On the direct and dissociative excitation of the O(3s3S0) state by electron impact on atomic and molecular oxygen

The excitation of the O (3s 3 S 0 to 2p 3 P; lambda 1304 AA) resonance transition by electron impact on atomic and molecular oxygen has been studied. The ratio of the cross sections for the specific atomic and dissociative processes, sigma A / sigma D , has been measured accurately from 60 to 300 eV by direct normalisation to the ratio of the specific O + and O 2 + ionisation cross sections. At 100 eV, sigma A / sigma D =3.00+or-9%. The experiment was carried out using an apparatus that combined a high-density diffuse gas source (n(O) approximately 10 12 cm -3 ), an electrostatically focused electron gun, a vacuum-ultraviolet monochromator and a quadrupole mass spectrometer for simultaneous optical and composition measurements. The present sigma A / sigma D values are smaller by a factor of 2.1 than the original results of Stone and Zipf (1974) and Mumma and Zipf (1971). When revised sigma A (1304 AA) values are calculated from these new data using contemporary calibration standards, it is found that the shape of the cross section for the excitation of the O (3s 3 S 0 ) state by electron impact on atomic oxygen agrees well with the earlier work but the absolute magnitude of sigma A (1304 AA) must be reduced by a factor of 2.8. The revised cross section values are in good agreement with recent quantum calculations when cascade excitation of the 3s 3 S 0 state is taken into account.