VUV Spectroscopy of Atoms, Molecules and Surfaces

24 Negative ions
ing the binding energy of 0.75 eV [8]. H− is also speculated to play a role
in the mechanism of formation of molecular hydrogen in the interior of the
interstellar medium [9]. Due to the absence of a dipole moment in the ground
state, H2 cannot simply be formed by the radiative association of two groundstate
H atoms; another type of particle is needed to carry away the excess
energy required for the binding [10, 11]. This particle could, for example, be
an electron emitted in a H− –H collision and it has recently been proposed to
look for signatures of H− in the VUV part of the absorption spectra of interstellar
clouds [9]. Also heavier atomic- and molecular negative ions have been
considered to play a role in the chemistry of the stars [12] and the interstellar
medium [13]. Recently, absorption spectra of small, linear carbon-chain
anions obtained by laboratory laser-spectroscopic measurements were found
to compare remarkably well with astrophysically observed diffuse interstellar
bands [14]. Negative molecular ions are, in addition, abundant 20–120 km
above the surface of the Earth in the form of O − 2 ,OH− ,ClO− ,O − 3 ,NO−2 ,
O − 4 ,NO− 3 ,CO− 3
etc. [15] and must be included in theoretical models of the
atmosphere [16, 17]. In the laboratory, negative ions are the key ingredients
in accelerator mass spectroscopy which has proven a very sensitive technique
in terms of detection efficiency and mass resolution for, e.g., carbon 14dating
[18] and investigations of rare- or heavy negative ions [19]. Negative ions are
also abundant in plasmas and discharges [20] where they play a role in laser
action and semiconductor device processing [21].
2.2 Negative-ion structure
From the above it is hopefully evident that negative ions are worth further
investigations, but one could start by asking whether the binding of an electron
to a neutral atom or -molecule is at all possible. Indeed, the structure
of a negative ion is very different from that of a neutral system, especially
for small atoms where the ratio of negative- to positive charge is large and
electron-correlation effects therefore more pronounced. The binding of an
extra electron to the ground state of a neutral atom is accomplished by it
inducing to the atom an electric dipole moment which acts back on it as
an attractive 1/r 4 short-range potential [20, 22]. In a classical picture the
neutral atom (or molecule) feels an electric field � E ∝ �r/r 3 from the electron
approaching a distance r away. This induces to the atom a dipole moment
�P ∝ α � E, α being the dipole polarizability, which gives rise to the attractive
Φ ∝ � P ·�r/2r 3 ∝ 1/r 4 potential [23]. Unlike the long-range Coulomb potential
acting on an electron of a neutral atom, the short-range potential implies a finite
number of bound states. In general, only a single bound state exists, but
the single-electron continuum may reveal a rich structure of doubly-excited