VUV Spectroscopy of Atoms, Molecules and Surfaces

4.5 CO − 2
the lowest-lying CO − 2 state is probably not reached. Electron capture by
CO + 2 (2Πg) ions will populate highly excited states of CO2, and subsequent
electron capture may likely lead to the formation of CO − 2 in an excited state
that happens to be long-lived. Starting out from the doublet CO + 2 state, the
long-lived CO − 2 state must be bound to a singlet or triplet state of CO2 and
therefore either be a doublet or quartet state. Whether these states of CO2
are bent or linear are open questions, since the charge-exchange
and CO − 2
process leaves a scope of at least 2×4.33 eV for their energies. Besides,
the mapping of electronic excited states of CO2 is far from being complete
with only the lowest excited singlet and triplet state energies being calculated
by Spielfiedel et al. [52]. They have considered the 1,3Σ− u , 1,3Σ + u , 1,3∆u and 1,3Πg states associated with the 1σ2 g1σ2 u2σ2 g2σ2 u3σ2 g3σ2 u4σ2 g1π4 u1π3 g2π1 u and
...1π4 u1π3 g5σ1 g linear electronic configurations, obtained by excitation of a
1πg electron from the CO2( 1Σ + g ) ground state of electronic configuration
. According to the Walsh rules, these linear configurations corre-
...1π4 u1π4 g
late with bent 1,3A2 and 1,3B2 states that either raise or lower the energy
with respect to the corresponding linear states [51]. If such a bent state turns
out to be the energetically most favourable, it is pushed 4–5 eV down with
respect to the corresponding linear state [52] and it is thus very difficult to
predict the order of states without performing detailed calculations. A complete
picture of the variation of equilibrium bend angle with excitation energy
can therefore not yet be given. All that can be argued is that between the
linear CO2 ground state and the bent CO + 2 ground state there is a low-lying
band of bent states and a high-lying band of linear states.
The present experimental result has stimulated theoretical investigations
of the lowest-lying excited states of CO − 2 and their excited CO2 parent states.
The quartet states 4A2 and 4B2 of CO − 2 associated with the lowest triplet
states 3B2 and 3A2 of CO2, turned out to be rather short-lived [53] and
further investigations therefore concentrated on higher-lying quartet states
and on doublet states. The lowest-lying excited 2A2 state of the isoelectronic
NO2 molecule is known to be metastable, decaying radiatively to the 2A1 ground state with a lifetime of ∼44 µs [54,55]. NO2is bent in the 2A1 ground state with an equilibrium angle of 133◦ —similar to the lowest-lying
CO − 2 state—while the 2A2 state is associated with a 110◦ geometry [56, 57].
By analogy, the observed metastable state of CO − 2 may be a 2A2 state of
strongly bent geometry decaying radiatively on a millisecond timescale to
the CO − 2 (2A1) state, followed by autodetachment to the CO2( 1Σ + g ) ground
state with a lifetime of ∼90 µs.
The possibility of an excited metastable state of CO + 2 surviving to the
charge-exchange cell and giving rise to a CO − 2 state which is not a doublet
or a quartet cannot be ruled out. In fact, an excited state of CO + 2 located
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