Picosecond Laser Induced Fluorescence Probing of NO₂ Photofragments

Abstract

The UV photolysis of nitromethane has been studied extensively for many years [1–12]. It is now generally accepted that the primary photodissociation process leads to formation of the free radicals CH₃ and NO₂ in the reaction: $CH_3 NO_2 \xrightarrow{{h\nu }}CH_3 + NO_2$ We have performed 264 nm photolysis of nitromethane in the gas phase and have observed formation of ground-state NO₂ product within the 5 ps pulse resolution time of our laser. This represents the first direct observation of the initial fragments of nitromethane photodecomposition with picosecond time resolution. We have employed a newly developed Nd:phosphate glass laser system, mode-locked and repetitively pulsed at 0.2 Hz [13]. The presence of NO₂ radicals was probed by its fluorescence induced with a second harmonic pulse at 527 nm. Thus using picosecond-delayed laser induced fluorescence (LIF) in the gas phase (0.1 to 10 Torr), we have been able to observe ground-state product formation in the collision free regime. The technique of exciting fragment molecules to a fluorescent state has proved to be a particularly well adapted approach to detecting gas phase species at low concentrations.