PHOTOIONIZATION OF POLARIZED AR-ASTERISK(4P, J=3) ATOMS NEAR-THRESHOLD

Using a crossed atomic and laser beams set-up with mass spectrometric ion detection we have studied the photoionization of laser-excited, al igned Ar(3p(5)4p(3)D(3)) atoms From the Ar+(P-2(3/2)) threshold up to photoelectron energies of epsilon = 0.846 eV. Absolute cross sections are reported for parallel (eta = 0 degrees) and perpendicular (eta = 90 degrees) polarization directions of the linearly polarized exciting and ionizing CW lasers over the range epsilon = 0-0.2 eV. The cross s ections exhibit nd'(J = 2. 3) autoionization resonances whose combined appearance varies with angle eta; this behaviour is traced to the eta dependence of the partial cross sections for the final (ion + electro n) channels with different total angular momentum, J(1). Using theoret ical formulae whose derivation is outlined we determine the energy dep endence of these partial cross sections from least squares fits to the data measured at eta = 0 degrees and 90 degrees. Experimental results are also presented for the asymmetry function, A(LD)(eta), which desc ribes the linear dichroism for the photoionization of aligned Ar(4p(3 )D(3)) atoms, and for the full II dependence of the ionization cross s ection as obtained for 10 selected photoelectron energies in the range (0.0170-0.846) eV. Many-electron calculations have been carried out w hich provide additional insight into the photoionization dynamics. In contrast to the situation for Ne(3p, J = 3), photoionization of Ar*(4 p, J = 3) is not well described by just two reduced matrix elements D- s, D-d. For Ar the term dependence of the Hartree-Fock wavefunctions and correlation effects lead to substantial differences between the re duced matrix elements for the five d-wave channels associated with the Ar+(P-2(3/2)) formation. Satisfactory overall agreement is observed b etween the experimental and theoretical cross sections and their polar ization dependence. The calculations also provide information on the w eak g-wave contributions and on the cross section for leaving the ion in the excited Ar+(P-2(1/2)) level; the latter amounts to about 1% rel ative to Ar+(P-2(3/2)) formation in agreement with experimental and MQ DT estimates.