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   COLLOQUIUM OF THE LABORATORY FOR COMPUTER DESIGN OF MATERIALS
                   School of Computational Sciences
                           (CSI 898-Sec 001)
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                   NEGATIVE IONS WITH DIFFUSE EXCESS ELECTRONS
                                 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 anio
ns, 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.
                      
                        Monday , April 4, 2005
                                  4:30 pm
               Room 206, Science & Tech. I, Fairfax Campus

Refreshments will be served at 4:15 PM.
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Find the schedule at http://www.scs.gmu.edu/lcdm/seminar/schedule.html
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