Fast Photodynamics of Aqueous Formic Acid

Femtosecond transient absorption spectroscopy is used to investigate the primary reaction dynamics of aqueous formic acid, HCOOH(aq), following the photoexcitation of the n → π* transition at 200 nm. Using probe pulses covering the spectral range from 200 to 620 nm, we find that the only detectable photoproduct is the OH radical, which suggests that HCOOH(aq) dissociates predominantly into the HCO + OH channel. The absence of signals assignable to the formyl radical is discussed, and the properties of hydrated formyl radicals are investigated using high-level electronic structure calculations.