Measurement of the 4d-photoionization cross section via two-photon and two-step excitation in sodium

We present a comparison of the photoionization cross sections of the 4d D3/2,5/22 excited levels of sodium by using the two-step resonant laser excitation and by two-photon non-resonant excitation from the ground state. Dye lasers, pumped with a Nd: YAG laser, have been used in conjunction with a thermionic diode ion detector to measure cross sections and the atomic densities as a function of laser energy. By applying the saturation technique, the measured values of the cross sections and atomic densities for the 4d D5/22 level by two-photon excitation are 12.2(2.4) Mb and 6.7×1010cm-3, respectively. Where as in the case of two-step excitation, the cross section and number density for the 4d D3/22 level via 3p P1/22 level and for the 4d D3/2,5/22 levels via 3p P3/22 level are determined as 9.6(1.9) Mb, 4.1×1010cm-3 and 12.8(2.5) Mb, 3.7×1010cm-3, respectively.