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 to ensure excitation in the perturbative regime, where the possibility of resonance enhanced multiphoton ionization (REMPI) can be investigated. O production is found to be resonance enhanced around 400 nm via three-photon excitation to the e'Δ( = 0) state, similar to results from REMPI studies using nanosecond dye lasers.

Femtosecond 2 + 1 Resonance-Enhanced Multiphoton Ionization Spectroscopy of the -State in Molecular Oxygen.

Coincidence electron-cation imaging is used to characterize the multiphoton ionization of O via the = 4,5 levels of the 3s(Π) Rydberg state. A tunable 100 fs laser beam operating in the 271-263 nm region is found to cause a nonresonant ionization across this wavelength range, with an additional resonant ionization channel only observed when tuned to the Π( = 5) level. A distinct 3s → p wave character is observed in the photoelectron angular distribution for the = 5 channel when on resonance.