Measurement of absolute differential cross sections for vibrational excitation of O₂ by electron impact

New measurements of absolute differential cross sections (DCSS) for vibrational excitation of O₂, with improved sensitivity and resolution, are presented. The 90 degrees DCSS are given as a function of energy up to 16 eV for elastic scattering and vibrational excitation of up to v=7. A weak continuous, probably non-resonant, background scattering is found in the v=1 cross section in addition to the well known sharp ² Pi _g resonances dominating vibrational excitation in the energy range up to 2.5 eV. The sharp ² Pi _g resonances interfere coherently with the background scattering both in the v=1 and in the elastic channels. New absolute values for the energy-integrated DCSS for ² Pi _g resonances are given. They exhibit oscillatory Franck-Condon factors in the v=4-7 exit channels. The broad resonance peaking at 9 eV, dominating vibrational DCSS at higher energies, causes excitation of high vibrational levels (up to the dissociation limit) of the X ³ Sigma _g^- ground state, but not of the a¹ Delta _g and b¹ Sigma _g⁺ electronically excited states, supporting its assignment as the 4 Sigma _u^- resonance, the selectivity being caused by spin selection rules. The excitation of high vs indicates a relatively narrow autodetachment width for this resonance. The angular dependence at 9 eV is, however, not a simple p_sigma wave as would be expected for a simple sigma _u* shape resonance. Preliminary experiments with a free jet cooled sample, concerned with rotational broadening of the ² Pi _g resonance, are reported.