A pulsed electron gun for ultrafast electron diffraction at surfaces

Abstract

The construction of a pulsed electron gun for ultrafast reflection high-energy electron diffraction experiments at surfaces is reported. Special emphasis is placed on the characterization of the electron source: a photocathode, consisting of a 10 nm thin Au film deposited onto a sapphire substrate. Electron pulses are generated by the illumination of the film with ultraviolet laser pulses of femtosecond duration. The photoelectrons are emitted homogeneously across the photocathode with an energy distribution of 0.1 eV width. After leaving the Au film, the electrons are accelerated to kinetic energies of up to 15 keV. Focusing is accomplished by an electrostatic lens. The temporal resolution of the experiment is determined by the probing time of the electrons traveling across the surface which is about 30 ps. However, the duration of the electron pulses can be reduced to less than 6 ps.

Ultrashort electron pulses for diffraction, crystallography and microscopy: theoretical and experimental resolutions

“…18 This approach is taken in our instruments of UED and UEC, resulting in an estimated kinetic energy spread of o0.3 eV; in ref. 18, the experimental DE i under similar conditions (for a gold photocathode) was measured to be 0.1 eV.…”

“…For single-electron UEM, it is clear that two effects have been suppressed by this development, namely the severe influence of space-charge not only on the temporal (longitudinal) broadening, but also on the spatial (lateral) dispersion, which introduces divergence and, hence, a loss in image resolution. The kinetic energy distribution, when using a cold photocathode, is limited by the excess photon energy available above the work function, which can be tuned to be less than 0.1 eV, 18 resulting in a femtosecond pulse duration. The spatial resolution achieved in single-electron pulsed mode has reached the atomic scale in UEM2 with features resolved below 3 Å , 30 which is comparable to the resolution capabilities of conventional electron microscopes.…”

Optical fiber-based photocathode

“…Ultrafast electron diffraction (UED) is a highly successful technique for structural dynamics investigation of phase transitions, electron-phonon coupling, and chemical reactions. [1][2][3][4][5][6][7][8][9] Metal photocathodes are currently the most commonly employed electron sources due to their robustness, ultrafast temporal response, and compatibility with high electric fields. They provide sub-picosecond electron pulses with coherence lengths up to a few nanometers.…”

Development of an Ultrafast Low-Energy Electron Diffraction Setup

“…However, many of the proposed ideas make compromises in either low signal intensity [142] or grazing incidence geometry [143], which ultimately complicates the quantitative analysis, as well as limits the achievable pulse duration.…”