Photoionization-cross-section measurement of the Rb85 5 P3/22 excited state using microwave cavity resonance spectroscopy

Abstract

We present microwave cavity resonance spectroscopy as a technique to determine excited-state photoionization cross sections of laser-cooled atoms. We demonstrate this technique by measuring the photoionization cross section of the $5^{2}P_{3/2}({F}^{\ensuremath{'}}=4)$ excited state of $^{85}\mathrm{Rb}$ to continuum by creating an ultracold plasma inside a 5-GHz resonant microwave cavity. We find a photoionization cross section of $\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}22}\phantom{\rule{4pt}{0ex}}{\mathrm{m}}^{2}$ for photon excess energies of 50, 100, 200, and 500 K above the ionization threshold. The measurements yield a cross section which is approximately 2 to 3 times smaller than the value provided by existing theory for the $5p$ excited state of $^{85}\mathrm{Rb}$ and two earlier measurements performed with other techniques for similar excess energies.