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)

The anion zero electron kinetic energy (ZEKE) spectra of the van der W aals clusters Ar2-3Br- and Ar2-3I- have been measured, and partially d iscriminated threshold photodetachment (PDTP) experiments have been pe rformed on Ar4-9Br- and Ar8-19I-. The experiments yield size-dependent adiabatic electron affinities (EAs) and electronic state splittings o f 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 th e neutral and anion pair potentials determined from previous work on t he diatomic rare gas-halide atom complexes [Y. Zhao, I. Yourshaw, G. R eiser, C. C. Arnold, and D. M. Neumark, J. Chem. Phys. 101, 6538 (1994 )]. A simple first-order degenerate perturbation theory model [W. G. L awrence and V. A. Apkarian, J. Chem. Phys. 101, 1820 (1994)] of the ne utral cluster potentials was found to agree well with the size-depende nt splitting of the halogen P-2(3/2) state observed in the ZEKE spectr a. However, the binding energies calculated from the pair potentials a lone were found to be inconsistent with the experimental electron affi nities, and it was necessary to include various nonadditive terms in t he simulated annealing calculations to obtain reasonable agreement wit h experiment. Many-body induction in the anion clusters was found to b e 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 b ound clusters. (C) 1996 American Institute of Physics.