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Journal of Physics B Atomic Molecular and Optical Physics

Polarization effects in two-colour two-photon detachment from Cl-

William G. Sturrus, L. P. Ratliff, Daniel J. Larson

Published Aug 14, 1992• 6 citations
Publisher

Abstract

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.

DOI
10.1088/0953-4075/25/15/003

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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.
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.
An accelerated beam of Cl− ions is photodetached by a single-mode Nd:YAG laser, and the resulting neutrals are detected. By measuring the saturation intensity of this process, one derives the cross section for four-photon detachment. This cross section is smaller by a factor of 6 than a recently calculated value. Comparison of the photodetachment saturation intensity of Cl− with those of other halogen negative ions shows that this corresponds to regular behavior among the halogen elements, suggesting limitations in the theoretical models.
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.
Multiphoton excitation with elliptically polarized light brings out more information in the photodetachment or photoionization processes than linearly polarized light. This information can be reached by recording photoelectron angular distributions for several values of the ellipticity angle, and fitting the whole set of data with an analytic formula. We give here the appropriate formula, valid for any number N of exciting photons, with the assumptions of (1) an initially even and momentumless (J equals 0) atom or ion and (2) that after excitation of an electron, the residual core be left in an odd state. These are the conditions met by multiphoton detachment from halogen negative ions. With elliptically polarized light of (lambda) equals 1064.2 nm, we have measured the angular distributions of three-photon detached electrons from the ion I-, and four-photon detached electrons from Cl- and F-. The experimental results are quantitatively analyzed and commented on.
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.