H atom formation from benzene and toluene photoexcitation at 248 nm.

The technique of excimer laser excitation/Lyman alpha H atom laser induced fluorescence was used to investigate the formation of H atoms from the 248 nm photoexcitation of benzene and toluene. The H atom signal dependence on laser excitation energy demonstrated that it is produced from two photon photolysis of the aromatics; absorption of the first photon populates the bound (1)B(2u) level followed by absorption from this level to a dissociative level, which produces H atoms, among other potential channels. Analysis of the data yields the second photon absorption cross section to produce H and is equal to 1.0 and 5.2x10(-19) cm(2) for benzene and toluene, respectively. In addition, the yield of H atoms was observed to be pressure dependent. This is because at sufficiently high pressures the nanosecond lifetime of the (1)B(2u) state can be pressure quenched and hence may compete with the absorption of the second photon. The yields of H atoms were determined as a function of pressure for a range of the laser energies and with various collider gases. The analysis of these data allowed the total absorption cross section for the second photon to be determined and is equal to 2.8 and 1.7x10(-17) cm(2) for benzene and toluene, respectively. In addition, the rate constants for quenching (1)B(2u) with various gases (He, Ar, N(2), and O(2)) were determined. This large absorption coefficient for the second photon implies that with a pulsed laser source of 248 nm it is difficult to avoid aromatic photodissociation. We highlight a few previous studies that may need to be reevaluated in the light of the results from this study.