Novel THz-streak Experiment for Time Resolved Measurement of Ultrafast Inner- and Interatomic Coulombic Decays

Abstract

In the present work, time-resolved measurement techniques with resolutions in the femtosecond range were used to understand electronic excitations better and thus the formation and breaking of chemical bonds. Electrical excitations occur on a time scale in the attosecond to femtosecond range, and therefore ultra-short pulses in the extreme ultraviolet (XUV) range are needed to excite these processes. Furthermore, pulses in the terahertz range are used for the time resolution, as the pulse duration and slope of the electric field match the required temporal resolution. Within the scope of this work, a THz streak camera was set up to investigate decay times of an excited system in the femtosecond range. The functionality of the setup was investigated by measuring Xenon Auger electrons. The lifetime known from measurements of the spectral width of the Auger peaks was used as a reference. In the experiment, the XUV pulses used to ionize the atom or molecule are superimposed with THz pulses. The time offset between the XUV and THz pulses is varied and the change in kinetic energy of the photoelectrons and Auger electrons is measured. Since the Auger electrons are emitted at a different phase of the THz field, there is a time offset between the two curves. This time offset provides information about the lifetime of the process.

In addition, a setup was designed to measure the Interatomic Coulomb Decay (ICD) in neon dimers in the next step. ICD is a process that occurs in many weakly bound systems. In this process, the excitation energy is passed on to a neighboring atom or molecule, so it is strongly dependent on the internuclear distance between the two atoms or molecules. In contrast to the purely atomic Auger decays, the lifetime of these ICD processes can not be determined from spectral measurements of the line width. Therefore, it is of particular interest to investigate these ICD processes with time-resolved measurement methods, such as a THz streak camera. With the help of the measurements performed with the Auger electrons, the temporal resolution of the experimental setup was determined in order to be able to derive predictions about the feasibility of the ICD measurements. For ICD processes with a significantly longer lifetime than in typical Auger decays, the analysis shows that the experimental requirements are fulfilled. Thus, time-resolved measurements of ICD processes are possible.