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 /sup 1/P/sup 0//sub 1/ state is measured versus wavelength. At threshold the cross section is (5 +- 2) x 10/sup -20/ m/sup 2/. In the experiment strong evidence for the existence of Ba/sub 2//sup +/ dimers was found.