Pulsed laser light of three different wavelengths, which is an output from three tunable dye lasers pumped by two excimer lasers, is introduced into a vacuum reaction chamber in the same direction and then the light irradiates an atomic beam perpendicularly. The photoionized ions in the irradiated volume are extracted by a static electric field applied perpendicular to the plane containing the laser beam and the atomic beam, and are detected by a secondary electron multiplier. The output wavelengths of the tunable pulse lasers for the 1st and 2nd excitation steps are fixed at: the resonance processes. The tunable pulse laser for the 3rd step with the band width 0.2 cm(-1) is single-scanned at the repetition rate of 12 similar to 25 Hz around the ionization limit. A series of sharp peaks corresponding to the Rydberg states of Ni, Ag, and Pd are observed. When the ion signals between the 1-step, 2-step, and 3-step photoionization processes are compared with each other, the ion yields are found to be exceedingly increased through the high Rydberg states (the 3-step process). The least squares fittings of the Rydberg and Rydberg-Ritz formula to the observed Rydberg series yield the ionization potentials of E(i)(Ni) = 61616.3 cm(-1), E(i)(Ag) = 61104.3 cm(-1), and E(i)(Pd) = 67238.1 cm(-1).
