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Shape resonances occur when photoelectrons are temporarily trapped by a potential energy barrier in the molecule. Because the shape of the potential depends upon the configuration of the molecule, it was initially thought that shape resonances might be a simple way to obtain information about molecular geometry. In particular, it was thought that the energy at which a shape resonance occurred could be correlated with the length of molecular bonds. An attempt was made 15 years ago to quantify the relationship between shape-resonance energy and bond length for a number of molecules, including acetylene. However, shape resonances are manifested as broad, somewhat ambiguous peaks in the photoelectron cross section, and the criteria for identifying shape resonances in general were unclear.