# Atomic, Molecular and Cluster Science with the Reaction Microscope Endstation at FLASH2
## 3 Selected Results
### Molecular Dynamics in Ar Dimers  
The pump-probe technique combined with fragment ion momentum imaging allows to track the evolution of intermediate states through observables in the final state as a function of the pump-probe delay [6]. The key observables to track molecular dynamics in REMI experiments are the yield of a specific coincidence channel and the kinetic energy release (KER) of this channel. The KER is the sum of the nuclear kinetic energies, which are generated from potential energy. While the system evolves along the potential energy curve of the intermediate state it accumulates the kinetic energy \(KER_{nl}(t_{d})\), until it is probed at the time \(t_{d}\) and at the internuclear distance \(R(t_{d})\). Then it accumulates further energy on the final state potential \(KER_{f}(t_{d})\) and we observe the sum \(KER(t_{d})=KER_{nl}(t_{d})+KER_{f}(t_{d})\) (see Figure 5). The capabilities to observe molecular dynamics with this setup is demonstrated exemplarily in an experiment investigating charge transfer (CT) in argon dimers using an XUV-pump IR-probe scheme [5]. The experiment was performed at FLASH1, using the same reaction microscope apparatus, but a different IR pump laser [40]. Argon dimers were ionized and excited to Ar\({}^{2+-}\)Ar states by absorption of three 27 eV photons. These states are still weakly bound, but the equilibrium nuclear distance is much smaller. So the nuclei start to move towards each other. At smaller internuclear distances, the potential energy curve crosses unbound states of the Ar\({}^{+-}\)Ar\({}^{+}\) type which are coupled nonadiabatically so charge transfer can happen there (see Figure 5). The system is subsequently probed by an intense 800 nm laser pulse [40] to the Ar\({}^{2+-}\)Ar\({}^{+}\) final state. This transition is only possible if charge transfer has taken place. Therefore the yield of this particular channel gives information on the transition probability of the charge transfer. In this process we actually have two intermediate states. The first can only contribute negligible kinetic energy as the potential is shallow. Thus the KER is accumulated mainly on the second intermediate state and the final state. The value of interest is the time constant when the system transfers its charge and thus changes from the first to the second intermediate. Restricting the KER to values corresponding to the distance \(R_{X}\), where crossings appear, allows to select those events that were probed directly after the charge is transferred (see Figure 6). With this restriction, the probe pulse directly tracks the transition rate depending on the time delay. This shows how the measurement of KERs combined with ion yields  
Figure 4: Two-dimensional kinetic energy spectrum of photoelectrons originating from the same laser pulse in two-photon double ionization (TPDI) in argon. The clustering at low \(E_{1}\) and low \(E_{2}\) are the signature of the sequential double ionization (SDI) channel. The blob on the diagonal is caused by false coincidences. Reproduced and modified from [30]. for varying pump-probe delays, allows to deduce the transition rates of the process of interest as a function of time. Figure 5: Potential energy curves of the argon dimer in the energy range 56 eV–68.5 eV. The XUV pump (purple arrow) populates the initial Ar\({}^{2+}(3\,\mathrm{s}^{-1}\,\mathrm{3}\,\mathrm{p}^{-1})-\)Ar state (black curve) at \(R_{\mathrm{eq}}\). The wave packet starts to evolve to smaller \(R\). At the crossings (orange-shaded area) with Ar\({}^{+}(3\,\mathrm{p}^{-2}\,\mathrm{nl})-\)Ar\({}^{+}(3\,\mathrm{p}^{-1})\) states (light-green curves), charge transfer (CT) happens. The IR probe (red arrows) interrogates the population of the Ar\({}^{+}(3\,\mathrm{p}^{-2}\,\mathrm{nl})-\)Ar\({}^{+}(3\,\mathrm{p}^{-1})\) states by ionization to final Ar\({}^{2+}(3\,\mathrm{p}^{-2})-\)Ar\({}^{+}(3\,\mathrm{p}^{-1})\) states (dark-grey curves). Specific potential energies and corresponding internuclear distances are indicated by horizontal and vertical dashed lines, respectively.