Direct mapping of recoil in the ion-pair dissociation of molecular oxygen by a femtosecond depletion method

International audienceTime-resolved dynamics of the photodissociation of molecular oxygen, O2, via the 3 Sigma u - ion-pair state have been studied with femtosecond time resolution using a pump-probe scheme in combination with velocity map imaging of the resulting O+ and O- ions. The fourth harmonic of a femtosecond titanium-sapphire (Ti:sapphire) laser (lambda ~= 205 nm) was found to cause three-photon pumping of O2 to a level at 18.1 eV. The parallel character of the observed O+ and O- images allowed us to conclude that dissociation takes place on the 3 Sigma u - ion-pair state. The 815 nm fundamental of the Ti:sapphire laser used as probe was found to cause two-photon electron photodetachment starting from the O2 ion-pair state, giving rise to (O(3P)+O+(4S)) products. This was revealed by the observed depletion of the yield of the O- anion and the appearance of a new O+ cation signal with a kinetic energy Etransl(O+) dependent on the time delay between the pump and probe lasers. This time-delay dependence of the dissociation dynamics on the ion-pair state has also been simulated, and the experimental and simulated results coincide very well over the experimental delay-time interval from about 130 fs to 20 ps where two- or one-photon photodetachment takes place, corresponding to a change in the R(O+,O-) interatomic distance from 12 to about 900 Å. This is one of the first implementations of a depletion scheme in femtosecond pump-probe experiments which could prove to be quite versatile and applicable to many femtosecond time-scale experiments