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Physical Review A

Path dependence in resonant multiphoton excitation to autoionizing states of barium

James S. Keller, John E. Hunter, R. Stephen Berry

Published Mar 1, 1991• 19 citations
Publisher

Abstract

We measure photoion spectra and angular distributions of photoelectrons from the two-color, two-photon resonant ionization of barium atoms in order to probe the effect of the photoionization pathway on autoionization processes. The (6s6p${)}^{1}$P${\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}_{1}$ and the (5d6p${)}^{1}$P${\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}_{1}$ states are used as intermediates in the photoexcitation of barium to energies in the range of a series of 5dnd autoionizing states. Dramatic differences in the profiles of autoionizing lines are observed for excitation pathways involving different intermediate states. Angular distributions of photoelectrons collected across the range of autoionizing resonances demonstrate strong variations that follow constraints imposed by the angular momentum of the autoionizing peaks and also suggest significant spin-orbit interactions in the photoionization step.

DOI
10.1103/physreva.43.2270

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We have observed the phase lag in the sinusoidal variation of the asymmetric photoelectron angular distributions into two ionization channels of atomic barium using an $\ensuremath{\omega}\text{\ensuremath{-}}2\ensuremath{\omega}$ excitation scheme. We excite atomic barium from the 6s6p 1P16s6p\text{ }{^{1}P}_{1}6s6p 1P1​ intermediate state electron to an energy between the 5d3∕2ϵ5{d}_{3∕2}ϵ5d3∕2​ϵ and the 5d5∕2ϵ5{d}_{5∕2}ϵ5d5∕2​ϵ thresholds with concurrent one-photon and two-photon interactions. The left-right asymmetry of the photoelectron angular distributions for the two energy-resolved product states, 6s1∕2ϵ6{s}_{1∕2}ϵ6s1∕2​ϵ and 5d3∕2ϵ5{d}_{3∕2}ϵ5d3∕2​ϵ, show sinusoidal modulation as a function of the relative phase of the $\ensuremath{\omega}\text{\ensuremath{-}}2\ensuremath{\omega}$ fields. The phase difference between these signals, or phase lag, varies rapidly with the laser frequency when the interaction is nearly resonant with autoionizing resonances in the continuum. An exceptionally large phase lag variation of over $2\ensuremath{\pi}$ was measured.
The line contour of the autoionizing resonance of 6P7P configuration in barium atoms was experimentally studied in the range of energies. 52,100 cm−1 with the help of a two‐step ionization of via intermediate states 6s6p (1P1), 6s8p (1P1). Using various combinations of polarization of the first‐ and second‐step laser, both linear and circular dichroism were meausred in the process of ionization from an excited state. Theoretical expressions governing linear and circular dichroism are derived. Using these expression, the values of two parameters used in the theory (the ratios of dipole matrix elements from the excited state, squared) were obtained from the experimental data.
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