Vacuum ultraviolet photoionization and dissociative photoionization of acetyl (CH3CO) and vinoxy (CH2CHO) radicals.

The vacuum ultraviolet photoionization and dissociative photoionization of isomeric radicals, acetyl (CH3CO) and vinoxy (CH2CHO), have been studied using synchrotron radiation double imaging photoelectron photoion coincidence (i2PEPICO) spectroscopy. Mass-selected threshold photoelectron spectra (ms-TPES) corresponding to the parent cations (m/z = 43, CH3CO+ and CH2CHO+) and their fragment ions (m/z = 15, CH3+) are measured and assigned based on theoretical calculations. The first and second bands of the ms-TPES of m/z = 43 are mainly unstructured due to the large molecular configuration changes upon photoionization, and they are attributed to contributions of the singlet ground state of CH3CO+ and CH2CHO+. The ms-TPES of m/z = 15 exhibit a rich structure with two vibrational series superimposed onto a broad envelope. The first vibrational series is attributed to the photoionization of the methyl radical (CH3), generated from secondary reactions in the flow tube reactor, while the second series is attributed to pre-dissociation of the triplet electronic state of CH2CHO+, and the statistical dissociation of ground state CH3CO+ in high vibrational levels contributes to the broad envelope. The dissociation mechanisms for the generation of CH3+ from CH3CO+ and CH2CHO+ are also discussed with the aid of a theoretical energy diagram and a breakdown diagram. In particular, a dissociation mechanism is proposed, in which CH2CHO+ in a triplet electronic state first undergoes an intersystem crossing into the singlet electronic state, followed by a structural rearrangement and dissociation into CH3+ along the singlet minimum energy path of CH3CO+.