Photodissociation of acetaldehyde and the absolute photoionization cross section of HCO.

Photodissociation of acetaldehyde (CH3CHO) at 266 nm produced CH3 and HCO radicals, and single-photon vacuum ultraviolet ionization was used to record velocity map ion images of both CH3+ and HCO+. Comparison of the translational energy distributions from both species indicates that secondary fragmentation of HCO is negligible for 266 nm photodissociation. Thus, the relative photoion signals for CH3+ and HCO+ in the mass spectrometer, combined with the recently measured absolute photoionization cross section of CH3, allowed the determination of the absolute photoionization cross section of HCO (σ(HCO) = 4.8 ± 1.5(2.0), 5.9 ± 1.6(2.2), and 3.7 ± 1.2(1.6) Mb at 10.257, 10.304, and 10.379 eV, respectively). The observed values are quite small but consistent with the similarly small value at threshold for the isoelectronic species NO. This behavior is discussed in terms of the character of the HOMO in both molecules.