ION-FORMING PROCESSES ON 248-NM LASER EXCITATION OF URACIL AND METHYLMONOSUBSTITUTED URACILS - A TIME-RESOLVED TRANSIENT CONDUCTIVITY STUDYIN AQUEOUS-SOLUTION

Uracil, thymine and 1-, 3-, and 6-methyluracil were studied by time-re solved optical and conductometric methods after 248 nm excitation with 20 ns laser pulses. The transient conductivity in argon-saturated aqu eous solution, showing a maximum increase (Delta kappa(max)) during th e pulse, is ascribed to the generation of hydrated electrons (e(aq)(-) ) and protons. Biphotonic photoionization as the primary process is in ferred from the almost linear dependence of Delta kappa(max) on the sq uare of the laser pulse intensity (I-L(2)). The quantum yield, obtaine d from either Delta kappa(max) or optical detection of e(aq)(-), e.g. Phi(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 kinet ics depend strongly on pH and the concentrations of laser-induced e(aq )(-) and H+, i.e. on I-L. At pH 6-7 the Delta kappa signal decays by s econd-order kinetics. Under argon the electron adds to the (methyl)ura cil and neutralization occurs by reaction of the radical anion with a proton, which originates from a fast decay of the radical cation. Virt ually the same conductivity pattern was found for the neutralization r eaction of OH- and H+ under N2O. In the acidic pH range the decay chan ges to first-order kinetics due to reaction of H+ with e(aq)(-) under argon or with OH- under N2O. In the alkaline pH range OH- release is i nvolved in the relaxation process resulting from the radical cation af ter excitation of the conjugate base. No indication of a specific spat ial correlation of the charged species, as proposed earlier by Grosswe iner for other systems, was found.