Unresolved electronic origin band transitions have been measured for the even-numbered linear carbon-chain radicals (13)C<inf>2</inf> n H/D with. Spectra of the fully 13C substituted 13C8H/D and 13C<inf>10</inf>H/D isotopologues are presented for the first time. 12C<inf>10</inf>H/D and 12C1<inf>2</inf>H/D spectra are presented with improved signal-to-noise ratio to realise a systematic study of the band profiles. Despite the fact that individual rotational transitions cannot be resolved, it is possible to derive effective values for the upper state lifetime. The obtained natural lifetime line broadening parameters of 0.8 ± 0.1 cm-1 for n = 4, 4.0 ± 0.5 cm-1 for n = 5 and 4.8 ± 0.5 cm-1 for n = 6 indicate that internal conversion processes occur in these carbon-chain molecules.