VUV Spectroscopy of Atoms, Molecules and Surfaces

66 Lifetimes of molecular negative ions
Figure 4.1: Potential-energy curves for the 4Πu and 4Σ − g states of N −
2 and the associated
3 +
Σ u and 3Πg parent states of N2 calculated by Sommerfeld and Cederbaum [36].
beam through the charge-exchange-cell, in agreement with a negative result
previously reported by Bae et al. [37]. From further investigations at the
Aarhus Tandem Accelerator an upper limit of 5×10−10 for the conversion
efficiency of N + 2 ions to N−2 ions could be estimated [38]. There, the N−2 ion
formation process was studied with a stripping technique which allowed the
masses of N − 2 and Si− to be separated. Long-lived N − 2 ions were detected
upon sputtering of TiN and BN surfaces, but could not be generated from
NaN3. Considering the differing chemical environment of N atoms bound in
the three compounds, it seems likely that the long-lived N − 2 ions are formed
by interaction between two sputtered N atoms and not from a precursor like,
e.g., N − 3 present in NaN3.
A ∼25 pA beam of N − 2 ions generated by sputtering of TiN was used
for the lifetime measurement. Since the TiN compound was expected to
be contaminated with Si from pump-oil deposits, a reference measurement
was performed on a pure Si− beam produced by sputtering of a pure Si
surface. Present in the Si− decay were a stable component and a short-lived
component fitted to 2.0±0.2 ms, with the stated uncertainty reflecting the
span of lifetimes obtained when disregarding data-points corresponding to
the first few round-trips in the ring. Since Si− is known to possess the three
stable states 4S, 2D and 2P with binding energies of 1.39 eV, 0.53 eV and
∼30 meV, respectively [39], the long-lived component must be atttributed to
the 4S and 2D states with the 2 ms component left for the 2P state. A typical
decay for N − 2 is shown on a semilogarithmic scale on figure 4.2, illustrating a