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Physical Review Letters

Control of Near-Threshold Detachment Cross Sections via Laser Polarization

R. C. Bilodeau, Michael Scheer, Harold K. Haugen

Published Sep 11, 2001• 9 citations
Publisher

Abstract

The behavior of near-threshold cross sections for dissociation of a target into a pair of particles, as described by Wigner's threshold law, can depend sensitively on the angular momentum of the particles. In this Letter, we investigate the near-threshold nonresonant two-photon detachment process in the negative ion of gold. The expected s-wave threshold behavior is observed with linearly polarized light. Closure of the s-wave channel is realized by using circular polarization, allowing the first observation of a d-wave threshold. Practical applications are discussed, including extensions which could prove valuable for investigations of negative ions with near-threshold structure.

DOI
10.1103/physrevlett.87.143001

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The behavior of near-threshold cross sections for dissociation of a target into a pair of particles, as described by Wigner's threshold law, can depend sensitively on the angular momentum of the particles. In this Letter, we investigate the near-threshold nonresonant two-photon detachment process in the negative ion of gold. The expected s-wave threshold behavior is observed with linearly polarized light. Closure of the s-wave channel is realized by using circular polarization, allowing the first observation of a d-wave threshold. Practical applications are discussed, including extensions which could prove valuable for investigations of negative ions with near-threshold structure.
The differential and total cross sections for two-photon detachment of H - are computed analytically within the asymptotic model for both linearly and circularly polarized light. The total cross section for unit flux is of the order 10 -48 cm 2 . The differential cross section for linearly polarized light is nearly isotropic near the two-photon absorption threshold but is sharply peaked along the polarization direction just below the threshold for one-photon detachment. The circular polarization efficiency is very small near the two-quantum threshold but is close to its theoretical maximum near the one-quantum threshold.
Two-colour photodetachment of a negative ion modelled by a three-dimensional delta-function potential is theoretically investigated. The high-frequency low-intensity field is closely tuned to the detachment energy of the ion, the low-frequency high intensity field creates a ponderomotive potential of 2.6% of the detachment energy and at the same time sidebands. The two fields are circularly polarized with either equal or opposite polarizations. In accordance with the Wigner threshold law, for equal and opposite polarizations, the sideband-induced thresholds are above or below the one-photon detachment threshold, respectively. The one-photon threshold is markedly visible for opposite polarizations at the position shifted by the full ponderomotive potential. For equal polarizations and low frequencies it is not noticeable at all.
A polarization dependence of a two-colour two-photon detachment cross section has been observed and calculated for the first time. In a Penning ion trap, electrons were detached from Cl - by an intense infrared laser beam at 1064 nm and a weak ultraviolet laser beam at 343 nm. Both beams were circularly polarized. The ratio of the two-colour two-photon cross section for photons with the same circular polarization to that for photons with opposite circular polarization was measured to be 1.085 +0.012 -0.007 . Also presented is a simple calculation of the ratio, the result of which is consistent with the observation.
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.
An analytic momentum-space wave function for an electron in both laser and static uniform electric fields is presented. It is used to obtain analytic multiphoton detachment cross sections in a static, uniform electric field which include effects of static-field-induced electron-photon interactions. These general results are not restricted to weak laser intensities or to weak static-field strengths and depend only on (a) the electric dipole approximation and (b) the approximation that final-state electron-atom interactions are ignored. Four specific predictions of our general formulas for the most interesting case of linearly polarized light polarized along the static-field direction are presented for weakly bound electrons initially in an s state. First, the effects of a weak static electric field on N-photon detachment cross sections near threshold are shown to be described by two modulation factors, one for odd N and one for even N, which depend only on a scaled energy. Second, for photodetachment in a static electric field, effects of static-field-induced electron-photon interactions are demonstrated. Third, the lifetime against field ionization is presented. Fourth, the cross section for electric-field-induced stimulated emission is presented. Numerical results for the latter three effects are presented for the ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ ion. The simpler case of circularly polarized light directed along the static-electric-field direction is treated briefly. In particular, we show for this case that the static- and laser-field effects are uncoupled and that the near-threshold, weak static-electric-field modulation factors for the N-photon detachment cross sections are dependent on N.
We report on the first experimental observation of a new threshold behavior observed in the 5(2)G partial channel in photodetachment of K(-). It arises from the repulsive polarization interaction between the detached electron and the residual K(5(2)G) atom, which has a large negative dipole polarizability. In order to account for the observation in the K(5(2)G) channel, we have developed a semiclassical model that predicts an exponential energy dependence for the cross section. The measurements were made with collinear laser-ion beams and a resonance ionization detection scheme.