Ion Pair States
While
the
description
of a diatomic molecule as a pair of ions seems appropriate for ionic
compounds
(such as NaCl) we would expect this to be a very poor description for
covalent
compounds, such as S2. Nevertheless a pair of
ions
will experience an attractive force, but the attraction will not be
enough
to overcome the energy required to prepare the ions. This leads to an
excited
electronic state of a covalently bound molecule that is best described
as a pair of ions. The diagram shows the potential energy curves
appropriate
for molecules like S2, where the ground state is
S=S,
but there exists an excited electronic state that is well described by
S+S–.
Such
states are likely to be important in understanding the properties of
plasmas
and can give guidance to modeling the properties of ionic crystals.
Ion
pair states usually have bond lengths much longer than the ground
state,
so special techniques are required to reach these states. I use double
resonance spectroscopy for this purpose (using two lasers to excite two
transitions sequentially) and have
applied it to S2. I have also used a
related method on ClO.