Femtosecond photochemistry in a quantum solvent

Nanometer-sized helium droplets provide a unique and fascinating approach to synthesize and investigate novel molecules, molecular aggregates and clusters under very controlled conditions at temperatures close to absolute zero. Spectroscopic investigations of particles inside helium droplets with continuous-wave lasers are an established technique to learn about their static properties. In contrast, methods using ultrashort laser pulses, which allow for real-time studies of dynamical processes, are currently under development. In the previous project (P 29369) we were able to demonstrate that ultrafast dynamics of a single atom located inside a helium droplet can be observed with femtosecond (10-15 s) temporal resolution by using time-resolved photoelectron and ion spectroscopy. In this project we will increase the complexity by investigating metal dimers (In2 , Al2 , ) inside helium droplets, which additionally exhibit intramolecular dynamics due to their vibrational degree of freedom. In particular, we will characterize the influence of the superfluid helium environment on vibrational dynamics (wave packets) and investigate how this influence depends on experimental parameters, such as the excitation energy or molecular mass. We aim at a mechanistic model describing the influence of the quantum fluid helium on the nuclear motion of molecules, which will provide a basis for the design and interpretation of future experiments with novel systems inside helium droplets. The project will be carried out at the Ultrafast Laser Laboratories at the TU Graz Institute of Experimental Physics, in collaboration with researchers from University of Barcelona, Spain, and VU Amsterdam, Netherlands.

A comprehensive understanding of fundamental light-matter interaction processes is key for the development of photonic applications such as solar energy conversion or photo catalysis. Since the primary photophysical and photochemical processes typically proceed on femto- and picosecond timescales, their investigation requires time-resolved femtosecond laser spectroscopy. For investigations of single molecular building blocks and small atomic/molecular aggregates in the sense of a bottom-up approach, superfluid helium nanodroplets represent a particularly promising approach. Their small diameter of a few nanometers and the low temperature of less than one Kelvin above absolute offer unique conditions to generate and study nanostructured molecular aggregates in a controlled environment. This project builds on first demonstrations of femtosecond spectroscopy inside He droplets, and develops photoelectrons and in particular -ions as reliable observables for photoinduced molecular dynamics in the droplet interior. While photoelectrons provide insight into the electronic structure of molecules, photo-ions inform about their nuclear structure. It could be demonstrated that correlated electron-ion detection and ion/electron imaging, which are standard techniques in gas-phase experiments, are also applicable to He droplets. These techniques provide deeper insight into photoinduced dynamics in molecules embedded inside them. With these developed detection capabilities, various nuclear dynamics of small molecules and atomic aggregates were investigated. For example, the peculiar solvation properties of Mg atoms inside He droplets, leading to the formation of a metastable "foam" configuration with 1 nm interatomic distance, were exploited to investigate cluster formation. During the 1 ps bond-formation phase, we discover a novel energy upconversion process: Energy pooling collisions of photoexcited Mg atoms, populating highly excited states. Being a rare observation of bond-formation in real time, this study highlights the potential of He droplets for exploring the dynamics of novel processes. With the example of photodissociation of single iodine molecules (I2) inside He droplets, it could be demonstrated that Coulomb explosion imaging provides insight into the nuclear dynamics of photochemical processes. This study opens the door for investigations of intermolecular proton and hydrogen transfer processes. In conclusion, this project well demonstrates the capabilities of He droplets as nano-cryo-reactors for investigation of energy and charge transport processes in molecular aggregates, which are not accessible otherwise.