Absolute cross sections for photoionization of laser-excited BaI states measured on a thermionic diode

We describe a method that allows the measurement of absolute photoionization cross sections with a simple experimental setup. In a barium oven designed as a thermionic diode, ions are produced by use of a two-step pulsed-laser excitation and ionization combined with optogalvanic detection. The variation of the photoionization cross section of the excited BaI 6s6p ${}_{}{}^1{}_{}{}^{}°_1^{}{}_{}{}^{}$ state is measured versus wavelength. At threshold the cross section is (5±2)×${10}^{\mathrm{-}20}$ ${\mathrm{m}}^2$. In the experiment strong evidence for the existence of ${\mathrm{Ba}}_2^{+}$ dimers was found.