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

Coincidence electron-cation imaging is used to characterize the multiphoton ionization of O2 via the v = 4,5 levels of the 3s(3Πg) 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 3Πg(v = 5) level. A distinct 3s → p wave character is observed in the photoelectron angular distribution for the v = 5 channel when on resonance.