  Shape resonances in photoionization of atoms and molecules arise from a
particular geometry of the ionic potential which traps the receding
photoelectron in a quasi-bound state in a particular partial wave. This
mechanism allows us to connect the photoionization cross section in the
resonant region with the photoelectron scattering phase in this partial wave by
a simple formula $\sigma \propto \sin^2\delta_\ell$. Due to this relation, the
phase $\delta_\ell$ can be extracted from an experimentally known cross section
and then converted to the photoelectron group delay (Wigner time delay)
$\tau_{\rm W} = \partial \delta_\ell/\partial E$ which is measurable by
recently developed laser interferometric techniques. Such a direct connection
of the photoionization cross section and the time delay is a fundamental
property of shape resonances which provides a comprehensive test of novel
measurements against a large body of older synchrotron data.
