Electronic spectra of positively charged carbon clusters-C_2n ⁺ (n = 6-14)

Electronic spectra are measured for mass-selected C_2n ⁺(n = 6-14) clusters over the visible and near-infrared spectral range through resonance enhanced photodissociation of clusters tagged with N₂ molecules in a cryogenic ion trap. The carbon cluster cations are generated through laser ablation of a graphite disk and can be selected according to their collision cross section with He buffer gas and their mass prior to being trapped and spectroscopically probed. The data suggest that the C_2n ⁺(n = 6-14) clusters have monocyclic structures with bicyclic structures becoming more prevalent for C₂₂ ⁺ and larger clusters. The C_2n ⁺ electronic spectra are dominated by an origin transition that shifts linearly to a longer wavelength with the number of carbon atoms and associated progressions involving excitation of ring deformation vibrational modes. Bands for C₁₂ ⁺, C₁₆ ⁺, C₂₀ ⁺, C₂₄ ⁺, and C₂₈ ⁺ are relatively broad, possibly due to rapid non-radiative decay from the excited state, whereas bands for C₁₄ ⁺, C₁₈ ⁺, C₂₂ ⁺, and C₂₆ ⁺ are narrower, consistent with slower non-radiative deactivation.