Multiple-scattering X alpha calculation of spectral energies involving the π and π* orbitals of C2H2

Bound-state and continuum multiple-scattering (MS) X alpha calculations on C 2 H 2 yield values accurate to within 0.8 eV for the pi IP, the average pi to pi * excitation energy, the C 1s to pi and C 1s to pi * X-ray excitation energies, the pi to pi * C 1s shake-up satellite energy, the pi * attachment energy and the pi 4 to pi 3 sigma * 2 Feshbach resonance energy. The derivative electron transmission spectrum of C 2 H 2 is shown for the first time. Quantitative relationships between the different spectral properties are established using the transition state approach.