Spectroscopy and Two-Photon Dissociation of Jet-Cooled Pyruvic Acid

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.