Knowledge Bank

We have carried out a detailed study of mixed clusters of benzene and water formed in a supersonic expansion. These clusters are intriguing because they offer a microscopic view of the interactions between two compounds which form immisible liquid solutions. The structure and dynamics of these clusters are probed using resonance-enhanced multiphoton ionization coupled with time-of-light mass spectroscopy. Fluorescence lifetimes for the complexes are also recorded where possible. One of the unique features of these complexes is their extremely efficient fragmentation upon photoionization which has led past studies to misassign the spectra. We have determined geometries for the $C6H6-H2O(1:1)$ and $C6H6-(H2O)2(1:2)$ complexes using a combination of rotational band contour analysis and vibronic selection rules. The 1:1 complex has a structure in which the oxygen atom of the water molecule is above the plane of the benzene ring near the six-fold axis. It is configured in a hydrogen-bonding geometry with the benzene cloud. The current bestlit geometry of the 1:2 complex retains the 1:1 geometry but places a second $H2O$ molecule at a position where it can hydrogen bond with the first $H2O$ molecule much as it would in the free $H2O$ dimer[Figure] The hydrogen bonding geometries are responsible for the efficient fragmentation of the ion since upon photoionization of the benzene molecule, the hydrogen-bonded H2O molecule finds itself in a geometry in which its positive H atom is pointing in toward the positively charged benzene cation. The Franck-Condon factors to the ionization continuum are thus strongly weighted toward energies well above the adiabatic ionization threshold for the complex, where fragmentation is energetically feasible. The absorption features for the 1:3 and 1:4 complexes are remarkably similar to the 1:2 complex both in appearance and in frequency shift relative to free benzene. We conjecture that these higher-order complexes are in fact mimcing an immiscible liquid in which the additional water molecules are clumping together on one side of the ring rather than solvating the benzene ring.