ELECTRON-EXCITATION CROSS-SECTION FOR THE 2S(2)2P(3) 4S(DEGREES)-]2S(2)2P(3) 2D(DEGREES) (FORBIDDEN) AND S-4(DEGREES)-]2S2P(4)-4P (RESONANCE) TRANSITIONS IN O-II

Experimental and theoretical excitation cross sections are reported fo r the first forbidden transition S-4(0) --> 2S(2)2p(3 2) D-0 (lambda l ambda 3726, 3729) and the first allowed (resonance) transition S-4(0) --> 2s2p(4 4) P (lambda 833) in O II. Use is made of electron energy l oss and merged-beams methods. The electron energy range covered is 3.3 3 (threshold) to 15 eV for the S --> D transition, and 14.9 (threshold ) to 40 eV for the S --> P transition. Care was taken to assess and mi nimize the metastable fraction of the O II beam. An electron mirror wa s designed and tested to reflect inelastically backscattered electrons into the forward direction to account for the full range of polar sca ttering angles. Comparisons are made between present experiments and 1 1-state R-matrix calculations. Calculations are also presented for the S-4(0) --> 2S(2)2p(3) P-2(0) (lambda 2470) transition.