OBSERVATION OF AN INDIRECT PATHWAY IN THE FEMTOSECOND STUDY OF ALKYL NITRITE PHOTODISSOCIATION IN THE S-1 STATE

The present work has addressed the question of the dissociation time o f four alkyl nitrites upon photoexcitation in the S-1 state (methyl ni trite, n-, and t-butyl nitrites and i-amyl nitrite). The time resolved Laser Induced Fluorescence technique has been used in the femtosecond regime under bulk conditions. The photodissociation has been initiate d at 351 nm by 150 fs pump pulses, and has been probed using a two-pho ton process at 467 nm by 200 fs pulses. The LIF signal has qualitative ly the same shape for the four nitrites: it passes through a transient peak before reaching a plateau. The two-photon process that induces t he detected fluorescence is nonresonant for detection of the dissociat ion product NO through the A <-- X transition. Conversely, the two-pho ton process is resonant or quasiresonant for detection of the excited nitrite molecule in the S-1 level before it dissociates. This leads to an enhanced detection efficiency of the non-dissociated excited molec ule versus that of the NO fragment. A simple kinetic model has been de veloped to account for this detection scheme. It shows that the transi ent peak is observable only if the lifetime of the excited molecule is comparable or larger than the temporal width of the lasers. in that c ase, the model allows the determination of an effective lifetime of th e excited molecule from the measured LIF signal. Lifetimes have been f ound in the range of 125 fs for the four nitrites investigated. This h as allowed us to answer a controversy on the dissociation mechanism of methyl nitrite and to confirm that an indirect pathway exists in the photodissociation of this molecule. (C) 1995 American Institute of Phy sics.