  We employ doubly-resonant two-photon excitation into the 74S Rydberg state to
spectroscopically measure the dynamic scalar polarizability, alpha 0, and
tensor polarizability, alpha 2, of rubidium 5P3/2. To reach the necessary high
intensities, we employ a cavity-generated 1064 nm optical-lattice light field,
allowing us to obtain intensities near 2x10^11 W/m^2. In the evaluation of the
data we use a self-referencing method that renders the polarizability
measurement largely free from the intensity calibration of the laser light
field. We obtain experimental values alpha 0 =-1149 (pm 2.5 percent) and alpha
2 = 563 (pm 4.2 percent), in atomic units. Methods and results are supported by
simulations.
