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AIP conference proceedings

Linear and circular dichroism in two-step photoionization of barium atoms

Sergei V. Bobashev, N. A. Cherepkov, A. Yu. Elizarov, V. K. Prilipko, В. В. Коршунов

Published Jan 1, 1995• 1 citation
Publisher

Abstract

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.

DOI
10.1063/1.47589

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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.
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{)}^{1})1P${\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}_{1}$ and the (5d6p)1{)}^{1})1P${\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}_{1}$ 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.
Stimulated by a recent measurement of coherent control in photoionization of atomic barium, we have calculated one- and two-photon ionization cross sections of the aligned 6s6p 1P16s6p\text{ }{^{1}P}_{1}6s6p 1P1​ state of barium in the energy range between the 5d3/25{d}_{3/2}5d3/2​ and 5d5/25{d}_{5/2}5d5/2​ states of Ba+{\text{Ba}}^{+}Ba+. We have also measured these photoionization spectra in the same energy region, driving the one- or two-photon processes with the second or first harmonic of a tunable dye laser, respectively. Our calculations employ the eigenchannel RRR-matrix method and multichannel quantum defect theory to calculate the rich array of autoionizing resonances in this energy range. The nonresonant two-photon process is described using lowest-order perturbation theory for the photon-atom interactions, with a discretized intermediate-state one-electron continuum. The calculations provide an absolute normalization for the experiment, and they accurately reproduce the rich resonance structures in both the one- and two-photon cross sections, and confirm other aspects of the experimental observations. These results demonstrate the ability of these computationally inexpensive methods to reproduce experimental observables in one- and two-photon ionization of heavy alkaline earths, and they lay the groundwork for future studies of the phase-controlled interference between one- and two-photon ionization processes.
We have observed and measured the phase lag in the phase-dependent variation of the asymmetric photoelectron angular distribution of atomic barium. For these measurements, we photoionize the 6s6p(1)P(1) intermediate state of barium with concurrent one-photon and two-photon (omega-2omega) interactions. The laser interactions ionize the atoms in the vicinity of the series of autoionizing states converging upon the 5d(2)D(5/2) threshold. We study the variation of the phase lag as a function of the laser frequency. The variation shows strong correlation to the location of the autoionizing resonances, with full range exceeding 2pi, confirming the critical role of these resonances.
The process of formation of doubly charged ions upon multiphoton ionization of barium atoms by linearly and circularly polarized radiation of dye lasers is studied in the frequency range from 16800 to 18700 cm −1 . A large number of resonance maxima in the yield of Ba + and Ba 2+ ions were observed upon tuning the radiation frequency. It is unambiguously proved that most resonance maxima in the yield of Ba 2+ ions are associated with the resonance transitions in the spectrum of Ba + ions. This fact confirms the cascade mechanism of formation of Ba 2+ ions in the frequency range of localization of these maxima.
An experiment is reported on two-step photoionization of Cs via the 6P 3/2 intermediate state using circularly polarized radiation from a GaAs laser for the first step (8521 AA) and from an Ar-ion laser for the second step. Values for the ratio of ion production rates for right- and left-handed circular polarization in the second step and for the spin polarization of the photoelectrons are presented. The measurements are interpreted on the basis of a calculation which takes into account the hyperfine splitting of the levels involved in the first transition. A description in terms of fine-structure splitting of the 6P state is shown to be totally inadequate, even when allowing for possibly large spin-orbit effects in the continuum. The process is evaluated as a possible source of a spin-polarized electron beam.