  The effect of thermal shape fluctuations on the recurrent fluorescence of
boron cluster cations, B$_N^+$ ($N=9-14$), has been investigated numerically,
with a special emphasis on B$_{13}^+$. For this cluster, the electronic
structures of the ground state and the four lowest electronically excited
states were calculated using time-dependent density functional theory, and
sampled on molecular dynamics trajectories of the cluster calculated at an
experimentally relevant excitation energy. The sampled optical transition
matrix elements for B$_{13}^+$ allowed to construct its emission spectrum from
the thermally populated electronically excited states. The spectrum was found
to be broad, reaching down to at least 0.85 eV. This contrasts strongly with
the static picture, where the lowest electronic transition happens at 2.3 eV.
The low-lying electronic excitations produce a strong increase in the rates of
recurrent fluorescence, calculated to peak at $4.6 \times 10^{4}$ s$^{-1}$,
with a time-average of $8 \times 10^{3}$ s$^{-1}$. The average value is one
order of magnitude higher than the static result, approaching the measured
radiation rate. Similar results were found for the other cluster sizes.
Furthermore, the radiationless crossing between the ground-state and the first
electronic excited state surfaces of B$_{13}^+$ was calculated, found to be
very fast compared to experimental time scales, justifying the thermal
population assumption.
