Investigation of effective ionization rate coefficients of Al I and Zr I in a plasma using laser produced atomic beams

Abstract

Al I and Zr I atomic as well as Zr II ionic beams were produced by laser irradiation of Al and Zr targets. The beams were injected into a plasma, and effective ionization rate coefficients were derived by analyzing the beam densities both with and without the plasma present. Because of the large number of intermediate levels, the Zr I atoms are ionized much faster than the Al I atoms under the same plasma conditions, in spite of the fact that the ionization energy from the ground state for Zr I is higher than that for Al I. The results are compared with theoretical effective ionization rate coefficients assuming a simplified two-step ionization process. For Zr I, the measured effective ionization rate coefficients were found to be two to five times the sum of the excitation rate coefficients for electric dipole-allowed transitions and for Al I one to three times. These results may represent a limiting case, in which the effective ionization rate coefficient is nearly equal to the sum of all excitation rate coefficients including electric dipole-allowed as well as multipole and forbidden transitions.