The spectroscopy of O2+ is well understood and has been studied at high resolution using laser photofragment apparatus [39,40,4,116,123,124,125,126,127]. It is therefore considered to be a `benchmark molecule', from which experimental parameters can be deduced. A recent paper by Ashman et al. [128] to characterise an ion cyclotron trap (ICR) apparatus used O2+ as the molecular ion of choice to characterise the temperature of the molecule inside the trap from the predissociation spectroscopy.
The electronic transition of interest for these experiments is the
.
All electronic transitions thus far observed in photofragment spectroscopy result in transitions to form
.
Rotational levels of the lower electronic state are split into four
fine structure components through the spin-orbit interaction, these
are
,
and
are labelled F1, F2, F3 and F4 respectively.
Each rotational component of the ground state is split by lambda doubling, with only negative parity levels present in the homonuclear
.
Each component is separated by approximately 50 cm-1 from the others (the exact value depending upon the vibrational and rotational states involved).
The upper (b
)
electronic state has four spin components, labelled:
,
,
and
.
Due to the zero spin of oxygen nuclei and nuclear spin statistics, only odd valued N
are present.
Transitions between the two electronic states are labelled using the following nomenclature: N
Where
is P, Q or R for values of -1, 0, and +1 respectively. The majority of lines observed in the visible region predissociation spectra of O2+ are due to to transitions from v
=3, 4 and 5 to v
=3, 4 and 5.