Disentangling Electronic Spectra of Linear and Cyclic Hydrogenated Carbon Cluster Cations, C₂H⁺ (n = 3-10)

Electronic spectra are measured for protonated carbon clusters (C₂H⁺) containing between 7 and 21 carbon atoms. Linear and cyclic C₂H⁺ isomers are separated and selected using a drift tube ion mobility stage before being mass selected and introduced into a cryogenically cooled ion trap. Spectra are measured using a two-color resonance-enhanced photodissociation strategy, monitoring C₂⁺ photofragments (H atom loss channel) as a function of excitation wavelength. The linear C₇H⁺, C₉H⁺, C₁₁H⁺, C₁₃H⁺, C₁₅H⁺, and C₁₇H⁺ clusters, which are predicted to have polyynic structures, possess sharp 1¹Σ⁺ ← X̃¹Σ⁺ transitions with well-resolved vibronic progressions in C-C stretch vibrational modes. The vibronic features are reproduced by spectral simulations based on vibrational frequencies and geometries calculated with time-dependent density functional theory (ωB97X-D/cc-pVDZ level). The cyclic C₁₅H⁺, C₁₇H⁺, C₁₉H⁺, and C₂₁H⁺ clusters exhibit weak, broad transitions at a shorter wavelength compared to their linear counterparts. Wavelengths for the origin transitions of both linear and cyclic isomers shift linearly with the number of constituent carbon atoms, indicating that in both cases, the clusters possess a common structural motif.