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Physical Review Letters

Shifts of photodetachment thresholds and induced shape resonances of H− in an electric field

Bin Zhou, C. D. Lin

Published Dec 7, 1992• 7 citations
Publisher

Abstract

The photodetachment of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ near the H(N=4) and H(N=5) thresholds in an external electric field is studied theoretically. The recently observed photodetachment threshold shifts of Halka et al. are interpreted in terms of the shifts of the barrier heights of the hyperspherical potential curves as the electric field strength is varied. It is shown that higher members of the zero-field Feshbach resonances can emerge as field-induced shape resonances in an electric field.

DOI
10.1103/physrevlett.69.3294

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Journal of Physics B Atomic and Molecular Physics•Oct 14, 1983•20 citations
The photodetachment of H\ensuremath−{\mathrm{H}}^{\mathrm{\ensuremath{-}}}H\ensuremath− near the H(N=4) and H(N=5) thresholds in an external electric field is studied theoretically. The recently observed photodetachment threshold shifts of Halka et al. are interpreted in terms of the shifts of the barrier heights of the hyperspherical potential curves as the electric field strength is varied. It is shown that higher members of the zero-field Feshbach resonances can emerge as field-induced shape resonances in an electric field.
An adiabatic treatment in hyperspherical coordinates is used to predict the existence of resonant structures in ${\mathrm{Ps}}^{\ensuremath{-}}$ (Ps denotes positronium) photodetachment. This highly correlated system displays a weak series of narrow Feshbach resonances lying just below the Ps(n=2)\mathrm{Ps}(n=2)Ps(n=2) threshold, while a single strong shape resonance dominates the photodetachment spectrum above this threshold as in ${\mathrm{H}}^{\ensuremath{-}}$. Use of a "prediagonalized" representation of hyperspherical harmonics dramatically improves the convergence of the adiabatic calculation.
We have implemented an integrable method with B-spline basis which allows us to calculate photodetachment cross sections of the ion in the presence of a constant electric field. The behaviour of the Feshbach resonances lying below the N = 2 and N = 3 thresholds is analysed in detail. Further insight on the relative importance of autoionization and tunnelling is provided by using a stabilization procedure. Our results permit us to explain most of the experimental findings and show the existence of resonance structures previously unobserved.
A multichannel study of the photodetachment of a negative ion such as ${\mathrm{S}}^{\mathrm{\ensuremath{-}}}$ in the presence of a constant electric field is presented. The analysis is carried out as a series of frame transformations for the outgoing photoelectron. An earlier analysis in the absence of the external field is now augmented by the further transformations from spherical to cylindrical symmetry that are required. The interplay between the various thresholds due to fine structure in the negative ion and residual atom and the effects of the external field are analytically treated. The net effect is to eliminate the thresholds, to induce modulations in the detachment cross section, and to introduce field-assisted-tunneling contributions to the photodetachment. Numerical illustrations are provided. Our treatment closely parallels similar treatments of photoionization of neutral atoms in an electric field and of photodetachment in a magnetic field.
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.