Nonresonant photofragmentation/ionization dynamics of O2 using picosecond and femtosecond laser pulses at 248 nm

Abstract

Photodissociation/ionization of molecular oxygen in a cold molecular beam is studied using a short-pulse laser beam at 248 nm and velocity map imaging. Both photoelectron and O+ images are recorded for laser pulsewidths of 5 and 0.5 ps. Most of the observed ionization dynamics takes place after absorption of four laser photons, equivalent to 20 eV excitation energy, in an above threshold ionization process. Two main channels are identified: postdissociative ionization creating O(3P)+O**3s″(1P, 3P) atoms where O** is an electronically excited autoionizing atom, and molecular (auto)ionization to create a range of highly vibrationally excited ground electronic state O2+ ions. The observed O+ signals then arise from resonance-enhanced two-photon dissociation of O2+ or autoionization of O** atoms, while the electron signals arise from ionization of O2 or autoionization of O**. The latter channel can be used to directly scale the photoelectron and O+ signal strengths. The O+ images show strong differences for 0.5 ps and 5 ps pulse which could arise from ac Stark shifting of levels involved in the A 2Πu←X 2Πg transition of O2+. With resonance enhancement, two-photon dissociation of O2+ is the dominant process producing O+ ions. The O+ angular distributions show an anisotropy that is more extreme than a simple two-step dissociation, which is attributed to alignment effects.