Generalized oscillator strengths for C 1s excitation of acetylene and ethylene

The generalized oscillator strength profiles for discrete C 1s excited states of C2H2 and C2H4 have been derived from angle-dependent inelastic electron scattering cross-sections measured with 1300 eV final electron energy. The measured GOS profiles for the strong C 1s→π* transition in each species are compared to theoretical calculations computed within the first Born approximation, using ab-initio generalized multi structural wave functions. These wave functions include relaxation, correlation and hole localization effects. Theory predicts large quadrupole contributions to the π* GOS of each species, analogous to those previously reported for computed GOS profiles for O 1s→π* excitation of CO2. We find good agreement between experiment and theory as to the shape of the π* GOS but, when the relative GOS extracted from the experimental data is normalized to the optical oscillator strength at K2=0, the magnitude is in better agreement with the GOS computed for only the dipole channel than for the sum of the dipole and quadrupole channels.