The VMI end-station of ELI ALPS makes it possible to obtain energy- and angle resolved information on ions and electrons resulting from the photoionization or photofragmentation of atoms, molecules or nanoparticles during the interaction with short laser or extreme ultraviolet (XUV) pulses. Exploiting the symmetry of the setup and interaction, the VMI-ES allows simple reconstruction of the three-dimensional momentum distribution from two-dimensional images through inversion algorithms. In the VMI-ES the interaction volume is separated from the gas source by a differential pumping section, allowing for ultra-high vacuum in the detection chamber and high signal-to-noise ratio acquisition (see Figure 1).
Figure 1. (a) Technical drawing of the VMI-ES showing the main vacuum chambers. (b) The VMI-ES during measurement on using HR GHHG Gas beamline as light source.
The VMI-ES has two VMI detector assemblies allowing the detection of electrons and ions in two different energy regimes, as detailed in Table 1. The VMI-ES can be used in combination with several light sources of ELI-ALPS with vertical polarization.
Table 1. Measured parameters of VMI-ES.
Representative data obtained during the factory acceptance tests of the VMI-ES by above-threshold ionization (ATI) of xenon with a 1 kHz infrared laser source demonstrating the spectral resolution can be seen in Figure 2 and Figure 3.
Figure 2. Raw image of the xenon ATI photoelectron momentum map measured with the low-energy VMI.
Figure 3. As per Figure 2, a) the photoelectron spectra obtained after inversion. b) A Gaussian peak fitting the spectral region bounded by the red horizontal lines. The sigma values of the Gaussian distributions correspond to a FWHM of 1.13 eV, which corresponds to a ΔE/E of 2.3% at 50 eV.