  We develop a formalism for photoionization (PI) and potential energy curves
(PECs) of Rydberg atoms in ponderomotive optical lattices and apply it to
examples covering several regimes of the optical-lattice depth. The effect of
lattice-induced PI on Rydberg-atom lifetime ranges from noticeable to highly
dominant when compared with natural decay. The PI behavior is governed by the
generally rapid decrease of the PI cross sections as a function of
angular-momentum ($\ell$), and by lattice-induced $\ell$-mixing across the
optical-lattice PECs. In GHz-deep lattices, $\ell$-mixing leads to a rich PEC
structure, and the significant low-$\ell$ PI cross sections are distributed
over many lattice-mixed Rydberg states. In lattices less than several
tens-of-MHz deep, atoms on low-$\ell$ PECs are essentially $\ell$-mixing-free
and maintain large PI cross sections, while atoms on high-$\ell$ PECs trend
towards being PI-free. Characterization of PI in GHz-deep Rydberg-atom lattices
may be beneficial for optical control and quantum-state manipulation of Rydberg
atoms, while data on PI in shallower lattices are potentially useful in
high-precision spectroscopy and quantum-computing applications of
lattice-confined Rydberg atoms.
