Observation and assignment of the first photoelectron band of the CH3CO (XA) radical

HeI photoelectron spectra have been recorded for the reaction of fluorine atoms with acetaldehyde at different reaction times. A structured band associated with a short-lived primary reaction product was recorded at a reactant mixing distance of 6 mm above the photon beam. The vertical ionization energy was measured as (8.39 ± 0.05 eV), and regularly spaced vibrational structure was observed in this band with an average vibrational separation of (1605 ± 30 cm⁻¹). The band was assigned to ionization of the CH₃CO (X²A) radical on the basis of first vertical ionization energies computed for the two possible primary products, CH₃CO and CH₂CHO, using ab-initio calculations. The calculations carried out on CH₃CO (X²A), and its singlet ground cationic state, revealed that the observed vibrational structure can be assigned to excitation of the CO stretching mode in the ion. The measured adiabatic ionization energy, 7.21 ± 0.05 eV, is higher than the value expected (6.94 ± 0.03 eV) from evidence in the literature and calculated values in this work. This difference is assigned to poor Franck–Condon factors in the region of the band onset which means that the true adiabatic ionization energy was not observed.