Ultrafast energy and charge transfer dynamics in molecular systems at surfaces
Charge injection and intramolecular excitation denote two important processes triggered by optical excitation at a molecule-electrode interface. An intramolecular excitation can decay via charge transfer to the metal substrate as well as by energy transfer upon electron-hole pair creation in the substrate.
Time-resolved two-photon photoemission (2PPE) spectroscopy will be applied to directly monitor the energetics and population dynamics of the excited states giving also a handle to energy conversion into conformational changes or dissipation to the molecular environment. We expect deeper insight into the competition of energy and charge transfer to intramolecular energy conversion, a key problem in molecular electronics and organic solar cells.
We will study dyes and molecular switches like azobenzene coadsorbed to monolayers of solvent molecules. Fig. 3 illustrates electron solvation in amorphous ice and its spectral signatures detected in two-photon photoemission. Solvent molecules not only screen the excited molecule from the substrate but also can promote charge localization at the solvated molecule upon injection of photo-excited electrons. This mechanism has been demonstrated for solvated electrons in ice monolayers reacting with adsorbates.
The goal of this project is to unravel the electron dynamics upon excitation of photochromic molecules at surfaces in order to separate excitonic couplings and nuclear motion and to clarify the influence of interfaces and solvent environments on energy transfer into the reaction coordinates (e.g. isomerization).
Complementary studies of energy pathways involved in photochemical reactions and isomerization will be performed for the same molecules in liquids with static and time-resolved RIXS (P2). The studies of inter-conversion dynamics of free conformers (P1) and of the transient structure of photochromic molecular switches in the gas phase (P4) and in liquids (P2) will serve as a solid basis to our investigations of these photo-chromophores at surfaces. This will provide a comprehensive picture of isomerization dynamics for different degrees of coupling to the environment; Methodological synergy is expected with P9.