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 (CO₂ elimination) was dominant for all three wavelengths, producing primarily ground state singlet methylhydroxycarbene, ¹CH₃COH, as well as a minor yield of electronically excited ³CH₃COH. Excitation at 369.74 and 354.45 nm produced ¹CH₃COH with internal energies lying below and above the calculated potential energy barriers for isomerization to vinyl alcohol (C₂H₃OH) and acetaldehyde (CH₃CHO). This suggests that the bimolecular chemistry of CH₃COH may be important in the atmosphere. At 322.5 nm, C–C bond fission producing HOCO + CH₃CO was also observed as a minor channel. While decarboxylation producing ¹CH₃COH + CO₂ on the singlet potential energy surface (PES) is clearly dominant, observation of ³CH₃COH + CO₂ and HOCO + CH₃CO confirms that the triplet PES also plays a role in pyruvic acid photochemistry in the actinic region.