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 D-2(3/2,5/2) 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 D-2(5/2) level by two-photon excitation are 12.2(2.4)Mb and 6.7 x 10(10) cm(-3), respectively. Where as in the case of two-step excitation, the cross section and number density for the 4d D-2(3/2) level via 3p P-2(1/2) level and for the 4d D-2(3/2,5/2) levels via 3p P-2(3/2) level are determined as 9.6(1.9) Mb, 4.1 X 10(10) cm(-3) and 12.8(2.5) Mb, 3.7 x 10(10) cm(-3), respectively. (C) 2006 Elsevier Ltd. All rights reserved.