Disentangling Multiphoton Ionization and Dissociation Channels in Molecular Oxygen Using Photoelectron-Photoion Coincidence Imaging.

Multiphoton excitation of molecular oxygen in the 392-408 nm region is studied using a tunable femtosecond laser coupled with a double velocity map imaging photoelectron-photoion coincidence spectrometer. The laser intensity is held at ≤∼1 TW/cm 2 to ensure excitation in the perturbative regime, where the possibility of resonance enhanced multiphoton ionization (REMPI) can be investigated. O 2 + production is found to be resonance enhanced around 400 nm via three-photon excitation to the e' 3 Δ u ( v = 0) state, similar to results from REMPI studies using nanosecond dye lasers. O + production reaches 7% of the total ion yield around 405 nm due to two processes: autoionization following five-photon excitation of O 2 , producing O 2 + (X( v )) in a wide range of vibrational states followed by two- or three-photon dissociation, or six-photon excitation to a superexcited O 2 ** state followed by neutral dissociation and subsequent ionization of the electronically excited O atom. Coincidence detection is shown to be crucial in identifying these competing pathways.