VUV Spectroscopy of Atoms, Molecules and Surfaces

Chapter 4
Lifetimes of molecular
negative ions
4.1 Introduction
Most of the negative ions that are unstable in the ground state exist in
metastable states associated with excited parent states of the corresponding
neutral system or, for molecules, with the ground state of the neutral
molecule. At present no clear definition of metastability exists, and I will,
following Blondel [1], regard a state as metastable if it is long-lived enough
that it can be detected along a beam-line. By this definition, states with
lifetimes as short nanoseconds may be considered metastable but the emphasis
of the present chapter is on the µs–ms range. For a state to be that
long-lived, decay via photon- and electron emission must be forbidden by the
radiative- and autodetachment selection rules, imposing for molecular negative
ions some restrictions on the electronic potential-energy curve associated
with the metastable state. First, the minimum of the curve must be attained
below the energy of a (possibly excited) electronic state of the neutral system
evaluated at the same bond length and (eventual) bend angle, i.e. the vertical
electron affinity must be positive. Second, unless the minimum of the
potential-energy curve of the negative-ion state is energetically located below
that of the neutral state (referred to as positive adiabatic electron affinity),
the minima of the potential-energy curves must be located at sufficiently different
bond lengths and/or bend angles that the detachment probability can
be considered small. This means that a barrier must be surpassed in order
for autodetachment to occur, or stated another way: the Frank-Condon overlap
between the states must be small. Third, if the potential-energy curve