Photoionization-cross-section measurement of the ⁸⁵Rb 5 ²P_3/2 excited state using microwave cavity resonance spectroscopy

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 ²P_3/2(F′ = 4) excited state of ⁸⁵Rb to continuum by creating an ultracold plasma inside a 5-GHz resonant microwave cavity. We find a photoionization cross section of ∼5 × 10⁻²² m² 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 ⁸⁵Rb and two earlier measurements performed with other techniques for similar excess energies.