Valence Photoionization and Autoionization of the Formyl Radical

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We have used 308 nm photolysis of acetaldehyde to measure a photoionization spectrum of the formyl (HCO) radical between 8 and 11.5 eV using an 11 meV FWHM photoionization energy resolution. We have confirmed that the formyl radical is the carrier of the spectrum by generating an identical spectrum of the HCO product in the Cl + H 2 CO reaction. The spectrum of HCO and its deuterated isotopologue (DCO) have several resolved autoionizing resonances above the Franck–Condon envelope, which we assign to autoionization after initial excitation into neutral 3sσ and 3p Rydberg states converging to the first triplet excited state of HCO + (ã 3 A′). The quantum defects for these states are δ 3sσ = 1.06 ± 0.02 and δ 3p = 0.821 ± 0.019. We report absolute photoionization cross-section measurements of σ HCO PI (9.907 eV) = 4.5 ± 0.9 Mb, σ HCO PI (10.007 eV) = 4.8 ± 1.0 Mb, σ HCO PI (10.107 eV) = 6.0 ± 1.2 Mb, σ HCO PI (10.107 eV) = 5.7 ± 1.2 Mb, and σ HCO PI (10.304 eV) = 10.6 ± 2.2 Mb relative to the photoionization cross section of the methyl radical. The absolute cross-section measurements are a factor of ∼1.5 larger than those determined in past studies, although the presence of strong autoionizing features supports a dependence on photoionization energy resolution. We propose that the semiempirical model of Xu and Pratt for estimation of free radical photoionization cross sections is more accurate when applied with a reference species containing the same atoms as the free radical rather than isoelectronic species with different atoms.