Electronic and Infrared Spectra of HC_2nO⁺ (n = 3-7) Chains

Electronic and infrared spectra of HC_2nO⁺ (n = 3-7) chains contained in a cryogenically cooled ion trap are measured using resonance-enhanced photodissociation spectroscopy. The HC_2nO⁺ chains, which possess X̃³Σ^- ground electronic states, are prepared through spin-forbidden association reactions between HC_2n-1⁺ chains and CO molecules, both of which have X̃¹Σ⁺ ground states. Infrared spectra of N₂-tagged HC_2nO⁺ (n = 4-7) chains measured over the 550-1820 cm^-1 range exhibit weak bands associated with chain stretch and bend vibrational modes, with more intense C≡C and CO stretch bands predicted to occur in the 2200-2300 cm^-1 region. Electronic spectra of bare HC_2nO⁺ (n = 3-6) chains measured over the 400-900 nm range are dominated by the C̃³Σ^- ← X̃³Σ^- band systems, with the origin band's wavelength shifting by ≈+80 nm for each added C₂ subunit, consistent with theoretical predictions. The origin bands of HC₆O⁺, HC₈O⁺, and HC₁₀O⁺ chains coincide in wavelength with λ4762.2, λ5828.6, and λ6862.6 diffuse interstellar bands (DIBs) observed along the sightline toward HD 183143, although bands in the laboratory spectra are broader than the DIBs.