We use double-resonant two-photon laser spectroscopy to measure the dynamic scalar polarizability of the rubidium
5D3/2 level, ${\ensuremath{\alpha}}_{5{D}_{3/2}}^{S}$, at a wavelength of $\ensuremath{\lambda}=1064$ nm. Since $\ensuremath{\lambda}$ is shorter than the photoionization (PI) limit of the Rb
5D3/2 level, this state undergoes significant broadening due to PI. The 1064-nm field is applied in the form of a deep optical lattice ($\ensuremath{\sim}{10}^{5}$ photon recoils) generated by an in-vacuum field-enhancement cavity. In our spectroscopic method, we use known dynamic polarizabilities to eliminate the need to measure the light intensity. Our method yields, in atomic units, ${\ensuremath{\alpha}}_{5{D}_{3/2}}^{S}=\ensuremath{-}524(17)$, in agreement with estimates. Additionally, we extract the
5D3/2 photoionization cross section $\ensuremath{\sigma}$ at 1064 nm from spectral linewidths; we find $\ensuremath{\sigma}=44(1)$ Mb.