Valence-Shell Photoionization of C₄H₅: The 2-Butyn-1-yl Radical

We present new high-resolution data on the photoionization of the 2-butyn-1-yl radical (CH₃C≡C-^•CH₂) formed by H atom abstraction from 2-butyne by F atoms. The spectra were recorded from 7.7 to 11 eV by using double-imaging, photoelectron-photoion coincidence spectroscopy, which allows the unambiguous correlation of photoelectron data and the mass of the species. The photoionization spectrum shows significant resonant autoionizing structure converging to excited states of the C₄H₅⁺ cation, similar to what is observed in the closely related propargyl radical (HC≡C-^•CH₂). The threshold photoelectron spectrum, obtained with a resolution of 17 meV, is also reported. This spectrum is consistent with previous measurements of the first photoionization band but has been extended to higher energy to allow the observation of bands corresponding to excited electronic states of the ion. A refined value of the adiabatic ionization energy is extracted: IE(C₄H₅) = 7.93 ± 0.01 eV. A determination of the absolute photoionization cross section of the 2-butyn-1-yl radical at 9.7 eV is also reported: σ_ion(C₄H₅) = 6.1 ± 1.8 Mb.