Angular Dependence of Electron Impact Excitation Cross Sections of O2

The electron-impact excitation spectrum of O2 has been studied at 20 and 45 eV impact energies and at scattering angles ranging from 10° to 90°. The angular behavior of the differential scattering cross sections for excitation of the a 1Δg, b 1Σg +, B 3Σu − states, for the 9.97 eV (``longest'' band), and the 10.29 eV (``second'' band) transitions, for the broad feature at 6.1 eV energy-loss (previously assigned to the excitation of the A 3Σu + state), and for elastic scattering has been determined. The experimentally measured relative differential and integral cross sections for these processes have been approximately normalized to the absolute scale. The intensities of the different transitions in optical and electron-impact spectra are compared and the importance of spin-orbit coupling and exchange processes are discussed. It is found that the energy-loss feature at 6.1 eV in the electron-impact spectrum is mostly due to the excitation of the c 1Σu − state. This new assignment is supported by the angular and energy dependence of the scattering cross section (and by the intensity pattern found in various electron impact processes).