The UV photodissociation of pyruvic acid (PA) is studied in molecular beams using time-of-flight (TOF) mass spectroscopy and time-sliced velocity map imaging (VMI) following excitation to the first absorption band (S1 ← S0) at 330–380 nm. CH3CO, HOCO, CO, CH3, and H are detected as photodissociation products. The photofragment yield (PFY) spectrum of the H product is recorded at 350–380 nm in He and Ar carrier gases. The spectrum shows sharp vibrational features reflecting the significant rotational cooling achieved in the molecular beam. It matches well the broad features observed in the room temperature absorption spectrum and indicates that the S1 state lives longer than a picosecond. The origin band of the S1 ← S0 transition is identified at 26 710 cm–1, and progressions in the CH3 and C–C torsional modes are tentatively assigned. Kinetic energy release (KER) and angular distributions of CH3CO, HOCO, CO, CH3, and H fragments indicate that additional photon absorption from S1 to the S2/S3 states is facile and is followed by rapid dissociation to the observed fragments. On the basis of the energetics of the different dissociation pathways and analyses of the observed KER distributions, three-body fragmentation processes are proposed as major contributors to the formation of the observed products.

Pyruvic acid, a representative alpha-keto carboxylic acid, is one of the few organic molecules destroyed in the troposphere by solar radiation rather than by reactions with free radicals. To date,...