2021A&A...656A.100R

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 limits 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.