We have studied dissociative multiphoton ionization of NO 2 by time-resolved velocity map imaging in a two-color pump-probe experiment using the 400 and 266 nm harmonics of a regeneratively amplified titanium-sapphire laser. We observe that most of the ion signal appears as NO ϩ with ϳ0.28 eV peak kinetic energy. Approximately 600 fs period oscillations indicative of wave packet motion are also observed in the NO ϩ decay. We attribute the signal to two competitive mechanisms. The first

more »

... three-photon 400 nm absorption followed by dissociative ionization of the pumped state by a subsequent 266 nm photon. The second involving one-photon 400 nm absorption to the 2 B 2 state of NO 2 followed by two-photon dissociative ionization at 266 nm. This interpretation is derived from the observation that the total NO ϩ ion signal exhibits biexponential decay, 0.72 exp(Ϫt/90Ϯ10)ϩ0.28 exp(Ϫt/4000Ϯ400), where t is the 266 nm delay in femtoseconds. The fast decay of the majority of the NO ϩ signal suggests a direct dissociation via the bending mode of the pumped state.