C 1sionization inC2H2studied 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 σ1s orbital in the molecule ${\mathrm{C}}_2$ ${\mathrm{H}}_2$. 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 σ1s 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.