Different mechanisms both lead to the production of the naphthalene–OH adduct in the 355 nm and 266 nm laser flash photolysis of the mixed aqueous solution of naphthalene and nitrous acid

The 355 nm and 266 nm nanosecond laser flash photolysis of the mixed aqueous solution of naphthalene (Np) and nitrous acid (HNO2) were performed to investigate the photo-initiated microscopic reactions in this binary component system. Following the 355 nm laser flash, HNO2 was readily photolyzed to NO and OH, and the latter fragment added to the Np ring to form the Np–OH adduct at the rate constant of (1.1 ± 0.1) × 1010 M−1 s−1. Following the 266 nm laser flash, Np was populated first to its lowest excited singlet state (S1) and then to its lowest excited triplet state (T1) via intersystem crossing. Np(S1) and Np(T1) both formed the [Np···HNO2]* exciplex with HNO2, and the fast dissociation of this exciplex again produced the Np–OH adduct. The second-order quenching rate constants of Np(S1) and Np(T1) by HNO2 were measured to be (6.0 ± 0.2) × 109 M−1 s−1 and (4.8 ± 0.2) × 109 M−1 s−1, respectively. It was very interesting to conclude that although both the 355 nm and the 266 nm irradiation of the mixed aqueous solution of Np and HNO2 produced the Np–OH adduct, the mechanisms leading to this species varied essentially for the two cases. This work suggested an unexpected way of releasing OH by HNO2 other than its direct photolysis, namely via forming exciplex with the excited Np (and possibly other polycyclic aromatic hydrocarbons, PAHs) in the environment.