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.
Categories: physics.atom-ph quant-ph