Electron-impact dissociation of O 2: Kinetic energy and angular distributions of highly excited Rydberg atoms

Abstract

Oxygen atoms in high-Rydberg states, O** (15⩽ n⩽30), were produced from oxygen molecules by electron impact at 50–100 eV. The time-of-flight and angular distributions of the O** atoms were measured at angles of 60–120° with respect to the electron-beam axis. The O** atoms with released kinetic energies E k>8 eV ( fast fragments) showed angle-independent distributions, whereas those with 5< E k<8 ( slow fragments) showed angular distributions having a minimum at 90°. An analysis based on the core-ion model, Dunn's selection rules, and the “practical approximations” developed by Zare and Herschbach led to the following conclusions: At impact energies higher than 50 eV, the O** atoms are produced mainly from the molecular Rydberg states which converge to the excited states of O 2 2+ lying in a range of 43–56 eV above the ground state of O 2 in the Franck-Condon region. The precursors of the fast fragments are ascribed to the Rydberg states converging to the 3Σ g − and 3Π g states of O 2 2+, while those of the slow fragments are ascribed to the Rydberg states converging to the 3Σ u − state.