Gas-phase electronic spectra of HC 2 n +1 H + ( 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 HC 2 n H + polyacetylene chains have been extensively characterized spectroscopically, the corresponding odd HC 2 n +1 H + chains have received far less attention. Here we use two-colour resonance enhanced photodissociation spectroscopy to measure electronic spectra for HC 2 n +1 H + ( n = 2-6) chains contained in a cryogenically cooled quadrupole ion trap. The HC 2 n +1 H + 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 HC 2 n +1 H + species are linear, consistent with predictions from electronic structure calculations. The HC 2 n +1 H + 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 HC 11 H + 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 HC 11 H + bands do not correspond to catalogued DIBs, casting doubt on the role of HC 11 H + as a carrier for the λ 5450 DIB. There are no identifiable correspondences between catalogued DIBs and bands for the other HC 2 n +1 H + chains, allowing upper limits to be established for their column densities in diffuse interstellar clouds.