# Opportunities for Two-Color Experiments in the Soft X-ray Regime at the European XFEL
## 3 Scientific Relevance
An efficient and informative experimental method to monitor the intramolecular processes is given by high-resolution Auger spectroscopy, ideally in combination with ion spectroscopy performed in a coincidence arrangement. The 3\(d\) Auger spectrum of iodine is located in the kinetic energy range around 350-450 eV. Similar to the 3\(d\) hole relaxation in atomic Xe [19] and molecular CH\({}_{3}\)I [20], the most prominent Auger lines in iodine are also assigned to transitions to doubly charged states with electron configurations 4_d\({}^{-2}\)_ and 4_d\({}^{-1}\)4_p\({}^{-1}\)_. These lines are well separated from the corresponding Cl 2\(p\) Auger spectrum (_L\({}_{23}VV\)_) at kinetic energies between 165 and 175 eV [21] and other ionization processes taking place at the photon energies considered here. An illustration of all possible ionization processes, including also the valence ionization of all atoms at both wavelengths is given in Figure 4.  
Due to the high intensity of the FEL pulses sequential ionization processes are possible and likely to happen. As a consequence, the electron spectrum of the neutral parent molecule (depicted in Figure 4) will be overlaid with emission lines arising from the ionization of the ionic species and of the dissociation fragments. In order to separate the emission from different species coincidences between electrons and ionic fragments can be used for a more detailed analysis. In fact, coincidence experiments  
Figure 3: Schematic representation of the two-color pump–probe process in molecular IC\({}_{n}\)H\({}_{2n}\)Cl. The pump pulse with photon energy h\(v_{1}\) (630 eV) is ionizing selectively the 3\(d\) shell in the iodine atom. The subsequent electronic relaxation and charge migration is monitored with the probe pulse h\(v_{2}\) (250 eV) of variable temporal delay \(\Delta\)T between both pulses via selective ionization in the 2\(p\) shell of the chlorine atom and the corresponding Auger decay. Typical pulse durations for pump and probe pulses are in the order of 2 fs. will be one of the major experimental tools available at the SQS instrument, and are feasible due to the high number of X-ray pulses (up to 27,000 per second) at the European XFEL. This high repetition rate allows one to record data of high statistics for coincident measurements between electrons and ionic fragments coming unambiguously from the same molecule. In a typical experimental scenario, first the Auger spectra would be recorded at the individual wavelength 250 eV and 630 eV, respectively, in order to obtain the one-photon reference spectra. In addition, electron-ion coincidence will provide charge and fragment resolved electron spectra would at these photon energies. In a second step, the Cl 2\(p\) Auger spectrum--caused by the 250 eV photon pulse--will be monitored in the presence of the additional the 630 eV pulse. When the 630 eV pulse comes after the 250 eV pulse, the spectrum will be unchanged compared to the single color spectrum. When both pulses are overlapping or the 630 eV comes earlier, the observation of the chlorine Auger spectrum for different delays between both pulses provide the information about the intermolecular processes. Changes of the kinetic energy position and of the intensity distribution within the Cl 2\(p\) Auger spectrum are the monitor to follow charge migration processes inside the molecule to determine, for example, the time required to transmit the information about the creation of a 3\(d\) core hole on the iodine site to the chlorine atom. For small molecules this time scale is in the order of a few femtoseconds [22], so probably difficult to access with pulses of about 2 fs duration each. For longer carbon chains the time scale is expected to increase to about 10 fs or more and therefore well suited to be studied with the set-up at the SQS instrument. Furthermore, by selecting in coincidence mode a fragment containing the iodine atom or the iodine atom itself, also information on charge transfer processes is made available.