## 2 Apparatus
### Simulations
#### 2.3.2 High Resolution Electron Modes  
For the high resolution electron modes, electrons and ions are detected on their respective sides of the spectrometer. To maximize the possible total voltage difference over the spectrometer, the voltage on the electron side flight tube (and thus, the front of the electron MCP) was set to \(V_{\rm F}^{\rm e}=5\) kV, so that the phosphor screen could still be operated at its full voltage bias of 5 kV relative to the front of the MCP without exceeding the limit of the electrical feedthroughs. The repeller voltage was set to \(V_{\rm R}=-9.2\) kV to allow for detection of electrons with kinetic energies up to 90 eV, while allowing for extraction and imaging of ions with kinetic energies up to 10 eV. Figure 2: Simulation results for the high resolution ion modes. (**a**) shows the resulting mass resolution at mass \(m=100\) u as a function of extractor voltage, \(V_{\rm F}^{\rm e}\), for ions with zero initial kinetic energy (black) and ions with an initial velocity of \(v_{0}=1000\) m/s in the direction of the molecular beam (blue); (**b**) shows the resulting VMI energy resolution for three different extractor voltages (\(V_{\rm E}^{\rm e}\)) using the 27-particle model (blue, green and orange lines) and the Monte Carlo model for the optimum setting (black line); (**c,d**) show the simulated detector image and the 3D momentum distribution after inversion, for the optimum voltages; (**e**) shows the kinetic energy spectrum calculated from the 3D momentum distribution. Figure 3a shows the resulting energy resolution for electron VMI for three different settings for the extractor voltage, \(V_{\text{E}}^{\text{e}}\), calculated using the 27-particle model (blue, green and orange lines). The optimum extractor voltage, for which the best resolution was achieved for a photoelectron energy of 30 eV, was found to be \(V_{\text{E}}^{\text{e}}=-5.775\) kV (\(\eta=0.76\)), and for this setting, the resulting energy resolution from the Monte Carlo simulation is shown (black line), indicating an energy resolution better than 2% for photoelectron energies between 30 and 85 eV. In Figure 3b,c, the corresponding simulated detector image and the 3D momentum distribution after inversion are shown, respectively. Figure 3d shows the photoelectron energy spectrum calculated from the 3D momentum distribution. Figure 3: Simulation results for high resolution electron VMI mode. (**a**) shows the resulting energy resolution for three different extractor voltages (\(V_{\text{E}}^{\text{e}}\)) using the 27-particle model (blue, green and orange lines) and the Monte Carlo model for the optimum setting (black line); (**b**,**c**) show the simulated detector image and the 3D momentum distribution after inversion, for the optimum voltages; (**d**) shows the photoelectron energy spectrum calculated from the 3D momentum distribution. With the electron side conditions optimized for electron VMI and the voltage on the ion side flight tube set to \(V_{\rm F}^{\rm i}=-10\) kV to detect ions with kinetic energies up to 10 eV, the ion side can be tuned either for TOF (Operation Mode 2(a)) or VMI (Operation Mode 2(b)) by varying the ion extractor electrode voltage, \(V_{\rm E}^{\rm i}\). Figure 4a shows the resulting mass resolution in Operation Mode 2(a) for ions with an initial velocity of 1000 m/s in the direction of the molecular beam, for different extractor voltages, \(V_{\rm E}^{\rm i}\). It is clear that it is not possible to reach Wiley-McLaren conditions within the voltage limits of the setup, due to the initial drift region between the interaction point and the mesh through which the ions have to travel. In addition, the best mass resolution of \(m/\Delta m=110\), achieved for \(V_{\rm E}^{\rm i}=-10\) kV, is a factor of 18 worse than that achieved in Operation Mode 1(a).