  We present calculations of the photoionization (PI) cross sections of
rubidium and cesium Rydberg atoms for light with wavelengths ranging from the
infrared to the ultraviolet, using model potentials from [M. Marinescu, H. R.
Sadeghpour, and A. Dalgarno, Phys. Rev. A 49, 982 (1994)]. The origins of
pronounced PI minima are identified by investigating the free-electron
wavefunctions. These include broad PI minima in the $nS$ to $\epsilon P$ PI
channels of both Rb and Cs, with free-electron energy $\epsilon$, which are
identified as Cooper minima. Much narrower PI minima in the $nD$ to $\epsilon
F$ channels are due to shape resonances of the free-electron states. We
describe possible experimental procedures for measuring the PI minima, and we
discuss their implications in fundamental atomic physics as well as in
practical applications.
