Abstract
We have constructed an experimental setup for gas phase electron diffraction with femtosecond resolution and a high average beam current. While gas electron diffraction has been successful at determining molecular structures, it has been a challenge to reach femtosecond resolution while maintaining sufficient beam current to retrieve structures with high spatial resolution. The main challenges are the Coulomb force that leads to broadening of the electron pulses and the temporal blurring that results from the velocity mismatch between the laser and electron pulses as they traverse the sample. We present here a device that uses pulse compression to overcome the Coulomb broadening and deliver femtosecond electron pulses on a gas target. The velocity mismatch can be compensated using laser pulses with a tilted intensity front to excite the sample. The temporal resolution of the setup was determined with a streak camera to be better than 400 fs for pulses with up to half a million electrons and a kinetic energy of 90 keV. The high charge per pulse, combined with a repetition rate of 5 kHz, results in an average beam current that is between one and two orders of magnitude higher than previously demonstrated.
Highlights
The field of ultrafast gas electron diffraction (UGED) came to life with the pioneering experiments by the groups of Zewail and Weber in the 1990s and early 2000s that achieved a temporal resolution of a few picoseconds
The setup uses an radio frequency (RF) cavity to compress the electron pulses and a tilted laser pulse to compensate for the group velocity mismatch
We have shown that the device can compress electron pulses with up to half a million electrons; we have found the best configuration for UGED experiments to be pulses with $105 electrons/pulse which can be compressed to a duration of 350 fs and delivered on the target with a small divergence angle and a transverse diameter of 300 lm FWHM to match the size of the gas jet
Summary
Gas electron diffraction has a long history in the determination of the structure of isolated molecules in the gas phase, in part due to the high scattering cross section of electrons in comparison to x-rays. There has been significant progress in time-resolved electron diffraction, which has been used to capture short-lived molecular structures. The field of ultrafast gas electron diffraction (UGED) came to life with the pioneering experiments by the groups of Zewail and Weber in the 1990s and early 2000s that achieved a temporal resolution of a few picoseconds. From on, the group of Zewail produced a series of groundbreaking electron diffraction results to capture the structures of transient molecular states with very high precision. This was made possible by numerous technological improvements that resulted in a temporal resolution of a few picoseconds combined with high spatial resolution. The group of Zewail produced a series of groundbreaking electron diffraction results to capture the structures of transient molecular states with very high precision.15–23 This was made possible by numerous technological improvements that resulted in a temporal resolution of a few picoseconds combined with high spatial resolution. Recent experiments using relativistic electron pulses with MeV energy have reached a resolution of 220 fs.32–35 In this case, relativistic effects significantly reduce the Coulomb broadening of the pulse duration and mostly eliminate the velocity mismatch. Relativistic effects significantly reduce the Coulomb broadening of the pulse duration and mostly eliminate the velocity mismatch Operating at these high energies, requires a significantly larger infrastructure compared to the table-top electron guns operating at keV energies. We describe the experimental setup, present measurements of the electron pulse duration, and demonstrate the capability to capture diffraction patterns with a short integration time
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