The C60:C60+ ratio in diffuse and translucent interstellar clouds
  Context. Insight into the conditions that drive the physics and chemistry in
interstellar clouds is gained from determining the abundance and charge state
of their components. Aims. We propose an evaluation of the C60:C60+ ratio in
diffuse and translucent interstellar clouds that exploits electronic absorption
bands so as not to rely on ambiguous IR emission measurements. Methods. The
ratio is determined by analyzing archival spectra and literature data.
Information on the cation population is obtained from published characteristics
of the main diffuse interstellar bands attributed to C60+ and absorption cross
sections already reported for the vibronic bands of the cation. The population
of neutral molecules is described in terms of upper limit because the relevant
vibronic bands of C60 are not brought out by observations. We revise the
oscillator strengths reported for C60 and measure the spectrum of the molecule
isolated in Ne ice to complete them. Results. We scale down the oscillator
strengths for absorption bands of C60 and find an upper limit of approximately
1.3 for the C60:C60+ ratio. Conclusions. We conclude that the fraction of
neutral molecules in the buckminsterfullerene population of diffuse and
translucent interstellar clouds may be notable despite the non-detection of the
expected vibronic bands. More certainty will require improved laboratory data
and observations.
