Book of Abstracts Book of Abstracts - Universität Konstanz

6
Diffuse Electron States
Kit H. Bowen, Jr.
Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218, USA
The extra electron in valence (conventional) anions resides in an orbital which is firmly
grounded on the nuclear framework of the system, i.e., the extra electron is localized. Likewise,
when such anions are solvated and become anion-molecule complexes, the extra electron
usually remains localized with the anion. During photodetachment of the excess electron from
such a cluster anion, this sub-anion acts as the chromophore, and the photoelectron spectrum
reflects the characteristics of that anion, albeit stabilized and spectrally shifted to higher electron
binding energy.
In other cluster anions, however, the excess electrons are more delocalized, even though
they remain closely associated with their nuclear frameworks. There is, however, still another
class of molecular and cluster anions in which the excess electrons are neither localized nor
closely associated with their nuclear frameworks, and these are anions with highly diffuse,
excess electron distributions. Examples include dipole bound anions, quadrupole bound anions,
double Rydberg anions, and some sizes of water cluster anions. While these species share the
characteristic of having spatially diffuse excess electrons, they are otherwise bound by different,
albeit idealized interactions. The excess electron in dipole bound anions is chiefly bound by the
dipolar field of the corresponding neutral system, with the excess electron ballooning out into
space at the positive end of the molecular or cluster system. Similarly, the excess electron in
quadrupole bound anions is primarily bound by the quadrupolar field of the corresponding
neutral system. Double Rydberg anions are the negative ions of Rydberg radicals. Each one can
be thought of as a closed shell, cationic core which is enveloped by two Rydberg-like, highly
diffuse electrons. Also, many water cluster anion systems appear to exhibit diffuse excess
electron states of at least two types, with a third type being associated with internalizing
electron states.
In this talk, we will survey these different types of diffuse electron states, but we will focus
on double Rydberg anions, in part because they are the least well-known diffuse, excess electron
species. In particular, we will discuss the ammonia-based double Rydberg anions, (NnH3n+1) - ,
where n = 1- 7. Using anion photoelectron spectroscopy, we have measured electron affinities
of these species, found examples of solvated double Rydberg anions, and accessed the first
electronic states of several neutral Rydberg radicals by photodetaching electrons from their
corresponding double Rydberg anions. Insight into the nature of these species was further aided
by comparisons with the absorption measurements of Fuke on neutral Rydberg cluster-like
radicals and by several theoretical studies, especially those of Ortiz.