MEASUREMENT OF ABSOLUTE DIFFERENTIAL CROSS-SECTIONS FOR VIBRATIONAL-EXCITATION OF O-2 BY ELECTRON-IMPACT

New measurements of absolute differential cross sections (DCSS) for vi brational excitation of O-2, 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 upsilon = 7. A weak continuous, probably non-resonant, background sca ttering is found in the upsilon = 1 cross section in addition to the w ell known sharp (2) Pi(g) resonances dominating vibrational excitation in the energy range up to 2.5 eV. The sharp (2) Pi(g) resonances inte rfere coherently with the background scattering both in the upsilon = 1 and in the elastic channels. New absolute values for the energy-inte grated DCSS for (2) Pi(g) resonances are given. They exhibit oscillato ry Franck-Condon factors in the upsilon = 4-7 exit channels. The broad resonance peaking at 9 eV, dominating vibrational DCSS at higher ener gies, causes excitation of high vibrational levels (up to the dissocia tion limit) of the X(3) Sigma(g)(-) ground state, but not of the a(1) Delta(g) and b(1) Sigma(g)(+) electronically excited states, supportin g its assignment as the (4) Sigma(u)(-) resonance, the selectivity bei ng caused by spin selection rules. The excitation of high v's indicate s a relatively narrow autodetachment width for this resonance. The ang ular dependence at 9 eV is, however, not a simple p(sigma) wave as wou ld be expected for a simple sigma(mu) shape resonance. Preliminary ex periments with a free jet cooled sample, concerned with rotational bro adening of the (2) Pi(g) resonance, are reported.