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Physical Review Letters

Photodetachment of He− in the Vicinity of the Two-Electron Escape Threshold

V. V. Petrunin, M. H. Jacobsen, Lars Bojer Madsen, Sergei A. Aseyev, T. Andersen

Published Jan 9, 2003• 1 citation
Publisher

Abstract

Recording the yield of $\mathrm{H}\mathrm{e}(1snl^{3}L)$ Rydberg states for $n=11--14$, we measure the photodetachment cross sections of metastable ${\mathrm{H}\mathrm{e}}^{\ensuremath{-}}(1s2s2p^{4}P^{o})$ ions in the vicinity of the two-electron escape threshold. We observe a large number of double Rydberg ${\mathrm{H}\mathrm{e}}^{\ensuremath{-}}$ quartet state resonances and report energies and widths of intrashell states in the $n=13--15$ manifolds. Sharp thresholds are measured at $\mathrm{H}\mathrm{e}(^{3}P^{o})$ and $\mathrm{H}\mathrm{e}(^{3}D^{e})$ Rydberg states with preference for population of the former, whereas the $\mathrm{H}\mathrm{e}(^{3}S^{e})$ states are not populated, in agreement with qualitative theoretical arguments.

DOI
10.1103/physrevlett.90.013002

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Recording the yield of He(1snl3L)\mathrm{H}\mathrm{e}(1snl^{3}L)He(1snl3L) Rydberg states for n=11−−14n=11--14n=11−−14, we measure the photodetachment cross sections of metastable He\ensuremath−(1s2s2p4Po){\mathrm{H}\mathrm{e}}^{\ensuremath{-}}(1s2s2p^{4}P^{o})He\ensuremath−(1s2s2p4Po) ions in the vicinity of the two-electron escape threshold. We observe a large number of double Rydberg He\ensuremath−{\mathrm{H}\mathrm{e}}^{\ensuremath{-}}He\ensuremath− quartet state resonances and report energies and widths of intrashell states in the n=13−−15n=13--15n=13−−15 manifolds. Sharp thresholds are measured at He(3Po)\mathrm{H}\mathrm{e}(^{3}P^{o})He(3Po) and He(3De)\mathrm{H}\mathrm{e}(^{3}D^{e})He(3De) Rydberg states with preference for population of the former, whereas the He(3Se)\mathrm{H}\mathrm{e}(^{3}S^{e})He(3Se) states are not populated, in agreement with qualitative theoretical arguments.
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A joint experimental and theoretical study of three-photon ionization of the 1s2p3Po1s2p{\phantom{\rule{0.16em}{0ex}}}^{3}{P}^{o}1s2p3Po(${M}_{L}=0,\ifmmode\pm\else\textpm\fi{}1$) states of helium is presented. The ion yield is recorded in the 690--730 nm wavelength range for different laser pulse energies, using an excited helium beam produced by photodetachment of helium negative ions. Two series of asymmetric peaks due to two-photon resonances with 1snp1snp1snp and 1snf1snf1snf Rydberg states are observed. In one series, the peaks have tails towards higher frequencies, while in the other series the tails change direction for higher Rydberg states. An effective Hamiltonian is built in the dressed state picture, and a numerical model simulating the traversal of the helium atom across the laser pulse is developed. The simulated and observed ion yields are in good qualitative agreement. The observed behavior is shown to result from the contributions of two different resonantly enhanced multiphoton ionization processes, depending on the magnetic quantum number ML{M}_{L}ML​ of the initial state. The asymmetry reversal is explained by the strong 1s2p1s2p1s2p--1s3s1s3s1s3s dynamic Stark mixing for ML=0{M}_{L}=0ML​=0.
A Feshbach resonance associated with the 1s3s4s4S{1s3s4s}^{4}S1s3s4s4S state of He${}^{\ensuremath{-}}$ has been observed in the He(1s2s3S){(1s2s}^{3}S)(1s2s3S) + ${e}^{\ensuremath{-}}(\ensuremath{\epsilon}s)$ partial photodetachment cross section. The residual He(1s2s3S){(1s2s}^{3}S)(1s2s3S) atoms were resonantly ionized and the resulting He+{}^{+}+ ions were detected in the presence of a small background. A collinear laser-ion beam apparatus was used to attain both high resolution and sensitivity. We measured a resonance energy Er=2.959255(7){E}_{\mathrm{r}}=2.959255(7)Er​=2.959255(7) eV and a width $\ensuremath{\Gamma}=0.19(3)$ meV, in agreement with a recent calculation.
