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Journal of Physics B Atomic and Molecular Physics

Threshold shifts in negative-ion photodetachment

Stanley L. Haan, J. Cooper

Published Sep 14, 1984• 36 citations
Publisher

Abstract

The authors study threshold shifts theoretically in negative-ion photodetachment, using the approach of Rzezewski, Lewenstein and Eberly (1982), by considering two model couplings for the discrete-state continuum interaction specific to outgoing p and s electrons. They apply their results to the photodetachment of Pt - and predict a threshold shift which is directly proportional to laser intensity and is approximately 2.3*10 -10 I cm -1 (W cm -2 ) -1 . This shift appears too small to be measured by present day techniques.

DOI
10.1088/0022-3700/17/17/018

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Near-threshold single-photon photodetachment processes are considered for simple model systems representing negative ions exposed to monochromatic radiation. The model systems considered feature an initial discrete state, the photoelectron continuum, and in some cases an excited or autodetaching discrete state. Population trapping through the formation of a bound dressed state and the photoelectron spectra are examined. Two photoelectron spectra are defined : a primary spectrum which results from decay while the laser interaction is on and a secondary spectrum which results from the destabilization of the bound dressed state if the interaction is turned off again. Careful attention is given to the passage of the poles of the Green's operator through the low-energy boundary of a continuum, and it is concluded that such poles move nonmonotonically through the boundary of an s-wave continuum, but monotonically through the boundary of a p-wave or higher-l-wave continuum. Two example systems, one featuring a single discrete state and an s-wave continuum and one featuring two discrete states and a p-wave continuum, are considered in detail.

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Near threshold photodetachment of electrons from negative ions is investigated theoretically. Various profiles of the laser pulse time envelope f(t) are considered. The Stark shift ( delta E 0 ) max of the ground level E 0 is assumed to be large mod ( delta E 0 ) max mod tau >>1, where tau is the pulse duration. The saturation regime found earlier is shown to be realisable rigorously only in the case of the exponential envelope f 0 (t)=exp(- mod t mod / tau ). For any other f(t) deviations from the saturation are shown to be larger the more significantly f(t) deviates from f 0 (t). In the case of the Gaussian curve f(t) the photodetachment probability w depends only slowly (logarithmically) on the peak field strength E 0 . The function w(E 0 ) is shown to depend not only of f(t) but also on the detuning from the threshold Delta . For some model functions f(t) the dependence w(E 0 ) is found analytically for all Delta , mod Delta mod <or approximately= mod ( delta E 0 ) max mod . For large positive Delta the photodetachment probability is found for any envelope f(t).

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Photodetachment of an electron from a negative ion in the near-threshold region. II. Pulses of radiation with smooth time envelopes

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For pt.I see ibid., vol.16, p.3641 (1983). The photodetachment of an electron from a negative ion is investigated in the near-threshold region. Pulses of electromagnetic radiation with a smooth envelope are considered. The strong Stark shift of the ground level is shown to influence essentially the regimes of the field-induced decay. The probability of photodetachment is found analytically for the envelope described by the curve f(t)=exp(- mod t mod / tau ). The cases with various relations between the Stark shift, pulse duration tau and detuning from the threshold are analysed. The probability w is found both for a positive and a negative polarisability alpha ( omega ) of the ground state. A partial stabilisation (when alpha ( omega )>0) and destabilisation (when alpha ( omega )<0) of the negative ion due to the strong laser field are discussed.

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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 $s$ 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.

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Photodetachment spectroscopy of quasibound states of the negative ion of lanthanum

C. W. Walter, N. D. Gibson, N. B. Lyman +1 more

The negative ion of lanthanum, $\mathrm{L}{\mathrm{a}}^{\text{--}}$, has one of the richest bound state spectra observed for an atomic negative ion and has been proposed as a promising candidate for laser-cooling applications. In the present experiments, $\mathrm{L}{\mathrm{a}}^{\text{--}}$ was investigated using tunable infrared photodetachment spectroscopy. The relative signal for neutral atom production was measured with a crossed ion-beam--laser-beam apparatus over the photon energy range 585--920 meV (2120--1350 nm) to probe the continuum region above the La neutral atom ground state. Eleven prominent peaks were observed in the $\mathrm{L}{\mathrm{a}}^{\text{--}}$ photodetachment cross section due to resonant excitation of quasibound transient negative ion states in the continuum, which subsequently autodetach. In addition, thresholds were observed for photodetachment from several bound states of $\mathrm{L}{\mathrm{a}}^{\text{--}}$ to both ground and excited states of La. The present results provide information on the excited state structure and dynamics of $\mathrm{L}{\mathrm{a}}^{\text{--}}$ that depend crucially on multielectron correlation effects.

