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Journal of Physics B Atomic Molecular and Optical Physics

A comment on the validity of an approximation method for two-colour photodetachment

Saverio Bivona, R. Burlon, C Leone

Published Oct 14, 1996• 1 citation
Publisher

Abstract

We show that by using the approximation method of Bivona et al , two-colour photodetachment cross sections are obtained that behave according to the Wigner threshold law even if the dressing effect of the low-frequency radiation field on the bound state is ignored. Within the range of the low frequency and intensity of the radiation field considered, it is found that the values of the cross sections calculated by using the field-free bound state are not significantly modified when the corrections to the wavefunction of the bound state due to the driving action of the low-frequency field are accounted for.

DOI
10.1088/0953-4075/29/19/021

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We show that by using the approximation method of Bivona et al , two-colour photodetachment cross sections are obtained that behave according to the Wigner threshold law even if the dressing effect of the low-frequency radiation field on the bound state is ignored. Within the range of the low frequency and intensity of the radiation field considered, it is found that the values of the cross sections calculated by using the field-free bound state are not significantly modified when the corrections to the wavefunction of the bound state due to the driving action of the low-frequency field are accounted for.
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.
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.
The behavior of the photodetachment cross section of the ${S}^{\ensuremath{-}}$ ion near thresholds in a magnetic field is derived. This cross section is shown to have an oscillatory dependence on photon energy. The calculation is based on first-order time-dependent perturbation theory, and final-state interactions are ignored. The weights of individual transitions between the various Zeeman sublevels for different light polarizations are given in terms of the appropriate angular momentum coupling coefficients. Broadening of the features of the cross section due to the motion of the ions is included in the calculation. The Wigner-law behavior known to be valid for zero magnetic field is shown to result from the magnetic field dependent cross section in the limit of low magnetic field. The theory is shown to be in good agreement with experimental results. The extension of the theory to other atomic negative ions is discussed.