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

Time-resolved dynamics of the photodissociation of molecular oxygen, O ₂, via the ³∑_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 (λ≈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∑_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 E_transl (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.