Unraveling the Primary Photochemistry of Pyruvic Acid: Direct Observation of Three Competing Channels

The photodissociation of gaseous pyruvic acid at three representative wavelengths, 369.74, 354.45, and 322.50 nm, was studied under collision-free molecular beam conditions. Kinetic energy distributions of the neutral products were measured using tunable vacuum ultraviolet photoionization followed by mass spectrometric analysis. Decarboxylation (CO 2 elimination) was dominant for all three wavelengths, producing primarily ground state singlet methylhydroxycarbene, 1 CH 3 COH, as well as a minor yield of electronically excited 3 CH 3 COH. Excitation at 369.74 and 354.45 nm produced 1 CH 3 COH with internal energies lying below and above the calculated potential energy barriers for isomerization to vinyl alcohol (C 2 H 3 OH) and acetaldehyde (CH 3 CHO). This suggests that the bimolecular chemistry of CH 3 COH may be important in the atmosphere. At 322.5 nm, C–C bond fission producing HOCO + CH 3 CO was also observed as a minor channel. While decarboxylation producing 1 CH 3 COH + CO 2 on the singlet potential energy surface (PES) is clearly dominant, observation of 3 CH 3 COH + CO 2 and HOCO + CH 3 CO confirms that the triplet PES also plays a role in pyruvic acid photochemistry in the actinic region.