Absolute measurement of the photoionization cross section of the excited7Sstate of cesium

We report the first measurement of the absolute cross section for photoionization of the $7S$ state of cesium. The measurement employed a new technique in which the density of the excited-state atoms was determined by the amount of fluorescence. The cross section for photoionization by 540-nm light is 1.14(10)×${10}^{-19\mathrm{}}$ ${\mathrm{cm}}^2$. We also propose a second new technique for the absolute measurement of photoionization cross sections which is based on modulated fluorescence.