C₆₀^+ - looking for the bucky-ball in interstellar space

The laboratory gas phase spectrum recently published by Campbell et al. has reinvigorated attempts to confirm the presence of the C$_{60}^+$ cation in the interstellar medium, thorough an analysis of the spectra of hot, reddened stars. This search is hindered by at least two issues that need to be addressed: (i) the wavelength range of interest is severely polluted by strong water- vapour lines coming from the Earth's atmosphere; (ii) one of the major bands attributed to C$_{60}^+$, at 9633 \AA, is blended with the stellar Mg{\sc ii} line, which is susceptible to non-local-thermodynamic equilibrium effects in hot stellar atmospheres. Both these issues are here carefully considered here for the first time, based on high-resolution and high signal-to-noise ratio echell\'e spectra for 19 lines of sight. The result is that the presence of C$_{60}^+$ in interstellar clouds is brought into question.