Anion photoelectron spectroscopy of I 2 2 and I 2 2 – Arn „ n 5 1 – 14 , 16 , 20 ... clusters

We report the mass-selected anion photoelectron spectra of I2 2 and the weakly bound clusters I2 2 •Arn (n51 – 14, 16, 20! measured at a photon energy of 4.657 eV. The experiment yields size-dependent vertical and adiabatic detachment energies for the formation of the ground state and five valence-excited states of the neutral cluster, which correspond to the (g 1 ~X!, P2u (A8), P1u ~A!, P02u (B8), P1u (B9), and P01u ~B! states of bare I2. The detachment energies are successively blue-shifted with increasing cluster size, indicating a stronger stabilization of the anionic cluster relative to the neutral counterpart. The blue shift is of similar extent for the electronically excited states A8 and A and approximately 10% less for the X state. The I2 2 and I2 2 •Ar spectra are simulated employing a Franck–Condon analysis, from which we estimate the ion vibrational temperature and determine the I2 –Ar binding energy (D055364 meV). The results are discussed with respect to possible cluster geometries and the evolution of the total and stepwise solvation energies. For I2 2 •Ar6 we present evidence that all Ar atoms are bound around the waist of the I–I bond. © 1998 American Institute of Physics. @S0021-9606~98!02935-3#