A collinear beam apparatus has been used to study photodetachment of Cs${}^{\ensuremath{-}}$. Partial cross sections were measured using state-selective detection based on a resonance ionization scheme. Several resonances were observed in the Cs(102S10{\phantom{\rule{0.16em}{0ex}}}^{2}S102S), Cs(62F6{\phantom{\rule{0.16em}{0ex}}}^{2}F62F), Cs(62G6{\phantom{\rule{0.16em}{0ex}}}^{2}G62G), and Cs(62H6{\phantom{\rule{0.16em}{0ex}}}^{2}H62H) final-state channels below the Cs(102P10{\phantom{\rule{0.16em}{0ex}}}^{2}P102P3/2{}_{3/2}3/2​) channel opening. A model was developed to account for the interference between overlapping resonances. This model is essentially a generalization of the Fano [U. Fano, Phys. Rev. 124, 1866 (1961)] and Shore [B. W. Shore, Phys. Rev. 171, 43 (1968)] parametrizations for single resonances. Resonance parameters were extracted by fitting the model to the data sets. The openings of the Cs(102S10{\phantom{\rule{0.16em}{0ex}}}^{2}S102S) and Cs(62F6{\phantom{\rule{0.16em}{0ex}}}^{2}F62F) channels, where the polarizabilities of the atomic states are large and positive, showed rapid onsets. In the case of the Cs(62G6{\phantom{\rule{0.16em}{0ex}}}^{2}G62G) and Cs(62H6{\phantom{\rule{0.16em}{0ex}}}^{2}H62H) channels, on the other hand, the photodetachment cross sections increased slowly with energy. For the Cs(62H6{\phantom{\rule{0.16em}{0ex}}}^{2}H62H) channel this is the expected behavior, since it is the result of a large and negative polarizability of the 62H6{\phantom{\rule{0.16em}{0ex}}}^{2}H62H state. In addition, the excitation of the Cs(62H6{\phantom{\rule{0.16em}{0ex}}}^{2}H62H) state with respect to the Cs ground state was found to be 28 356.3(2) ${\mathrm{cm}}^{\ensuremath{-}1}$, in agreement with a previous experiment.
Four-photon double detachment of the helium negative ion is investigated experimentally and theoretically for photon energies where the transient helium atom is in the 1s2s3S1s2s\phantom{\rule{0.16em}{0ex}}^{3}S1s2s3S or 1s2p3Po1s2p\phantom{\rule{0.16em}{0ex}}^{3}P^{\mathrm{o}}1s2p3Po states, which subsequently ionize by absorption of three photons. Ionization is enhanced by intermediate resonances, giving rise to series of peaks in the He+{\mathrm{He}}^{+}He+ spectrum, which we study in detail. The He+{\mathrm{He}}^{+}He+ yield is measured in the wavelength ranges from 530 to 560 nm and from 685 to 730 nm and for various polarizations of the laser light. Double detachment is treated theoretically as a sequential process, within the framework of RRR-matrix theory for the first step and effective Hamiltonian theory for the second step. Experimental conditions are accurately modeled, and the measured and simulated yields are in good qualitative and, in some cases, quantitative agreement. Resonances in the double detachment spectra can be attributed to well-defined Rydberg states of the transient atom. The double detachment yield exhibits a strong dependence on the laser polarization which can be related to the magnetic quantum number of the intermediate atomic state. We also investigate the possibility of nonsequential double detachment with a two-color experiment but observe no evidence for it.
We have measured the near-threshold cross sections for the photodetachment of ${\mathrm{Li}}^{\ensuremath{-}}$ into the ${e}^{\ensuremath{-}}(ks,d)\text{\ensuremath{-}}\mathrm{Li}(2{p}_{1∕2})$, ${e}^{\ensuremath{-}}(ks,d)\text{\ensuremath{-}}\mathrm{Li}(3p)$, ${e}^{\ensuremath{-}}(kp)\text{\ensuremath{-}}\mathrm{Li}(3s)$, and ${e}^{\ensuremath{-}}(kp)\text{\ensuremath{-}}\mathrm{Li}(4s)$ continuum channels and the photodetachment of ${\mathrm{K}}^{\ensuremath{-}}$ into the ${e}^{\ensuremath{-}}(ks,d)\text{\ensuremath{-}}\mathrm{K}(4{p}_{3∕2})$ continuum channel. The data was analyzed using both a top-of-barrier model and a modified effective range theory. Both approaches explicitly take into account the significant polarization potential in the final continuum channels. The study involving both experiment and theory enabled us to investigate the effect of the polarization on near-threshold photodetachment. Furthermore, since photodetachment and electron scattering are related in the half-collision concept, we were able to determine low-energy electron scattering quantities such as phase shifts, elastic cross sections, and sss wave scattering lengths for electron scattering on excited Li and K atoms. The results allowed us to predict a virtual state in the ${e}^{\ensuremath{-}}(ks)\text{\ensuremath{-}}\mathrm{Li}(3p)$ system.