Many-body effects in weakly bound anion and neutral clusters: Zero electron kinetic energy spectroscopy and threshold photodetachment spectroscopy of ArnBr− (n=2–9) and ArnI− (n=2–19)

Abstract

The anion zero electron kinetic energy (ZEKE) spectra of the van der Waals clusters Ar2-3Br− and Ar2-3I− have been measured, and partially discriminated threshold photodetachment (PDTP) experiments have been performed on Ar4-9Br− and Ar8-19I−. The experiments yield size-dependent adiabatic electron affinities (EAs) and electronic state splittings of the halogen atom in the neutral clusters formed by photodetachment. These results are compared with simulated annealing calculations using model potentials for the anion and neutral clusters, making use of the neutral and anion pair potentials determined from previous work on the diatomic rare gas–halide atom complexes [Y. Zhao, I. Yourshaw, G. Reiser, C. C. Arnold, and D. M. Neumark, J. Chem. Phys. 101, 6538 (1994)]. A simple first-order degenerate perturbation theory model [W. G. Lawrence and V. A. Apkarian, J. Chem. Phys. 101, 1820 (1994)] of the neutral cluster potentials was found to agree well with the size-dependent splitting of the halogen 2P3/2 state observed in the ZEKE spectra. However, the binding energies calculated from the pair potentials alone were found to be inconsistent with the experimental electron affinities, and it was necessary to include various nonadditive terms in the simulated annealing calculations to obtain reasonable agreement with experiment. Many-body induction in the anion clusters was found to be the dominant nonadditive effect. The exchange quadrupole effect—i.e., the interaction of the exchange induced electron charge distribution distortion among argon atoms with the halide charge—was also found to be important. This comparison between experiment and theory provides a sensitive probe of the importance of nonadditive effects in weakly bound clusters.