Gas-phase electronic spectra of HC₂H⁺ (n = 2-6) chains

Highly unsaturated carbon chains are generated in combustion processes and electrical discharges, and are confirmed constituents of the interstellar medium. In hydrogen-rich environments smaller carbon clusters tend to exist as linear chains, capped on each end by hydrogen atoms. Although the HC₂H⁺ polyacetylene chains have been extensively characterized spectroscopically, the corresponding odd HC₂H⁺ chains have received far less attention. Here we use two-colour resonance enhanced photodissociation spectroscopy to measure electronic spectra for HC₂H⁺ (n = 2-6) chains contained in a cryogenically cooled quadrupole ion trap. The HC₂H⁺ chains are formed either top-down by ionizing and fragmenting pyrene molecules using pulsed 266 nm radiation, or bottom-up by reacting cyclic carbon cluster cations with acetylene. Ion mobility measurements confirm that the HC₂H⁺ species are linear, consistent with predictions from electronic structure calculations. The HC₂H⁺ electronic spectra exhibit three band systems in the visible/near infrared spectral range, which each shifts progressively to longer wavelength by ≈90 nm with the addition of each additional CC subunit. The strongest visible HC₁₁H⁺ band has a wavelength (λ = 545.1 nm) and width (1.5 nm) that match the strong λ 5450 diffuse interstellar band (DIB). However, other weaker HC₁₁H⁺ bands do not correspond to catalogued DIBs, casting doubt on the role of HC₁₁H⁺ as a carrier for the λ 5450 DIB. There are no identifiable correspondences between catalogued DIBs and bands for the other HC₂H⁺ chains, allowing upper limits to be established for their column densities in diffuse interstellar clouds.