  We use double-resonant two-photon laser spectroscopy to measure the dynamic
scalar polarizability of the rubidium 5$D_{3/2}$ level,
$\alpha^{S}_{5D_{3/2}}$, at a wavelength of $\lambda=1064$~nm. Since $\lambda$
is shorter than the photoionization (PI) limit of the Rb 5$D_{3/2}$ level,
$\alpha^{S}_{5D_{3/2}}$ depends on both bound-bound and bound-free transition
matrix elements. The level also undergoes significant broadening due to PI. The
$1064$-nm field is applied in the form of a deep optical lattice
($\sim10^{5}$~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, $\alpha^{S}_{5D_{3/2}}=-524(17)$, in agreement with estimates.
Additionally, we extract the $5D_{3/2}$ photoionization cross section $\sigma$
at $1064$~nm from spectral linewidths; we find $\sigma=44(1)$~Mb.

Atomic Physics;Quantum Physics