arXiv:2010.01122•Physical Review A•Oct 13, 2020•3 citations
The authors study threshold shifts theoretically in negative-ion photodetachment, using the approach of Rzezewski, Lewenstein and Eberly (1982), by considering two model couplings for the discrete-state continuum interaction specific to outgoing p and s electrons. They apply their results to the photodetachment of Pt - and predict a threshold shift which is directly proportional to laser intensity and is approximately 2.3*10 -10 I cm -1 (W cm -2 ) -1 . This shift appears too small to be measured by present day techniques.
Near-threshold single-photon photodetachment processes are considered for simple model systems representing negative ions exposed to monochromatic radiation. The model systems considered feature an initial discrete state, the photoelectron continuum, and in some cases an excited or autodetaching discrete state. Population trapping through the formation of a bound dressed state and the photoelectron spectra are examined. Two photoelectron spectra are defined : a primary spectrum which results from decay while the laser interaction is on and a secondary spectrum which results from the destabilization of the bound dressed state if the interaction is turned off again. Careful attention is given to the passage of the poles of the Green's operator through the low-energy boundary of a continuum, and it is concluded that such poles move nonmonotonically through the boundary of an s-wave continuum, but monotonically through the boundary of a p-wave or higher-l-wave continuum. Two example systems, one featuring a single discrete state and an s-wave continuum and one featuring two discrete states and a p-wave continuum, are considered in detail.
Photodetachment from negative ions in a static electric field has been studied using ${\mathrm{Cl}}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{S}}^{\mathrm{\ensuremath{-}}}$. These experiments were done with greater precision than previous studies and allow a quantitative comparison of data with theory. A 10-keV ion beam was sent through a region with fields of up to 1.5 kV/cm applied parallel to the beam velocity. A pulsed dye-laser beam, perpendicular to the ion beam, photoneutralized the ions. The relative cross sections were measured by detecting the resulting fast neutral atoms. Detachment below threshold and oscillations on the cross section above threshold were observed near the ${\mathrm{S}}^{\mathrm{\ensuremath{-}}}$ threshold at 16 269.5 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and near the ${\mathrm{Cl}}^{\mathrm{\ensuremath{-}}}$ threshold at 29 138.3 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The phase of the oscillations in the data is in good agreement with predictions for s-wave photodetachment in a static electric field, but the amplitude of the oscillations is found to be slightly reduced.
Near threshold photodetachment of electrons from negative ions is investigated theoretically. Various profiles of the laser pulse time envelope f(t) are considered. The Stark shift ( delta E 0 ) max of the ground level E 0 is assumed to be large mod ( delta E 0 ) max mod tau >>1, where tau is the pulse duration. The saturation regime found earlier is shown to be realisable rigorously only in the case of the exponential envelope f 0 (t)=exp(- mod t mod / tau ). For any other f(t) deviations from the saturation are shown to be larger the more significantly f(t) deviates from f 0 (t). In the case of the Gaussian curve f(t) the photodetachment probability w depends only slowly (logarithmically) on the peak field strength E 0 . The function w(E 0 ) is shown to depend not only of f(t) but also on the detuning from the threshold Delta . For some model functions f(t) the dependence w(E 0 ) is found analytically for all Delta , mod Delta mod <or approximately= mod ( delta E 0 ) max mod . For large positive Delta the photodetachment probability is found for any envelope f(t).
For pt.I see ibid., vol.16, p.3641 (1983). The photodetachment of an electron from a negative ion is investigated in the near-threshold region. Pulses of electromagnetic radiation with a smooth envelope are considered. The strong Stark shift of the ground level is shown to influence essentially the regimes of the field-induced decay. The probability of photodetachment is found analytically for the envelope described by the curve f(t)=exp(- mod t mod / tau ). The cases with various relations between the Stark shift, pulse duration tau and detuning from the threshold are analysed. The probability w is found both for a positive and a negative polarisability alpha ( omega ) of the ground state. A partial stabilisation (when alpha ( omega )>0) and destabilisation (when alpha ( omega )<0) of the negative ion due to the strong laser field are discussed.
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.
The negative ion of lanthanum, La–\mathrm{L}{\mathrm{a}}^{\text{--}}La–, has one of the richest bound state spectra observed for an atomic negative ion and has been proposed as a promising candidate for laser-cooling applications. In the present experiments, La–\mathrm{L}{\mathrm{a}}^{\text{--}}La– was investigated using tunable infrared photodetachment spectroscopy. The relative signal for neutral atom production was measured with a crossed ion-beam--laser-beam apparatus over the photon energy range 585--920 meV (2120--1350 nm) to probe the continuum region above the La neutral atom ground state. Eleven prominent peaks were observed in the La–\mathrm{L}{\mathrm{a}}^{\text{--}}La– photodetachment cross section due to resonant excitation of quasibound transient negative ion states in the continuum, which subsequently autodetach. In addition, thresholds were observed for photodetachment from several bound states of La–\mathrm{L}{\mathrm{a}}^{\text{--}}La– to both ground and excited states of La. The present results provide information on the excited state structure and dynamics of La–\mathrm{L}{\mathrm{a}}^{\text{--}}La– that depend crucially on multielectron correlation effects.