# Opportunities for Two-Color Experiments in the Soft X-ray Regime at the European XFEL
## 3 Scientific Relevance  
The two-color operation mode described above offers numerous advantages in various scientific applications utilizing the pump-probe scheme. The examples presented in the following illustrate how nuclear, electron or combined dynamics in small quantum systems can be investigated using time-resolved two-color spectroscopy; they are not intended to represent a comprehensive scientific case. In the first example, the two-color capability enables site selectivity. Pump and probe pulses are independently tuned to particular well-defined photon energies, so that the X-ray pulses interact with the system at different atomic sites. This can be achieved most clearly by resonant absorption, which in the case of the present setup would be the photoexcitation from an atomic core level to unoccupied orbitals. Excitation energies of the strongly bound 1s core electrons of various relevant elements are found in the wavelength range covered by the SASE3 undulator of the European XFEL, for example C 1s around 290 eV, N 1s around 400 eV, C 1s around 540 eV, F 1s around 690 eV and S 1s around 2.4 keV, to name only a few. Tuning the photon energy of the pump to a resonant transition allows one not only to excite specific elements in the system, but also populate desired unoccupied states, such as the \(\pi^{*}\) orbitals in many organic compounds. Site-selectivity can also be obtained by non-resonant photoionization, by tuning the pump pulse energy just above the binding energy of a specific core electron whereby that partial ionization cross-section becomes the dominant one in the system. Tuning now the photon energy of the probe pulse to another core excitation or above another core ionization threshold, the effects induced by the pump pulse are observed at a different location in the system. Varying the delay between the pulses allows one to follow in time the propagation of the effects, such as changes in the charge density or in the molecular geometry. Figure 2: Solutions to obtain zero-crossing in the temporal delay by implementing an optical delay line (**a**) or a fresh-slice technique (**b**). One category of compounds where site selectivity can easily be obtained is organic molecules containing elements other than H, C, N, or O--most commonly (in biomolecules) S and P, but also halogenated or metal-containing compounds. Halogenated linear hydrocarbons would represent a basic model system where the electronic charge propagation and dynamics can be investigated. For example, in a halogenated polyethane (PE) IC\({}_{n}\)H\({}_{2n}\)Cl, composed of a carbon chain terminated by iodine and chlorine (Figure 3), the pump can be tuned to \(\approx\)630 eV to most efficiently create 3\(d\) core holes at the iodine site. At this energy, the photoionization cross-section at the iodine site exceeds that at the chlorine more than ten times. Tuning the probe pulse to \(\approx\)250 eV shifts the site-selectivity to the chlorine atom as now the Cl 2\(p\) cross-section is over four times higher than the 4\(d\) cross-section at the iodine site. Depending on the charge propagation during pump-probe delay, the local charge at the chlorine site starts to change, which will be manifested as changes in the Cl electron binding energies. Observing the electron emission in the energy ranges specifically relevant to the chlorine atom adds further site-selectivity to the probe. In addition, at longer timescales molecular fragmentation proceeds and the probe would be able to explore chlorine-containing fragments. Absorption of the probe photons by these fragments can have a different effect than absorption by the still-intact parent molecule, such as more pronounced secondary fragmentation. Thus, a two-color time-dependent study of the fragmentation pattern can yield information not only on the charge migration but also on the dynamics of the spread of radiation damage.