Angular distributions and separation energies of predissociation photofragments of O2+ (b 4Σg−, v′=4, N′, F′)

Abstract

Kinetic energy distributions have been measured for O+ photofragments resulting from transitions O2 + (a 4Π u , v ″, J ″, Ω″) →O2 + (b 3Σ g −,v′=4,N′,F′) →O+(4 S 0) +O(3 P), where for the first time in the study of photodissociation all relevant quantum numbers involved are specified. Both 16,16O2 + and 16,18O2 + were studied. The measurements were made using a laser‐ion coaxial‐beams photofragment spectrometer, a single‐mode laser, and velocity tuning of the absorption wavelength. Experimental values of the anisotropy parameter β, which describes the angular distributions of the photofragments, were obtained by fitting the experimental energy distributions with calculated distributions, obtained from a Monte Carlocomputer simulation of ion trajectories in the apparatus. Values of β were thus determined with an uncertainty of ±0.05 or less. They are compared with theoretical values of β obtained by a generalization to Hund’s case (b) of the theory previously developed by R. N. Zare. In a similar manner, values of the kinetic energy of separation of the photofragments in the center‐of‐mass system, W, are determined with an accuracy, in the most favorable case, of ±0.14 meV. The experimental W values are used, along with other spectroscopic data, to improve the bond dissociation energy values of the ground X 3Σ g − state of O2, and of the a and b states of O2 +. It is shown that for W<50 meV the dissociation strongly favors the O(3 P 2) channel over 3 P 1 and 3 P 0. This result is compared with several possible limiting‐case models of the long‐range couplings involved in the dissociation process, and it is concluded that the dissociation proceeds in a completely adiabatic way. Finally, further possible refinements of the present work are discussed.