DIPOLE-BOUND EXCITED-STATES OF THE I-CENTER-DOT-CH3CN AND I-CENTER-DOT(CH3CN)(2) ION-MOLECULE COMPLEXES - EVIDENCE FOR ASYMMETRIC SOLVATION

Dipole-bound excited states are reported for the I- . CH3CN and I- .(C H3CN)(2) cluster ions, located just below their vertical electron deta chment energies (determined using negative ion photoelectron spectrosc opy). The absorption cross sections for excitation to these states are observed to increase with increasing dipole moments of the solvent mo lecules in the I- . M series (M=methyl iodide, acetone, acetonitrile). Photoexcitation at the peak of the transition to the dipole-bound sta te results exclusively in the dipole-bound fragment ion, M(-). The pho toelectron spectrum of the (CHCN-)-C-3 fragment was also recorded by s equential two-photon absorption in the I- . CH3CN parent, indicating t hat the excess electron is indeed weakly bound (less than or equal to 10 meV) with very little intramolecular distortion evident upon electr on detachment. The I- .(CH3CN)(2) cluster displays two absorption band s, one below each of the two features in the photoelectron spectrum. T he most intense band correlates with the weaker, lower binding energy photoelectron band. Excitation of this strong absorption band results in production of both CH3CN- and (CH3CN)(2)(-) fragments, while excita tion of the weaker absorption band only results in electron detachment . We interpret these results in the context of two structural isomers for I- .(CH3CN)(2): one with the solvent molecules surrounding the I- and another with both solvent molecules on the same side, asymmetrical ly solvating I- in a configuration with a large electric dipole moment in the neutral cluster over this anionic geometry. (C) 1995 American Institute of Physics.