## 3 Experimental Results
### Data Acquisition  
The successful implementation of the partial covariance analysis technique, discussed above, requires that the different single-shot observables can be reliably recorded in a synchronized fashion. In order to achieve this, a timestamp server was set up. After receiving the trigger from the laser master clock, the acquisition programs signal their readiness to the server. If all programs signal within a set timeout, the server responds with a timestamp. Otherwise, it orders the programs to cancel the acquisition and to wait for the next trigger pulse. Typically, about 5% of the shots are discarded by the server. To fully evaluate the performance of the instrument, the acquisition of four synchronized signals is required:  
* Ion time-of-flight trace
* Ion velocity map image
* Electron velocity map image
* XUV intensity from photodiode  
The ion TOF trace was obtained by decoupling the current from the back of the ion MCP and converting it into a voltage signal. The XUV photodiode generates a current, which is converted to a voltage signal by means of a transimpedance amplifier. These analog ion TOF and XUV photodiode voltage traces are subsequently converted into digital signals using a two-channel, 1-GHz sampling rate, analog-to-digital converter (Agilent Acquiris DP1400), which acquires 20,000 samples per laser shot in each of the channels, with a dynamic range of eight bits. The ion and electron VMI data are recorded with two one-megapixel Allied Vision Technology Pike F-145B cameras that have an 8 bit dynamic range and a maximum frame rate of 30 Hz. In terms of storage rate, this amounts to more than 2 MB of data per shot, or 20 MB/s. As a benchmark molecule, we used N\({}_{2}\). Figure 6 shows simultaneously-acquired data from N\({}_{2}\), acquired using voltage ratios corresponding to Operation Mode 2(b), i.e., optimized for electron and ion VMI, but with absolute voltages adapted for a maximum electron kinetic energy of 30 eV. The data were recorded for 25,000 laser shots, corresponding to just above 40 minutes of acquisition time and amounting to \(\sim\)50 GB of data. The molecules were ionized using the harmonics 13-29 generated in argon, corresponding to photon energies between 20 and 45 eV, as displayed in Figure 5. The typical on-target pulse energy was \(\sim\)5 nJ, which corresponds to approximately \(10^{9}\) photons per pulse over the whole bandwidth and \(10^{8}\) photons per pulse and harmonic. The count rates were on the order of 500 particles detected per shot for the ions, as well as the electrons. Considering that the maximum available photon energy is just above the double ionization threshold of N\({}_{2}\) (42.88 eV [50]), a number of electronic states in N\({}_{2}\) with ionization potential (IP) in the range can be accessed: X\({}^{2}\Sigma_{S}^{+}\), A\({}^{2}\Pi_{u}\), B\({}^{2}\Sigma_{u}^{+}\), C\({}^{2}\Sigma_{u}^{+}\), F\({}^{2}\Sigma_{S}^{+}\) and H\({}^{2}\Sigma_{S}^{+}\). The average ion mass spectrum in Figure 6d exhibits two main features, which were assigned to the N\({}^{+}\) and N\({}_{2}^{+}\) channels. While the former also contains a contribution coming from stable N\({}_{2}^{2+}\) ions, only photons from the 29 th harmonic or higher can contribute, and considering the low intensity at these photon energies (Figure 5), the contribution from this channel is expected to be negligible. The mass resolution was estimated to \(m/\Delta m=60\) for a mass of 28 u. Figure 6b shows the photoion momentum distribution of all the ionic fragments that were generated in the interaction region, averaged over 25,000 shots. The anisotropy of the distribution, peaked in the vertical direction, is due to the fact that the cross-section for excitation to the dissociative states is higher for molecules aligned along the laser polarization direction. The faint horizontal ripple pattern in the images is a result of ions scattering on the mesh in the repeller electrode. In Figure 6e, the corresponding photoion kinetic energy is displayed.