  Vibrational and electronic absorption spectra calculated at the
(time-dependent) density functional theory level for the bismuth carbide
clusters Bi$_{n}$C$_{2n}$$^+$ ($3 \le n \le 9$) indicate significant
differences in types of bonding that depend on cluster geometry. Analysis of
the electronic charge densities of these clusters highlighted bonding trends
consistent with the spectroscopic information. The combined data suggest that
larger clusters ($n > 5$) are likely to be kinetically unstable in agreement
with the cluster mass distribution obtained in gas-aggregation source
experiments. The spectral fingerprints of the different clusters obtained from
our calculations also suggest that identification of specific
Bi$_{n}$C$_{2n}$$^+$ isomers of should be possible based on infra-red and
optical absorption spectroscopy.
