VUV Spectroscopy of Atoms, Molecules and Surfaces

2.3 Status on negative-ion knowledge 27
d-orbital [35]. Since then, the stability of the heavier negative ions, Sr− and
Ba− , of this group has been established, as expected on account of the larger
dipole polarizability exhibited by their neutral parent atoms [35, 36]. Although
heavier than Ca, the elements of the alkaline-earth subgroup, Zn, Cd
and Hg, are unable to bind an extra electron in the ground state [3]. Apart
from nitrogen the rest of the most common elements of the periodic table
are able to form negative ions with electron affinities in the 0.1-3.6 eV range,
the highest values being attained by the halogens [3, 37]. The instability of
nitrogen originates from its half-filled outer p3 shell with parallel spins of all
the electrons; adding an extra electron with anti-parallel spin increases the
Coulomb repulsion energy between the electrons above that of the neutral
atom, due to the Pauli exclusion principle [38].
Considering the doubly-excited states of negative atomic ions and restricting
the discussion to levels accesible by one-photon absorption, most
information is available about H− ,He−and the negative ions of the alkaliand
alkaline earth atoms. These are the most interesting from a theoretical
point of view, especially H− which has been the subject of the largest number
of experimental and theoretical studies as described in chapter 3. He− is
known to exist in a metastable 1s2s2p 4P state with a ∼350 µs lifetime, bound
with respect to the excited 1s2s 3S state 19.8 eV above the 1s2 1S ground
state of He [39]. By photon absorption from the 4P state a total of 15 4S and 4P resonances, located 22.7–24.0 eV above the He ground state and encompassing
the He(n=3,4,5) thresholds, have been investigated by the group
of Hanstorp [40, 41]. In addition, a lower-lying 1s2p2 4P shape resonance
associated with the 1s2p 3P state of He has been observed by Walter and
Peterson [42]. Most of these resonances have been predicted by theory [41],
which in addition predicts a very weak, broad 4S resonance, not amenable
to experimental observation [43]. Ar− exists in a metastable ...3p54s4p 4S state with a 260 ns lifetime, bound by 32.5 meV with respect to the ...3p54s 3 6 P state 11.6 eV above the ...3p ground state of Ar [44]. A similar state
has been predicted for Xe− but not yet experimentally observed [45].
Among the alkali negative ions Li− is particularly interesting due to the
simililarity of its electronic structure with that of H− [46]. For the lower-lying
Li− resonances the number of available decay channels is larger—causing the
resonances to become broader—than those of H− due to the non-degeneracy
of the Li(nl) thresholds implied by the non-hydrogenic Li + core [47]. For the
higher-lying Li− resonances the influence of the core is less pronounced and
the Li− spectrum becomes increasingly similar to that of H− [46]. Dellwo et
al. have studied the energy region corresponding to the 1s22p 2P excited state
of Li, 1.85 eV above the 1s22s 2S ground state of Li, giving no clear indications
of resonance structure [48]. The majority of experimental information