# Spatial and Momentum Mapping Modes for Velocity Map Imaging Spectrometer
## 2 Apparatus  
Since the spectrometer was detailed in ref. [11], we provide only a summary here. Figure 1 shows a diagram of the spectrometer. It consists of five chambers: (i) the source chamber, (ii) the first differential chamber, (iii) the second differential chamber, (iv) the main chamber, and (v) the jet dump chamber. A piezo valve (Amsterdam Cantilever ACPV2, Amsterdam, The Netherlands) [24] with a pinhole of 100 \(\upmu\)m diameter is installed to produce a supersonic molecular beam in the source chamber. The piezo valve backing pressure is set at 1.0 bar. After passing the 1 st and the 2 nd differential chambers equipped with 1 mm diameter holes, the molecular beam enters the main chamber of the VMI spectrometer, where it encounters either the X-ray photon or femtosecond laser. The diameter of the gas jet is estimated as 1.2 mm according to the mechanical structure. A double-layered \(\mu\)-metal shield is installed to reduce the residual magnetic field at the interaction region to 8 mGs. Particles from the interaction region are focused by the lens system onto the two vacuum imaging detectors (VID275) from Photek, East Sussex, UK.  
Each detector consists of two multi-channel plates (MCPs) with an active diameter of 75 mm and a P46 phosphor screen. Two Andor Zyla 4.2 Plus cameras from Oxford Instruments, Oxford, UK (2048 \(\times\) 2048) are used to capture images of ions and electrons, respectively.  
The lens system consists of 27 electrodes with voltages that can be adjusted individually from outside the vacuum chamber. A work fixture is used to assemble all electrodes into a whole set so that the possible misalignment is below 0.1 mm. The detailed geometry of the lens system can be found in [PERSON] et al. [11]. By adjusting electrode voltages individually, we can add more working modes without changing the VMI lens structure, and  
Figure 1: The overall structure of the velocity map imaging spectrometer. The supersonic molecular beam is generated in the source chamber, passes the first and second differential chambers, interacts with the photon beam in the main chamber, and finally is pumped out in the dump chamber. The generated electrons and ions are guided to the detectors on each end by the VMI lens in the main chamber.  
different working modes can be easily switched back and forth using a computer-controlled high-voltage supply. Unfortunately, because the center the ion detector is worn out, we have to use the electron arm to demonstrate all the operation modes. As shown in Figure 2, the electron arm consists of 15 electrodes. Although it is already known that meshes can degrade the \((p_{x},p_{y})\) image due to the microlensing effect [2], we installed two ultrafine stainless-steel meshes at the repeller electrode and the ninth extractor electrode for three reasons: keep an effusive gas jet using a hollow needle, focus three-dimensional \((p_{x},p_{y},p_{z})\) for the future upgrade, and run both the electron arm and the ion arm simultaneously. The mesh has a density of 180 lines per inch and a transparency of 80% to effectively alleviate the microlensing effect.