Reappraisal of the Existence of Shape Resonances in the Series C2H2, C2H4, and C2H6

We have measured the main line (single hole) C 1s photoionization cross sections for C2H2, C2H4, and C2H6 in the immediate threshold region. These three molecules occupy a central place in the proposed correlation between shape resonance position and molecular geometry. We find no clearly identifiable shape resonances in the single hole cross sections of C2H4 and C2H6 and only weak evidence for one in C2H2.