ions in ionized O2/CH4 mixtures: Intermediacy of CH3OOH+ and CH2O+

H 2 O 2 + ions are shown to be formed in ionized O2/CH4 mixtures from the secondary reaction of CH2O+ with O2. By high-resolution mass spectrometric techniques, the peroxide HOOH structure and the H2CO structure of ionized formaldehyde are assigned to the [H2O2]+ ions and the [CH2O]+ reactants, respectively. The [CH4O2]+ intermediate of the reaction between O 2 + and CH4 is characterized as a mixture of the loosely bound [ CH 4 O 2 ] + complex and the theoretically predicted insertion product CH3OOH+. The CH3OOH+ intermediate, precursor of the most abundant ionic product CH2OOH+, has been characterized for the first time as a long-lived species.