MEASUREMENT OF ABSOLUTE DIFFERENTIAL CROSS-SECTIONS FOR THE EXCITATION OF THE PI,PI-ASTERISK TRIPLET-STATE OF ETHENE BY ELECTRON-IMPACT AT 0-DEGREES AND 180-DEGREES

A recently developed magnetically collimated electron impact spectrome ter with pulsed incident electron beam and time-of-flight selection of electrons scattered in the forward and backward directions was used t o measure differential cross sections (DCSs) for the excitation of the valence triplet state of ethene at scattering angles of 0 degrees and 180 degrees. The measurements are complemented by data at intermediat e angles measured with a conventional spectrometer using hemispherical electron-energy analyzers. A pronounced trend is found in the DCS mea sured as a function of incident electron energy at 0 degrees, 90 degre es, and 180 degrees: a band assigned as the (2)(pi,pi(2)) core excite d shape resonance becomes substantially wider and moves to higher ener gies with increasing angle of observation. The DCS have also been meas ured as a function of scattering angle, in the range 0 degrees-135 deg rees and at 180 degrees, at 1.5, 3, and 10 eV above threshold. The DCS was found to be strongly backward peaked at energies higher than 5 eV above threshold. The results at 0 degrees and 180 degrees are found t o be a sensitive test of the theory. All qualitative trends are reprod uced well by recent published Schwinger multichannel and complex Kohn theories, but quantitative differences are found in the absolute magni tude of the DCSs and the position of the resonances. (C) 1997 American Institute of Physics.