Photoelectron and photofragment velocity map imaging of state-selected molecular oxygen dissociation/ionization dynamics

Abstract

A substantial improvement in the photofragment imaging technique is illustrated in a study of molecular oxygen photodynamics. In this method, labeled velocity map imaging, electrostatic ion lenses are shown to allow mapping of all particles with the same initial velocity vector onto the same point on a 2D detector, irrespective of their position of creation in the ionization volume. This leads to a dramatic increase in image resolution. Velocity map imaging of photoelectrons from molecular ionization is also demonstrated and applied along with O+ imaging to identify the processes leading to O+ formation when using (2+1) resonantly enhanced multiphoton ionization (REMPI) detection for O2. Oxygen molecules prepared in the (v=2, N=2) level of the 3dπ(3Σ1g−) Rydberg state by two-photon excitation at 11.02 eV are excited by a third photon to an energy near v=24 of ground-state O2+ (equivalent to one-photon excitation at 75 nm). All energetically accessible excited oxygen atoms and an extensive range of vibrationally excited O2+ ions result, with the primary dissociation/ionization events taking place at the third-photon level. Competition between dissociation into excited atoms and formation of O2+ is gauged by comparing images for e− and O+ products. Trends in the photoelectron and O+ fragment angular distributions are discussed for each active channel.