Ultrafast Extreme Ultraviolet Laser Science

This research project aims at the first application of extreme ultraviolet (EUV) femtosecond (fs) spectroscopy to doped superfluid helium nanodroplets (HeN). Experiments, which will investigate processes with fs time resolution in an ultarcold (0.4 K) and superfluid environment, are planned to be accomplished at the Institute of Experimental Physics of Graz University of Technology. In order to acquire expertise in the sophisticated fields of ultrafast laser physics and high photon-energy spectroscopy by means of high harmonic generation (HHG), the applicant plans to spend a one-year research period at the Stanford University PULSE Institute (headed by Philip Bucksbaum), in the group of Markus Gühr. The collaboration is aiming at novel time-resolved investigations of photo-induced reaction dynamics. Femtosecond UV-pump/EUV-probe techniques will be used to investigate non-Born-Oppenheimer and catalytical processes in organometallic molecules, both in gas phase and isolated in a quantum cryogenic matrix. The novelty lies in the use of core electrons instead of valence electrons to study molecular dynamics. The research program in Stanford will provide the applicant with the necessary know-how in order to apply fs-EUV spectroscopy to cold aggregates and clusters produced and isolated in superfluid HeN. Upon his return to Graz the applicant will establish a new ultrafast laser system with HHG. He plans to build his habilitation at TUG on this new research branch. Continuing collaboration with Stanford will provide exchange of knowledge. Femtosecond laser spectroscopy is a powerful technique to study real-time dynamics in atoms, molecules, and clusters, as well as the dynamics of chemical reactions. The possibility of HHG expands the excitation wavelength to the vacuum UV and EUV and thus provides direct access to much deeper electron levels. These inner valence and core electrons essentially expand the experimenter`s scope because they often contain structural, magnetic, or chemical information, which is supplemental to that obtained from valence electron spectroscopy. HeN are attracting significant interest from many scientific communities due to their unique properties. Acting as a cold, superfluid bath, providing confinement for single particle isolation, and very easy and versatile doping feasibilities make them ideal and least perturbing hosts for spectroscopic investigations. Helium nanodroplet isolation spectroscopy has therefore hugely expanded within the last two decades. Although both techniques have been exploited extensively, no fs-HHG experiment has been performed with doped HeN up to now. This project thus aims at a substantial expansion of spectroscopic techniques for dynamic investigations at ultralow temperatures.

Summary for public relations work During his Erwin-Schrödinger Fellowship at the SLAC/Stanford University PULSE Institute in the group of Dr. Markus Gühr, the fellow constructed and built a novel spectrometer utilizing ultrashort extreme ultraviolet laser pulses and used it for time-resolved molecular dynamics measurements.The interaction of light with molecules and solids is of central interest for many biological and technological problems. For example, the transfer of light energy into chemical energy is of importance in vision and light harvesting. Selective and ultrafast transfer of light energy into heat is involved in the photoprotection of DNA. Despite their importance many of these processes are still not completely understood, in part because they take place on picosecond (10-12 s) and femtosecond (10-15 s) time scales and are thus not fully accessible with most conventional spectroscopic methods. With the invention of femtosecond laser systems this timescale has become accessible for direct investigations. Recent progress in the generation of high-order harmonics from femtosecond laser pulses as well as the development of free electron lasers has extended the available wavelength range from the visible and ultraviolet into the extreme ultraviolet and soft X-ray range. Especially high-order harmonic generation (HHG) provides the opportunity to perform time-resolved experiments with photon energies ranging from 10 to 100 electron volt in a conventional laboratory environment.During his stay the fellow acquired expertise in the fields of ultrafast laser physics and high photon-energy spectroscopy by means of HHG. He designed and constructed a photoelectron and photoion spectrometer utilizing ultrafast extreme ultraviolet probe pulses and performed femtosecond time-resolved experiments. The novelty of this setup is the application of metal filters for the spectral isolation of a single harmonic from the HHG spectrum. This is a very simple and cost efficient approach that allows a versatile combination of pump- and probe wavelength. Time-resolved pump-probe experiments with the polycyclic aromatic hydrocarbon perylene demonstrated the excellent spectral and temporal resolution of the system. Furthermore, molecular excited state dynamics after photoexcitation with ultraviolet light could be observed. The results of these new measurements are currently being interpreted in a close cooperation of the fellow with host institution and will lead to at least two publications.In addition to this laboratory based technique the fellow participated in two user beamtimes at the free electron laser LCLS (Linear Coherent Light Source, Stanford Linear Accelerator), as well as in two user beamtimes at the synchrotron ALS (Advanced Light Source, Lawrence Berkeley National Laboratory). It is expected that several publications will result from these projects, however, the data analysis is very complex and typically takes more than one year.After his return to the Graz University of Technology a new ultrafast laser laboratory will be established based on the experience the fellow acquired during his stay.