# Opportunities for Two-Color Experiments in the Soft X-ray Regime at the European XFEL
## Appendix B Time-Of-Flight Photo-Electron Spectrometer  
To provide non-invasive shot-to-shot diagnostics of spectrum and intensity of each color in multi-bunch mode, the gas-based Time-Of-Flight (TOF) Photo-Electron Spectrometer (PES) [35] can be used. The PES device consists of 16 eTOF drift-tubes oriented perpendicularly with respect to the X-raybeam. A target gas is injected in the interaction region via an effusive gas-jet. The drift-tubes have been partitioned into four groups (one per quadrant) where the retardation voltages can be set individually to optimize the resolution for different photon energies. With this approach, a subset of the drift-tubes can be optimized for the first color and another subset for the second color, see Figure A4a. Figure A4b shows single pulse TOF spectra with U1 tuned to 950 eV (retardation voltage: \(-\)60 V) and U2 tuned to 910 eV (retardation voltage: \(-\)20 V). These spectra are only from detectors [E] and [W], respectively, although we have the possibility to add signal from several detectors to improve statistics. Target gas is Neon with a \(1s\) binding energy of 870.2 eV and a natural line width \(\Gamma\) of 0.24 eV.  
For these photon energies, the anisotropy parameter \(\beta\) is 2.0 and the cross-section \(\sigma\) is \(2\times 10^{-1}\) Mb. Drift-tubes at the horizontal plane are selected to take advantage of the higher yield due to the horizontal polarization of the electromagnetic field and the \(\beta\) value of the Neon \(1s\) photo-line. The first peak in the spectrum is the prompt from scattered light with a TOF of 0.45 ns, from which we can define our time zero. The peak at around 10 ns is comprised of Ne \(2s\), \(2p\), and KLL Auger lines. On detector [E], we see the U1 photo-electrons, while the U2 photo-electrons have too low kinetic energy to reach the detector with this retardation voltage. On detector [W], we see the U1 and U2 photo-electrons, but those from U1 have a high kinetic energy which leads to poorer photon-energy resolution. Figure A4c shows the spectra converted from TOF to photon-energy. This data set was collected in single-bunch mode, which enables us to compare the spectra with the grating spectrometer. Indeed, the agreement is good and to some extent, we can even resolve individual SASE spikes. To study the XFEL bandwidth, Figure A4d shows the average intensity of 500 consecutive pulses. From the grating spectra, we find, for U1 and U2, a FWHM of 10.1 eV and 16.7 eV, respectively. The PES spectra give no significant additional line broadening, thus demonstrating that it is a reliable instrument for spectral diagnostics.  
For single color operation, absolute intensity measurements are performed by the gas-based X-ray Gas Monitor (XGM) device [36; 37]. An intrinsic challenge to the two-color operation mode is that the XGM information can no longer be used. To get around this obstacle, the PES can be calibrated in terms of intensity to the XGM when FEL operates in single color mode. This way, the PES can also play the pulse energy diagnostics role during two-color experiments. Figure A5 shows a contour plot of pulse energy correlation for 500 pulses measured with XGM and PES with U2 tuned to 910 eV. For PES, we used the added intensity of the three detectors for better statistics. We found the Pearson correlation coefficient to be 0.94 from which we estimate a pulse energy error of 7%. The absolute measurement uncertainty of the XGM is 7% to 10% of the measurement value.