Photoionisation-induced switch in aromatic molecule-solvent recognition

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This project aims at the infrared (IR) spectroscopic and quantum chemical characterization of aromatic ion-ligand complexes isolated in the gas phase, in order to determine the potential energy surface describing the involved intermolecular ion-ligand interaction in the electronic ground state. Target molecules are cations of fundamental benzene derivatives of biophysical relevance, which are microsolvated by a controlled number of neutral solvent molecules. In a collaborative effort, two unique but complementary experimental strategies are combined to characterize the intermolecular potential by generating the clusters with two different techniques in a molecular beam. The first approach employs electron impact ionisation for cluster generation, which enables the characterization of the global minima on the potential. In a second approach, the cation clusters are generated by resonant photoionisation (REMPI) of the neutral precursor. As the interaction potential of the neutral and ionic clusters are qualitatively different (due to the additional charge), higher-lying local minima on the potential of the cation ground state can be populated by REMPI and characterized by IR spectroscopy. Depending on the excess energy involved in the ionisation process and the magnitude of the barrier for isomerisation, the local minima will isomerise toward the global minima. The dynamics of this ionisation-induced isomerisation process will directly be measured by time-resolved IR spectroscopy. The shape of the intermolecular potential and the dynamics of this reorganization process will be investigated as a function of the substitution of functional groups, the excess energy involved in ionisation, and the type and degree of microsolvation.