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 of four alkyl nitrites upon photoexcitation in the S-1 state (methyl nitrite, 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 initiated at 351 nm by 150 fs pump pulses, and has been probed using a two-photon process at 467 nm by 200 fs pulses. The LIF signal has qualitatively the same shape for the four nitrites: it passes through a transient peak before reaching a plateau. The two-photon process that induces the detected fluorescence is nonresonant for detection of the dissociation product NO through the A <-- X transition. Conversely, the two-photon 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 molecule versus that of the NO fragment. A simple kinetic model has been developed to account for this detection scheme. It shows that the transient peak is observable only if the lifetime of the excited molecule is comparable or larger than the temporal width of the lasers. in that case, the model allows the determination of an effective lifetime of the excited molecule from the measured LIF signal. Lifetimes have been found in the range of 125 fs for the four nitrites investigated. This has 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 Physics.