Vacuum ultraviolet photodynamics of the methyl peroxy radical studied by double imaging photoelectron photoion coincidences.

The vacuum ultraviolet photoionization of the methyl peroxy radical, CHO, and unimolecular dissociation of internal energy selected CHO cations were investigated in the 9.7-12.0 eV energy range by synchrotron-based double imaging photoelectron photoion coincidence. A microwave discharge flow tube was employed to produce CHO via the reaction of methyl radicals (CH) with oxygen gas. After identifying and separating the different sources of CH from photoionization of CH or dissociative photoionization of CHO, the high resolution slow photoelectron spectrum (SPES) of CHO was obtained, exhibiting two broad bands superimposed with a complex vibrational structure. The first band of the SPES is attributed to the XA″ and aA' overlapped electronic states of CHO and the second is assigned to the bA' electronic state with the help of theoretical calculations. The adiabatic ionization energy of CHO is derived as 10.215 ± 0.015 eV, in good agreement with high-accuracy theoretical data from the literature. The vertical ionization energy of the bA' electronic state is measured to be 11.5 eV and this state fully dissociates into CH and O fragments. The 0 K adiabatic appearance energy (AE) of the CH fragment ion is determined to be 11.15 ± 0.02 eV.