The Primary Photodynamics of Aqueous Nitrate:  Formation of Peroxynitrite

We have examined the photochemical reactions occurring after irradiation at 200 nm of the aqueous nitrate ion, NO3 -(aq). Using femtosecond transient absorption spectroscopy over the range 194−388 nm, we have characterized the formation and subsequent relaxation of the primary photoproducts of nitrate photolysis. The dominant photoproduct is the cis-isomer of peroxynitrite, which accounts for 48% of the excited state molecules initially produced. A slightly smaller fraction, 44%, of the excited molecules return to the electronic ground state of NO3 - and relax to the vibrational ground state in 2 ps. The remaining 8% of the molecules initially excited react via the •NO + •O2 - or the NO- + O2 dissociation channels. Formation of NO2 - and •NO2 is not observed, suggesting that the previous observations of these species in steady-state photolysis are caused by reactions occurring on a longer time scale.