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 <=similar to 1 TW/cm2 to ensure excitation in the perturbative regime, where the possibility of resonance enhanced multiphoton ionization (REMPI) can be investigated. O2+ production is found to be resonance enhanced around 400 nm via three-photon excitation to the e ' 3 Delta 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 O2, producing O2+(X(v)) in a wide range of vibrational states followed by two-or three-photon dissociation, or six-photon excitation to a superexcited O2** 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.

Automated summary

Multiphoton excitation of molecular oxygen in the 392-408 nm region was investigated using a tunable femtosecond laser and a double velocity map imaging photoelectron-photoion coincidence spectrometer. The results revealed resonance enhanced multiphoton ionization around 400 nm, as well as the production of O+ through two different processes at 405 nm. Coincidence detection played a crucial role in identifying these pathways.