Photoionization of polarized (4 p,<b><i>J</i></b>= 3) atoms near threshold

Abstract

Using a crossed atomic and laser beams set-up with mass spectrometric ion detection we have studied the photoionization of laser-excited, aligned atoms from the threshold up to photoelectron energies of . Absolute cross sections are reported for parallel and perpendicular polarization directions of the linearly polarized exciting and ionizing CW lasers over the range . The cross sections exhibit (J = 2,3) autoionization resonances whose combined appearance varies with angle ; this behaviour is traced to the dependence of the partial cross sections for the final (ion + electron) channels with different total angular momentum, . Using theoretical formulae whose derivation is outlined we determine the energy dependence of these partial cross sections from least squares fits to the data measured at and . Experimental results are also presented for the asymmetry function, , which describes the linear dichroism for the photoionization of aligned atoms, and for the full dependence of the ionization cross section as obtained for 10 selected photoelectron energies in the range (0.017 - 0.846) eV. Many-electron calculations have been carried out which provide additional insight into the photoionization dynamics. In contrast to the situation for (3p, J = 3), photoionization of (4p, J = 3) is not well described by just two reduced matrix elements , . For the term dependence of the Hartree - Fock wavefunctions and correlation effects lead to substantial differences between the reduced matrix elements for the five d-wave channels associated with the formation. Satisfactory overall agreement is observed between the experimental and theoretical cross sections and their polarization dependence. The calculations also provide information on the weak g-wave contributions and on the cross section for leaving the ion in the excited level; the latter amounts to about 1% relative to formation in agreement with experimental and MQDT estimates.