Photoinduced Dynamics in a Quantum Fluid Environment

Nanometer sized superfluid helium droplets provide an ideal approach to synthesize and investigate exotic molecules or clusters at temperatures close to absolute zero. Electronic excitation and ionization of particles inside the droplets by the absorption of photons trigger various dynamical processes. In this project single atoms located inside helium droplets will be used to investigate these processes on femto- (10-15 s) and picosecond (10-12 s) time scales. The questions to be addressed are: 1) Which dynamical processes are initiated by photoexcitation and what are their time scales? 2) How does electronic relaxation after photoexcitation proceed? 3) Are molecules consisting of excited atoms and helium atoms formed? 4) What is the influence of the quantum fluid environment on the generated photoelectrons and photoions? To answer these questions, femtosecond laser pulses will be used at the Institute of Experimental Physics of Graz University of Technology. A first pulse triggers the dynamics and a second pulse probes the system after a controlled time delay. The detection of photoelectrons and photoions as function of the time-delay between the two pulses will provide information about the dynamics of the system. The project enables the investigation of dynamics after photoexcitation and -ionization in more complex molecular aggregates solvated in a superfluid environment. In particular, the applicability of helium nanodroplets for the investigation of photoinduced processes in molecules of technological or biological relevance will be explored.

A detailed understanding of light-matter interaction is key for the conversion of solar energy into other forms of energy, a topic of enormous social relevance due to the required transition from fossil fuels to renewable energy sources. For investigations of single molecular building blocks in the sense of a bottom-up approach, superfluid helium nanodroplets represent a particularly promising approach. The small diameter of a few nanometers and the low temperature of less than one Kelvin above absolute zero of these droplets offer unique environmental conditions to generate and study nanostructured molecular aggregates in a controlled environment. In this project, individual atoms and molecules were isolated inside helium droplets and studied for the first time using femtosecond laser spectroscopy. The femtosecond technique allows to study processes on femto- (10-15 s) and picosecond (10-12 s) time scales, a time resolution that is required due to the small dimensions and low masses of molecular systems. Within this project, we successfully observed for the first time the primary processes triggered by photoexcitation of the atoms and molecules embedded in the droplets. By combining time-resolved photoelectron detection and theoretical modeling using density functional theory, we were able to describe both the characteristic time scales, as well as the motions and energy flow within these systems in great detail. This mechanistic understanding provides a fundamental basis for future studies of more complex systems. As a first demonstration in this direction, it has also been possible to measure the vibrations of diatomic molecules inside droplets. Unexpectedly, we discovered that the influence of superfluid helium on the nuclear motions of solute molecules can be orders of magnitude smaller than in ordinary solvents. This extraordinary property of He droplets may enable future investigations of aggregates of large photoactive systems in view of energy and charge carrier transport.