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Physical Review A

Autoionizing states of negative ions in strong resonant laser fields: The negative rubidium ion

Henrik Stapelfeldt, Philip Trøst Kristensen, Ulric B. Ljungblad, T. Andersen, Harold K. Haugen

Published Aug 1, 1994• 17 citations
Publisher

Abstract

We have performed an experimental study of the atomic system ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$, for which the ground state 5${\mathit{s}}^{2}$ is coupled to the autoionizing state 5${\mathit{p}}_{1/2}$6s by means of a strong resonant laser field. The investigations are based on irradiation of ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$ with intense (I\ensuremath{\le}1.5\ifmmode\times\else\texttimes\fi{}${10}^{10}$ W/${\mathrm{cm}}^{2}$) laser pulses of duration \ensuremath{\sim}1 ns. Single-photon detachment of ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$ is suppressed due to the Fano interference between transitions directly to the continuum and transitions via the autoionizing state. In contrast, two-photon absorption is at the same time resonantly enhanced since no such destructive interference is present in this process. These observations constitute experimental evidence of general features predicted from theoretical considerations of autoionizing states in strong laser fields. Furthermore, we discuss the role of autoionizing states in multiphoton double-ionization processes. Electron spectroscopy has been used to show that double ionization of ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$ primarily is the result of a sequential removal of the two electrons via an excited Rb state. The perspectives for direct two-electron ionization in negative ions are briefly discussed.

DOI
10.1103/physreva.50.1618

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We have performed an experimental study of the atomic system Rb\ensuremath−{\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}Rb\ensuremath−, for which the ground state 5s2{\mathit{s}}^{2}s2 is coupled to the autoionizing state 5p1/2{\mathit{p}}_{1/2}p1/2​6s by means of a strong resonant laser field. The investigations are based on irradiation of Rb\ensuremath−{\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}Rb\ensuremath− with intense (I\ensuremath{\le}1.5\ifmmode\times\else\texttimes\fi{}1010{10}^{10}1010 W/cm2{\mathrm{cm}}^{2}cm2) laser pulses of duration \ensuremath{\sim}1 ns. Single-photon detachment of Rb\ensuremath−{\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}Rb\ensuremath− is suppressed due to the Fano interference between transitions directly to the continuum and transitions via the autoionizing state. In contrast, two-photon absorption is at the same time resonantly enhanced since no such destructive interference is present in this process. These observations constitute experimental evidence of general features predicted from theoretical considerations of autoionizing states in strong laser fields. Furthermore, we discuss the role of autoionizing states in multiphoton double-ionization processes. Electron spectroscopy has been used to show that double ionization of Rb\ensuremath−{\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}Rb\ensuremath− primarily is the result of a sequential removal of the two electrons via an excited Rb state. The perspectives for direct two-electron ionization in negative ions are briefly discussed.
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The multiphoton excitation of rubidium has been investigated over the 4600-6500-\AA{} wavelength region with a tunable dye-laser source having a linewidth better than 0.1 \AA{} and a space-charge ionization detector sensitive to a few ions per second. Multiphoton transitions have been observed to occur both through intermediate atomic states and through intermediate continuum states of the rubidium molecule. In the former case two-photon transitions have been observed from the 52S5^{2}S52S ground state of atomic rubidium to higher-lying n2Dn^{2}Dn2D levels for values of nnn ranging from 9 to 34 and to n2Sn^{2}Sn2S levels for values of nnn from 11 through 20. The fine-structure intervals of the n2Dn^{2}Dn2D levels for n=9toc13n=9 \mathrm{to}c 13n=9toc13 were measured together with the line-strength ratios and were found to be in good agreement with the predictions of a simple theoretical model. At the shorter wavelengths hybrid two-photon resonances were observed to be excited through resonant intermediate continuum states of Rb2{\mathrm{Rb}}_{2}Rb2​. As a result the dispersion curve for two-photon absorption in rubidium showed what appears to be resonant intermediate $p\ensuremath{\pi}(^{3}\ensuremath{\Sigma}_{u1})$ and 0g+{0}_{g}^{+}0g+​ terms dissociating to give a 5P325{P}_{\frac{3}{2}}5P23​​ atom and resulting in the strong development of features corresponding to the $5^{2}P_{\frac{3}{2}}\ensuremath{\rightarrow}n^{2}D_{\frac{3}{2},\frac{5}{2}}$ part of the diffuse series, for $n\ensuremath{\le}50$ in absorption and the $5^{2}P_{\frac{3}{2}}\ensuremath{\rightarrow}n^{2}S_{\frac{1}{2}}$ part of the sharp series. Components to 322D32^{2}D322D and 302S30^{2}S302S were recorded to a precision of 0.3 ${\mathrm{cm}}^{\ensuremath{-}1}$ and quantum defects for these previously unobserved terms were derived. The corresponding hybrid two-photon resonances involving intermediate states dissociating to give a 52P125^{2}P_{\frac{1}{2}}52P21​​ atom were not observed in the wavelength interval available in this experiment.
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