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Journal of Physics B Atomic and Molecular Physics

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

Mikhail Vladimirovich Fedorov, A V Kazakov

Published Oct 14, 1983• 20 citations
Publisher

Abstract

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.

DOI
10.1088/0022-3700/16/19/022

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Journal of Physics B Atomic Molecular and Optical Physics•Oct 14, 1989•12 citations
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.
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).
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.
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.
We present measurements of photodetachment cross sections of negative gold ions stored in a RF trap, in the energy range from about to above threshold. A number of different laser devices have been used. The results complement those described in a previous paper. The photodetachment spectrum exhibits a strong resonance, located at above ground level and a much fainter one at . These resonances can be represented as Fano profiles and reveal the presence of autodetaching excited levels of (most probably and ). Furthermore, the opening of a new photodetachment channel near 360 nm (corresponding to the first excited level of neutral Au) is accompanied by an increase of the photodetachment yield.
The near-threshold behaviour of four-photon ionisation is studied in an energy region where the third laser photon crosses the Rydberg threshold. Particular emphasis is given to the role played by the Stark shift (raising of the ionisation potential) in a Gaussian laser pulse. As a function of the laser frequency the ionisation probability shows (i) a region where individual Rydberg resonances are well resolved, followed by (ii) a smoothly decreasing ionisation probability in the bound-state region, and (iii) an almost energy-independent above-threshold ionisation signal.