Quantum effects in the threshold photoionization and energetics of the benzene-H2O and benzene-D2O complexes:: Experiment and simulation

The present study combines both experiment and molecular dynamics simulations in order to document the ionization behavior of the C6H6-H2O and C6H6-D2O complexes close to the ionization threshold, in particular its nonadiabatic character. Using the two-color two-photon resonant ionization laser technique, the ionization thresholds of these species have been measured together with the threshold for dissociative ionization. A binding energy has been deduced for the neutral species: D-0(C6H6-H2O) = 106 +/- 4 meV and D-0(C6H6-D2O) = 116 +/- 5 meV, which significantly increases the precision compared to literature. Using a semiempirical potential model, the minimum energy structures of the neutral and ionic species have been determined, and the potential energy surfaces have been analyzed using a two-dimensional approach. As a result, the formation of a stable C6H6(+)-H2O complex close to the threshold is found to be controlled by a pure quantum effect and is ascribed to the classically forbidden region of the neutral ground state wave function for the intermolecular vibrational motion. Using classical molecular dynamics simulations in order to sample this region, it has been shown that the neutral conformations involved in the production of stable ions at the ionization threshold exhibit a strong geometry change compared to the neutral equilibrium conformation; i.e., the water molecule is strongly shifted off the benzene C-6 axis and is also flipped over backward the benzene ring. The difference in the ionization energy of the C6H6-H2O and C6H6-D2O complexes, which cannot be explained by the difference in the neutral binding energies alone, supports this result.