  We consider the possibility that solid molecular hydrogen is present in
interstellar space. If so cosmic-rays and energetic photons cause ionisation in
the solid leading to the formation of H6+. This ion is not produced by
gas-phase reactions and its radiative transitions therefore provide a signature
of solid H2 in the astrophysical context. The vibrational transitions of H6+
are yet to be observed in the laboratory, but we have characterised them in a
quantum-theoretical treatment of the molecule; our calculations include
anharmonic corrections, which are large. Here we report on those calculations
and compare our results with astronomical data. In addition to the H6+
isotopomer, we focus on the deuterated species (HD)3+ which is expected to
dominate at low ionisation rates as a result of isotopic condensation
reactions. We can reliably predict the frequencies of the fundamental bands for
five modes of vibration. For (HD)3+ all of these are found to lie close to some
of the strongest of the pervasive mid-infrared astronomical emission bands,
making it difficult to exclude hydrogen precipitates on observational grounds.
By the same token these results suggest that (HD)3+ could be the carrier of the
observed bands. We consider this possibility within the broader picture of ISM
photo-processes and we conclude that solid hydrogen may indeed be abundant in
astrophysical environments.
