Nonresonant photofragmentation/ionization dynamics of O-2 using picosecondand femtosecond laser pulses at 248 nm

Photodissociation/ionization of molecular oxygen in a cold molecular beam i s 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 abso rption of four laser photons, equivalent to 20 eV excitation energy, in an above threshold ionization process. Two main channels are identified: postd issociative ionization creating O(P-3)+O**3s "(P-1, P-3) atoms where O** is an electronically excited autoionizing atom, and molecular (auto)ionizatio n to create a range of highly vibrationally excited ground electronic state O-2(+) ions. The observed O+ signals then arise from resonance-enhanced tw o-photon dissociation of O-2(+) or autoionization of O** atoms, while the e lectron signals arise from ionization of O-2 or autoionization of O**. The latter channel can be used to directly scale the photoelectron and O+ signa l strengths. The O+ images show strong differences for 0.5 ps and 5 ps puls e which could arise from ac Stark shifting of levels involved in the A (2)P i(u)<-- X (2)Pi(g) transition of O-2(+). With resonance enhancement, two-ph oton dissociation of O-2(+) is the dominant process producing O+ ions. The O+ angular distributions show an anisotropy that is more extreme than a sim ple two-step dissociation, which is attributed to alignment effects. (C) 20 00 American Institute of Physics. [S0021-9606(00)00713-3].