  We present studies of mixed Coulomb crystals of 40Ca+ and 40Ca2+ ions in a
linear Paul trap. Doubly charged ions are produced by photoionisation of
trapped 40Ca+ with a vacuum ultraviolet laser source and sympathetically cooled
via Doppler cooled 40Ca+ ions. We investigate experimentally and theoretically
the structural configurations and the vibrational modes of these mixed
crystals. Our results with 40Ca2+ are an important step towards experimental
realization of the proposals for mode shaping in a linear crystal and
spin-dependent configuration changes from zigzag to linear as proposed by Li et
al., Phys. Rev. A 87, (2013) 052304 and Phys. Rev. Lett 108, (2012) 023003
using ions excited to Rydberg states.
