C$_{60}^+$ and the Diffuse Interstellar Bands: An Independent Laboratory
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  In 2015, Campbell et al. (Nature 523, 322) presented spectroscopic laboratory
gas phase data for the fullerene cation, C$_{60}^+$, that coincide with
reported astronomical spectra of two diffuse interstellar band (DIB) features
at 9633 and 9578 \AA. In the following year additional laboratory spectra were
linked to three other and weaker DIBs at 9428, 9366, and 9349 \AA. The
laboratory data were obtained using wavelength-dependent photodissociation
spectroscopy of small (up to three) He-tagged C$_{60}^+-$He$_n$ ion complexes,
yielding rest wavelengths for the bare C$_{60}^+$ cation by correcting for the
He-induced wavelength shifts. Here we present an alternative approach to derive
the rest wavelengths of the four most prominent C$_{60}^+$ absorption features,
using high resolution laser dissociation spectroscopy of C$_{60}^+$ embedded in
ultracold He droplets. Accurate wavelengths of the bare fullerene cation are
derived based on linear wavelength shifts recorded for He$_n$C$_{60}^+$ species
with $n$ up to 32. A careful analysis of all available data results in precise
rest wavelengths (in air) for the four most prominent C$_{60}^+$ bands:
9631.9(1) \AA, 9576.7(1) \AA, 9427.5(1) \AA, and 9364.9(1) \AA. The
corresponding band widths have been derived and the relative band intensity
ratios are discussed.
