Measurement of the Absolute Photoionization Cross Section for the 5P_3/2 State of ⁸⁷Rb in a Vapor Cell Magneto-optic Trap

We report the measurement of the absolute photoionization cross section for the 5P_3/2 state of ⁸⁷Rb at wavelength of 473 nm, which results in the photoelectron energies of 33 meV above the ionization threshold, using cold atoms confined in a vapor−loaded magneto-optical trap. The ⁸⁷Rb 5P_3/2 photoionization cross section at 473 nm is determined to be σ_PI=10.5±2.2 Mb. Considering the spatial distribution of the trapped atoms, the average intensity I_PI of the ionization laser seen by an atom in the MOT instead of ionizing laser intensity I_PI is used in our calculations for the photoionization cross sections. The excited state fraction is also accurately estimated using the latest experimental result.