C 1s ionization in C2H2 studied by asymmetric (e,2e) experiments.

Abstract

The dynamics of core ionization by electron impact is investigated through the measurement of the triply differential cross section of the C \ensuremath{\sigma}1s 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 \ensuremath{\sigma}1s peak with respect to the expected value as measured by x-ray photoelectron spectroscopy. The amplitude of the shift amounts to 0.46\ifmmode\pm\else\textpm\fi{}0.23 eV at 9.6 eV excess energy, and it is too large to be explained only in terms of postcollision interactions.