Photodissociation mechanism of pyruvic acid studied via time-resolved XUV photoelectron spectroscopy

The photoexcitation of organic molecules by ultraviolet (UV) light, and subsequent dissociation dynamics into free radicals, influences chemical reactions in the atmosphere. These fragmentation dynamics can often involve complex processes such as proton-coupled electron transfer (PCET) that isomerize the molecule prior to dissociation. Pyruvic acid (PA) is a volatile organic compound (VOC) found in the atmosphere that undergoes PCET followed by decarboxylation upon exposure to solar UV wavelengths. Prior experimental studies of PA at 351nm have observed the decarboxylation photoproducts but did not time-resolve the reaction dynamics or measure the lifetime of the excited state. Recent theoretical work suggests that PCET on the excited electronic state mediates a population transfer back to the ground electronic state on picosecond timescales, after which decarboxylation takes place. Here, we present a time-resolved XUV photoelectron spectroscopy study of the photodissociation dynamics of PA probed at 20 eV. The experiment probed the evolving binding energies of the valence electronic states, offering insight into ultrafast population transfer mediated by PCET. We studied the system pumped at a range of wavelengths (320-370 nm) to excite below, at, and above the PCET barrier on the S1 state to study how excitation energy influences the photochemical dynamics. Our results provide insight into the electronic states involved in PCET in this atmospherically important molecule.