Electron gun for diffraction experiments off controlled molecules

Abstract

A dc electron gun, generating picosecond pulses with up to 8 × 10 6 electrons per pulse, was developed. Its applicability for future time-resolved-diffraction experiments on state-and conformer-selected laser-aligned or oriented gaseous samples was characterized. The focusing electrodes were arranged in a velocity-map imaging spectrometer configuration. This allowed to directly measure the spatial and velocity distributions of the electron pulses emitted from the cathode. The coherence length and pulse duration of the electron beam were characterized by these measurements combined with electron trajectory simulations. Electron diffraction data off a thin aluminum foil illustrated the coherence and resolution of the electron-gun setup.Diffractive imaging is a promising approach to unravel the microscopic details of chemical processes through the recording of so-called molecular movies in the gasphase, which trace the structural dynamics of individual molecules and nano-particles at the atomic level. Electron and x-ray diffraction are well established tools to investigate the structures of solid state samples [1], for example in transmission electron microscopy [2] or xray crystallography [3,4]. Furthermore, electron diffraction has found broad application for gas-phase structuredetermination in chemistry [5]. Recent developments have mainly focused on realizing time-resolved experiments in order to study structural dynamics, where xray and electron diffraction served as complementary approaches [6][7][8][9][10][11].To be able to record structural changes during ultrafast molecular processes of small complex molecules in the gas phase, signals from many identical molecules have to be averaged. Gas-phase investigations pose the challenge that the sample might comprise different isomers and sizes [12]. In addition, the molecules in the gas phase are typically randomly oriented. It is therefore important to provide samples, which are as clean and defined as possible, to allow for experimental averaging over multiple electron pulses. Clean molecular samples can be generated by their spatial separation according to shape [13][14][15] and size [16]. Controlling the spatial orientation of the molecules leads to an enhancement of the information that can be retrieved from a diffraction pattern, as proposed theoretically [17][18][19][20] and demonstrated experimentally for x-ray [21,22] as well as for electron diffraction [23,24]. Strong alignment or orientation is generally necessary [11,20] for three-dimensional structure reconstruction [24] and can be provided in cold supersonic molecular beams by strong-field laser alignment and mixed-field orientation [25][26][27][28][29]. The low density of these controlled gas-phase samples requires sources of large-cross-section particles or photons with large brightness, while still ensuring atomic resolution. Electron sources can meet these requirements even in table-top setups.The first sources for creating electron pulses short enough to study ultrafast processes in molecules o...

Terahertz Acceleration Technology Towards Compact Light Sources

“…To calibrate and optimize the spectrometer field configuration for both SMI and VMI, a fixed potential of -6 kV was applied to both the repeller plate and the sample holder while the ground plate was grounded. While scanning the extractor voltage from -5.8 kV to -4.3 kV, we observed the focusing of the electron bunch depending on the extractor voltage [285]. This behavior is explored based on the RMS of the electron bunch size in the x and y directions on the detector.…”

X-ray diffractive imaging of controlled gas-phase molecules: Toward imaging of dynamics in the molecular frame

“…The simulations of the diffraction pattern of 2,5diiodothiophene were carried out using the CMIdiffract code, which was developed within the CMI group to simulate the diffraction of x-rays or electrons of gas-phase molecules based on the independent atom model. 12,14,15,[28][29][30] The structure of 2,5-diiodothiophene, which was used to calculate the diffraction pattern, was calculated with GAMESS-US 31 at the MP2/6-311G** level of theory. Parameters such as molecular beam density, molecular beam width, and the degree of alignment were extracted from the experiment 22 and appropriately considered in the simulations, as were geometric con- strains such as the distance from the interaction zone to the CSPAD camera, the size of the detector, photon energy, and number of photons.…”

“…[6][7][8] Time resolved diffraction studies of small gas-phase molecules in the picosecond range were first employed by electron diffraction at the beginning of the 21st century 9,10 and have been used ever since with laboratory-based electron sources. 11,12 Recently, much higher time resolution of ∼100 fs was achieved by an accelerator-facility based relativistic electron gun. 13 The development of ultrashort and intense hard x-ray laser pulses generated by x-ray freeelectron lasers (XFELs) has also provided the possibility to image structure as well as structural changes of small gas-phase molecules via x-ray diffraction [14][15][16] on ultrafast (femtosecond) timescales.…”

Improved spatial separation of neutral molecules

“…17 Additionally, the deflector enables the separation of polar molecules from the seed gas, which is relevant, for instance, for gas-phase diffraction experiments. [18][19][20][21] Alternatively, eigenstates of small polar neutral molecules can be separated using switched electric or magnetic fields. 12,[22][23][24][25][26][27][28] Large molecular ions were separated according to their shape using ion-mobility techniques.…”