for studying dynamical processes that involve transient structures
where symmetries, if any, can possibly be broken. A new setup for the diffraction experiments that can provide enough
time resolution as well as high currents suitable for gas phase experiments is reported. Time resolution is obtained by
longitudinal compression of electron pulses by time-varying electric fields synchronized to the motion of electron pulses.