# Atomic, Molecular and Cluster Science with the Reaction Microscope Endstation at FLASH2
## 3 Selected Results  
### Angle-Resolved Wavelength Scan: XUV+IR in Helium  
This experiment makes use of the wavelength tunability of FLASH2 and the 4\(\pi\) angular acceptance of the reaction microscope. As will be shown in the following, it demonstrates a beneficial combination of the FEL and the IR laser.  
In this pump-probe measurement the XUV light of the FEL was used to excite helium, while a subsequent IR-laser pulse ionizes the excited atom. Scanning the XUV photon energy from 20.4 eV to just below the ionization threshold of 24.6 eV reveals the excitation transition series \(1s^{2}\to 1 snp\) in helium [31], in accordance with the dipole selection rules for linearily polarized light. The excited helium atom absorbs multiple photons (each 1.55 eV) of the subsequent IR pulse which bring the electron to the continuum. This process is shown in Figure 3a, where an increased photoelectron yield is observed at XUV energies that matches the specific excitation energy (\(1s2p\) at 21.2 eV, \(1s3p\) at 23 eV...). The distribution of the photoelectron kinetic energy is plotted along the ordinate and shows several peaks at specific XUV excitation energies. The underlying process is the above threshold ionization (ATI) and describes the behavior of an electron absorbing more photons than the minimal requirement to be emitted into the continuum [32; 33]. The additional electron energy is directly related to the IR-photon energy (in this case \(\approx 1.55\) eV).  
While in Figure 3a only the absolute electron kinetic energy is plotted, the REMI allows to measure the electron momentum in 3D as shown in Figure 3b. The XUV is polarized along the \(p_{x}\) direction and  
Figure 3: (**a**) XUV photon energy scan with delayed IR-laser pulse, plotted versus photoelectron kinetic energy. Resonances in helium are revealed for specific XUV energies and above threshold ionization (ATI) can be seen by the distribution of the photoelectron kinetic energy. (**b**) Electron momentum distribution at the specific region indicated by the black square. The XUV is polarized along \(p_{x}\), the IR along \(p_{z}\).  
the IR laser along the \(p_{z}\) direction. Figure 3b contains exclusively electrons recorded with parameters in the marked region of Figure 3a. First an XUV photon excites the helium atom into the \(1s4p\) state (\(23.7\,\mathrm{eV}\)) and upon the additional absorption of one IR photon, the 4p-electron is emitted into the continuum. Starting from the helium groundstate, the absorption of two photons can yield to an angular momentum of \(l=2\) for the photoelectron, with the corresponding characteristic number of nodes in the angular distribution. The two orthogonal linearly polarized light fields can lead to the population of angular momentum substates with \(m=0,\pm 2\). This can be understood as in a spherical basis each light field can be described by a superposition of left- and right-handed circularly polarized light, driving \(\Delta m=\pm 1\) transitions. For a detailed explanation see [34].  
This experiment demonstrates the capability to precisely measure electron angular distributions which is essential to investigate many different phenomena like quantum beating [35], electron correlation [36] or propensity rules [37].