Issue 16, 2024

Gas-phase electronic spectra of HC2n+1H+ (n = 2–6) chains

Abstract

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 HC2nH+ polyacetylene chains have been extensively characterized spectroscopically, the corresponding odd HC2n+1H+ chains have received far less attention. Here we use two-colour resonance enhanced photodissociation spectroscopy to measure electronic spectra for HC2n+1H+ (n = 2–6) chains contained in a cryogenically cooled quadrupole ion trap. The HC2n+1H+ 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 HC2n+1H+ species are linear, consistent with predictions from electronic structure calculations. The HC2n+1H+ 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[double bond, length as m-dash]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 HC2n+1H+ chains, allowing upper limits to be established for their column densities in diffuse interstellar clouds.

Graphical abstract: Gas-phase electronic spectra of HC2n+1H+ (n = 2–6) chains

Supplementary files

Article information

Article type
Paper
Submitted
12 Kol 2024
Accepted
08 Mme 2024
First published
09 Mme 2024

Phys. Chem. Chem. Phys., 2024,26, 12306-12315

Gas-phase electronic spectra of HC2n+1H+ (n = 2–6) chains

S. J. P. Marlton, C. Liu, P. Watkins and E. J. Bieske, Phys. Chem. Chem. Phys., 2024, 26, 12306 DOI: 10.1039/D4CP00625A

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