Production and decay of ground-state gaseous lead from the 308-nm photodissociation of gaseous lead diiodide

An extensive investigation of the production and decay of ground-state lead atoms from the 308-nm XeCl excimer laser photodissociation of PbI[sub 2](g) is reported. The laser fluence dependences of the Pb(g) and PbI(g) yields along with energetic considerations suggest that Pb(g) is produced by a two-photon process and PbI(g) is produced by a one-photon process. Pb decay profiles as a function of argon pressure, PbI[sub 2](g) number density, laser fluence, and cell temperature were determined. The rates of Pb decay were found to be approximately second order in Pb. The second-order rate constants were found to increase with PbI[sub 2](g) number density, to not change with argon pressure, and to decrease with increasing laser fluence and cell temperature. These results are consistent with a complex mechanism that can be described as the PbI[sub 2](g)-catalyzed recombinations of Pb and PbI with I involving an exothermic preequilibrium, I + PbI[sub 2] = PbI[sub 3], with [Delta]H [approx] [minus]22 [plus minus] 6 kcal/mol. The proposed mechanism is capable of reproducing the observed Pb(g) decay profiles under all conditions studied when reasonably large values of rate coefficients for the various elementary steps are used.