C 1S IONIZATION IN C2H2 STUDIED BY ASYMMETRIC (E,2E) EXPERIMENTS

The dynamics of core ionization by electron impact is investigated through the measurement of the triply differential cross section of the C 1s orbital in the molecule C2H2. The (e,2e) experiments have been performed under asymmetric conditions and at small scattering angles, with a scattered electron energy of 1500 eV and low energies of the ejected electrons (9.6 and 41.0 eV). The measured angular distributions are characterized by large-size recoil lobes, breaking of the symmetry around the momentum-transfer direction, and unusual deviations of the maxima of the recoil peaks towards smaller deflection angles. In the (e,2e) energy spectrum a shift is observed in the position of the C 1s peak with respect to the expected value as measured by x-ray photoelectron spectroscopy. The amplitude of the shift amounts to 0.46 0.23 eV at 9.6 eV excess energy, and it is too large to be explained only in terms of postcollision interactions. © 1990 The American Physical Society.