For studying the gas phase photoionization dynamics in molecules which are solid at room temperature, oven sources are commonly employed. Upon heating, however, the molecules can take different conformations and be excited in many different vibrational and rotational states. Therefore, it is advantageous to pick up the hot gas phase molecules by a supersonic cold jet of a rare gas, e.g. He or Ar. Collisions between the molecules and the rare gas atoms lead to an efficient cooling and the jet transports the molecules into the interaction region with the ionizing light. Two oven source designs were adapted to the setup of the ELYCHE high harmonic generation (HHG) beamline. One embedded the molecular powder in a graphite matrix for reducing the risk of self-condensation, in the second one the molecules were heated directly. The oven source was coupled to a pulsed solenoid valve, synchronized with the laser, through which the supersonic jet of the carrier gas was produced. A data acquisition program for recording time-of-flight mass spectra was furthermore developed and both oven source designs were tested successfully yielding similar results. They proved to be perfectly suited for transferring solid intermediate-sized molecules intact into the gas phase. It was taken care that the experiment can be switched easily between electron detection, helpful for time delay calibration, and ion detection and a highly versatile setup for gas phase attosecond spectroscopy was therefore built.
While attosecond experiments offer naturally an unprecedented time-resolution, the bandwidth of ultrafast pulses is intrinsically large. That is why in CHARISMA stationary high-resolution synchrotron experiments were performed to gain insights into the photoionization and fragmentation of potential target molecules. Threshold photoelectron-photoion coincidence (TPEPICO) experiments were performed on the VUVbeamline of the Swiss Light Source. Here, for each photoionization event producing a photoelectron the corresponding ion fragment can be assigned. By scanning the photon energy, high-resolution threshold photoelectron spectra revealing the ionization energies of different electronic states can be recorded, and the appearance energies of fragment ions can be determined state-selectively. This helps understanding which state contributes to the formation of an ionic fragment and if it is suited as an observable in an ultrafast experiment. Due to their similar structure to aromatic amino acids and proteins, respectively, the photoionization of the three cresol (hydroxytoluene) isomers and N-methyl acetamide were investigated. High quality experimental data were obtained, and the threshold photoelectron spectra and breakdown curves were interpreted with the help of quantum chemistry and statistical rate theory. A publication on these data will follow soon after the end of the project.
It has been shown previously that charge migration in a molecule can be tracked by ultrafast oscillations when recording the yield of a certain ionic fragment as a function of the pump-probe delay. In CHARISMA the new oven source and the knowledge on the steady state synchrotron photoionization of the potential target molecules were the pillars for conducting such experiments. The basis, however, is a well characterized attosecond light source. This characterization was done by performing angle-resolved attosecond spectroscopy in rare gases employing RABBITT (reconstruction of attosecond beatings by two-photon transitions) and quantum beat spectroscopy. These experiments served on one hand for benchmarking the beamline but introduced some original aspects due to the angular resolution, too. The rare gas experiments revealed several issues with the performance of the laser, which were solved during the project. This led to some delay in the work plan, but the performance could ultimately be improved significantly by re-designing some key elements of the beamline. The results for the rare gas experiments were finally of good quality and a publication will follow. The focus was then shifted to molecules and promising preliminary results were obtained for several target species. However, the Covid-19 lockdown in Italy reduced the effective experimental time of the CHARISMA project significantly and impacted the stability of the laser system, which reacted very sensible to such an unusually long downtime. The goal of CHARISMA to resolve charge migration in molecules has not yet been achieved. The researcher will continue to reach those in a future collaboration with the host and the improved experimental setup will guarantee high-quality results.
