Journal of Chemical Physics, Vol.107, No.7, 2357-2362, 1997
Photoelectron and Photofragment Velocity Map Imaging of State-Selected Molecular-Oxygen Dissociation/Ionization Dynamics
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 O-2. Oxygen molecules prepared in the (v=2, N=2):level of the 3d pi((3) Sigma(1g)(-)) 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 O-2 (equivalent to one-photon excitation at 75 nm). All energetically accessible excited oxygen atoms and an extensive range of vibrationally excited O-2(+) ions result, with the primary dissociation/ionization events taking place at the third-photon level. Competition between dissociation into excited atoms and formation of O-2(-) 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.
Keywords:ENHANCED MULTIPHOTON IONIZATION;DOUBLY EXCITED-STATES;ANGULAR-DISTRIBUTIONS;RYDBERG STATES;SPECTROSCOPY;PHOTOIONIZATION;O2;PHOTODISSOCIATION;THRESHOLD;ATOMS