Reappraisal of the Existence of Shape Resonances in the Series C{sub 2}H{sub 2} , C{sub 2}H{sub 4} , and C{sub 2}H{sub 6}

We have measured the main line (single hole) C 1s photoionization cross sections for C{sub 2}H{sub 2} , C{sub 2}H{sub 4} , and C{sub 2}H{sub 6} 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 C{sub 2}H{sub 4} and C{sub 2}H{sub 6} and only weak evidence for one in C{sub 2}H{sub 2} . {copyright} {ital 1997} {ital The American Physical Society}