Gas-phase electronic spectra of HCH ( = 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 HCH polyacetylene chains have been extensively characterized spectroscopically, the corresponding odd HCH chains have received far less attention. Here we use two-colour resonance enhanced photodissociation spectroscopy to measure electronic spectra for HCH ( = 2-6) chains contained in a cryogenically cooled quadrupole ion trap. The HCH 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 HCH species are linear, consistent with predictions from electronic structure calculations. The HCH 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 HCH band has a wavelength ( = 545.1 nm) and width (1.5 nm) that match the strong 5450 diffuse interstellar band (DIB). However, other weaker HCH bands do not correspond to catalogued DIBs, casting doubt on the role of HCH as a carrier for the 5450 DIB. There are no identifiable correspondences between catalogued DIBs and bands for the other HCH chains, allowing upper limits to be established for their column densities in diffuse interstellar clouds.