  Irradiation with high energy photons (10.2 - 11.8 eV) was applied to small
diamondoids isolated in solid rare gas matrices at low temperature. The
photoproducts were traced via UV absorption spectroscopy. We found that upon
ionization the smallest of these species lose a peripheral H atom to form a
stable closed-shell cation. This process is also likely to occur under
astrophysical conditions for gas phase diamondoids and it opens the possibility
to detect diamond-like molecules using their rotational spectrum since the
dehydrogenated cations possess strong permanent dipole moments. The
lowest-energy electronic features of these species in the UV were found to be
rather broad, shifting to longer wavelengths with increasing molecular size.
Calculations using time-dependent density functional theory support our
experimental findings and extend the absorption curves further into the vacuum
ultraviolet. The complete sigma - sigma* spectrum displays surprisingly strong
similarities to meteoritic nanodiamonds containing 50 times more C atoms.
