Ion-forming processes on 248 NM laser excitation of uracil and methyl-monosubstituted uracils: A time-resolved transient conductivity study in aqueous solution

Abstract Uracil, thymine and 1-, 3-, and 6-methyluracil were studied by time-resolved optical and conductometric methods after 248 nm excitation with 20 ns laser pulses. The transient conductivity in argon-saturated aqueous solution, showing a maximum increase ( Δκ max ) during the pulse, is ascribed to the generation of hydrated electrons (e aq − ) and protons. Biphotonic photoionization as the primary process is inferred from the almost linear dependence of Δκ max on the square of the laser pulse intensity ( I L 2 ). The quantum yield, obtained from either Δκ max or optical detection of e aq − , e.g. gf e− = 0.02 for uracil at pH 7 and I L = 12 MW cm −2 , varies by a factor of about two for the five pyrimidines. The neutralization kinetics depend strongly on pH and the concentrations of laser-induced e aq − and H + , i.e. on I L . At pH 6–7 the Δκ signal decays by second-order kinetics. Under argon the electron adds to the (methyl)uracil and neutralization occurs by reaction of the radical anion with a proton, which originates from a fast decay of the radical cation. Virtually the same conductivity pattern was found for the neutralization reaction of OH − and H + under N 2 O. In the acidic pH range the decay changes to first-order kinetics due to reaction of H + with e aq − under argon or with OH − under N 2 O. In the alkaline pH range OH − release is involved in the relaxation process resulting from the radical cation after excitation of the conjugate base. No indication of a specific spatial correlation of the charged species, as proposed earlier by Grossweiner for other systems, was found.