title: Direct mapping of recoil in the ion-pair dissociation of molecular oxygen by a femtosecond depletion method.
abstract: Time-resolved dynamics of the photodissociation of molecular oxygen, O(2), 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 approximately 205 nm) was found to cause three-photon pumping of O(2) 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 O(2) ion-pair state, giving rise to (O((3)P)+O(+)((4)S)) 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 A. 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.
authors.affiliation: Institute of Chemical Kinetics and Combustion, Institutskaja St. 3, Novosibirsk 630090, Russia and Novosibirsk State University, Pirogova St. 2, Novosibirsk 630090, Russia.
authors.forename: Alexey V
authors.initials: AV
authors.lastname: Baklanov
authors.forename: Liesbeth M C
authors.initials: LM
authors.lastname: Janssen
authors.forename: David H
authors.initials: DH
authors.lastname: Parker
authors.forename: Lionel
authors.initials: L
authors.lastname: Poisson
authors.forename: Benoit
authors.initials: B
authors.lastname: Soep
authors.forename: Jean-Michel
authors.initials: JM
authors.lastname: Mestdagh
authors.forename: Olivier
authors.initials: O
authors.lastname: Gobert
mesh_terms: D003116:Color; D010100:Oxygen / Q000737:chemistry*; D010782:Photolysis; D017785:Photons; D013997:Time Factors
chemical_list: D010100:Oxygen