THE NATURE AND EVOLUTION OF EXCESS ELECTRON BINDING IN CLUSTER ANIONS STUDIED VIA NEGATIVE ION PHOTOELECTRON SPECTROSCOPY. Jay H. Hendricks, H. L. deClercq, S. A. Lyapustina and K. H. Bowen, Johns Hopkins University, Baltimore MD, USA. (NIST address: Building 221, Room B312, NIST, Gaithersburg, MD 20899, 301-975-4371, email:


Clusters have a tremendous impact on nature and chemistry as they constitute the transition region between gas-phase and solid or liquid phases. The physical and energetic properties of clusters are intermediate and often unique between those of an individual clustering sub-unit and those of the bulk phase. Clusters, therefore, serve as ideal probes of the microscopic nature of the condensed phase. The technique of negative ion photoelectron spectroscopy (NIPES) is used to study a variety of cluster anion systems. The goal is to obtain a more complete understanding of the nature of excess electron binding as the cluster anions grow in size. The attachment of an excess electron to a neutral cluster enables it to be transported and mass-selected using ion-optical techniques. Photoelectron spectroscopy of the mass-selected ion beam allows the cluster energetics to be monitored as a function of cluster size. Recently, a new class of anions referred to as dipole-bound anions in which the excess electron is bound primarily by the dipole moment of the system have been observed. To insure that the excess electron s binding is due to its interaction with the dipolar field of the system, the cluster species under study are composed of molecular components which do not themselves form conventional (valence) anions. When the composite dipole moment of the resultant cluster is greater than 2.5 D, binding of the excess electron occurs. Here, direct spectroscopic evidence showing the existence of stable, ground state, dipole-bound anions is presented for the systems of uracil, thymine, HF dimer, water dimer, and ethylene glycol dimer dipole-bound anions. The photoelectron spectra of the larger sized mixed cluster systems of hydrogen chloride-water, and hydrogen cyanide-water show a transition from dipole-bound states to solvated electron states.