DOC - UPCON - Ultrafast Photochemistry in a Confined Quantum Solvent

The field of molecular engineering aims to develop novel macroscopic systems by manipulating the quantum properties of their individual building blocks on a molecular level. This approach enables the targeted development of future materials for application in a wide range of fields, from energy production to health care. Microscopy and spectroscopic techniques are available for the single molecule, molecular assembly and the macroscopic stage of the molecular engineering development chain. However, dynamical properties at the intermediate stage (molecular assembly), which are only accessible via femtochemistry methods, are an ongoing challenge. The goal of the proposed research project is to develop a novel method for ultrafast studies of molecular assemblies by demonstrating the first ultrafast photochemistry experiments in a confined quantum solvent, which represents a vital step in the advancement of molecular engineering. Superfluid helium nanodroplets will be used as the confining quantum solvent. By consecutive pickup of dopant atoms and molecules, the droplets allow the construction of arbitrary molecular assemblies such as core-shell complexes or a chromophore and its solvent environment. The droplets also possess excellent spectroscopic properties such as transparency to photon energies up 19.8 eV and cooling of dopants to their electronic / vibrational ground state. Before larger molecular assemblies can be designed and analysed inside the droplets, a thorough characterization of the interaction between the superfluid and chromophore molecules is required. To this end, the dynamics of simple dimer molecules as model systems (aluminium dimers and molecular iodine), which are well understood in gas phase spectroscopy, will be studied inside the superfluid. The experiments will reveal the quantum fluid influence on coherent nuclear motion, as well as on electronic states and their couplings. The low degree of complexity of these model systems makes high level quantum chemistry calculations tractable. As a first experiment on photochemistry in a confined quantum solvent, additional species like Ar or H2O will be combined with the dimer-superfluid system. The effect on the chromophore dynamics will be studied by continuously varying the amount of additional solvent atoms and molecules. The experimental methods which are used in this project are helium droplet beam generation and doping, ultrafast pump-probe photoelectron spectroscopy as well as time-resolved coincidence / covariance spectroscopy. Advanced detection techniques such as velocity map imaging (VMI) and coulomb explosion imaging (CEI) will provide insight into the nuclear dynamics and fragmentation behaviour. In order to interpret the experimental data, the research project will be supported by in-house experts on quantum chemistry calculations. Helium density functional theory (HeDFT) will be applied to determine shifts in excitation and ionization energies caused by the chromophore-superfluid interaction (bubble expansion). High level ab-initio quantum chemistry calculations will be used to model the motion of excited state wavepackets and predict time-resolved photoelectron spectra.