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AIP conference proceedings

Multistep excitation of alkaline earth atoms to autoionizing states

Thomas F. Gallagher

Published Jan 1, 1982• 1 citation
Publisher

Abstract

Using multistep visible laser excitation of autoionizing states, it is possible to excite atoms from the ground state to autoionizing states lying as much 10,000 cm−1 above the ground state. In addition to the fact that the approach allows one to easily reach states lying at high energies, it also considerably simplifies the interpretation of the observed spectra, becuase typically, one excitation is orders of magnitude stronger than the others. As a result, the Beulter‐Fano interference phoenomenon associated with the vuv excitation of autoionizing states is absent. We describe the multistep method and its application in a recent study of Ba 6pnd states, which lie ∼60,000 cm−1 above the ground state. In addition to illustrating the method, this study shows how the inherent simplicitiy of the approach allows one to observe the interaction between the 6p1/2nd and 6p3/2 series.

DOI
10.1063/1.33785

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One- and two-photon ionization cross sections of the laser-excited 6s6p P11 state of barium

John R. Tolsma, Daniel J. Haxton, Chris H. Greene +2 more

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 $6s6p\text{ }{^{1}P}_{1}$ state of barium in the energy range between the $5{d}_{3/2}$ and $5{d}_{5/2}$ states of ${\text{Ba}}^{+}$. 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 $R$-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.

arXiv:0906.1638•Physical Review A•Sep 1, 2009•9 citations
Using multistep visible laser excitation of autoionizing states, it is possible to excite atoms from the ground state to autoionizing states lying as much 10,000 cm−1 above the ground state. In addition to the fact that the approach allows one to easily reach states lying at high energies, it also considerably simplifies the interpretation of the observed spectra, becuase typically, one excitation is orders of magnitude stronger than the others. As a result, the Beulter‐Fano interference phoenomenon associated with the vuv excitation of autoionizing states is absent. We describe the multistep method and its application in a recent study of Ba 6pnd states, which lie ∼60,000 cm−1 above the ground state. In addition to illustrating the method, this study shows how the inherent simplicitiy of the approach allows one to observe the interaction between the 6p1/2nd and 6p3/2 series.
Multistep laser excitation makes it possible to excite one electron at at time in the excitation of doubly excited states. This suppresses the direct continuum excitation, removing the familiar Beutler–Fano interference profiles from the excitation of autoionizing states. Removing this complexity in the excitation process allows one to investigate inherently complex atomic systems, such as interacting autoionizing series. The ability to examine such systems experimentally has led to the development of theoretical techniques based on quantum-defect theory to interpret the experimental results. This powerful combination of complementary experimental and theoretical techniques has led to substantial advances in the study of autoionizing states.
Traditional schemes for multistep resonance photoionization of atoms let every employed laser beam interact with the atoms simultaneously. In such a situation, analyses via time-dependent Schr\"odinger equation show that high ionization probability requires all the laser beams must be intense enough. In order to decrease laser intensity, we proposed a scheme that the laser beam used to pump the excited atoms (in a higher bound state) into an autoionization state does not interact with the atoms until all the population is transferred by the other lasers from a ground state to the bound state. As an interesting example, we examined three-step photoionization of 235U{}^{235}\mathrm{U}235U with our scheme, showing that the intensity of two laser beams can be lowered by two orders of magnitude without losing high ionization probability.
Using a three-step laser saturation excitation technique, the saturation effects on the Ba 6p n s ( J = 1) and 6p n d ( J = 1, 3) autoionization spectra are observed systemically in zero field. These saturation spectra are introduced to determine the high n members of 6p nl ( l = 0, 2) autoionizing series and are used to analyse the channel interactions among the autoionizing series in zero field. Furthermore, the saturation excitation technique is applied to the electric field case, in which the saturation spectra of Ba 6p nk (| M | = 0, 1) autoionizing Stark states are measured. Most of these saturation spectra are observed for the first time so far as we know, which indicate the mixing of the autoionizing states in the electric fields.
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.