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Physical Review A

s-wave photodetachment in a static electric field

N. D. Gibson, B. J. Davies, Daniel J. Larson

Published Mar 1, 1993• 40 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.47.1946

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Europhysics Letters (EPL)•May 30, 2014•5 citations
Photodetachment from negative ions in a static electric field has been studied using Cl\ensuremath−{\mathrm{Cl}}^{\mathrm{\ensuremath{-}}}Cl\ensuremath− and S\ensuremath−{\mathrm{S}}^{\mathrm{\ensuremath{-}}}S\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 S\ensuremath−{\mathrm{S}}^{\mathrm{\ensuremath{-}}}S\ensuremath− threshold at 16 269.5 cm\ensuremath−1{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}cm\ensuremath−1 and near the Cl\ensuremath−{\mathrm{Cl}}^{\mathrm{\ensuremath{-}}}Cl\ensuremath− threshold at 29 138.3 cm\ensuremath−1{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}cm\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.
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.
The goal of the presented work is to investigate double detachment of\nnegative ions on nano- and femtosecond time scales. I have performed\nthree types of experiments.\nFemtosecond time resolved pump-probe experiments have been performed\non electron dynamics in the valence shell of carbon, silicon\nand germanium atoms. The atom under investigation was prepared\nby strong field photodetachment of its parent negative ion in a linearly\npolarized femtosecond laser pulse. The detachment process coherently\npopulates the fine structure levels of the atomic ground state\ngiving rise to an electronic wave packet moving in the valence shell\nof the atom. The wave packet is probed by strong field ionization at a\ntime delay controlled on a femtosecond time scale. The ionized electrons\nare detected in an electron imaging spectrometer. The energy\nand angular distribution of the ionized electrons provide information\non the phase of the wave packet. The results reveal a periodic spatial\nrearrangement of the electron cloud in the atom. The experimental\nprocedure and detection scheme developed provide the first direct visualization\nof an electronic wave packet moving in the ground state of\nan atom.\nThe process of rescattering in strong field detachment has also been\ninvestigated. The detached electron is accelerated in the laser field.\nWhen the E-field changes its direction the electron follows and may\nreturn and collide with the core. I have investigated the rescattering\neffect in negative fluorine ions and compared with previous results on\nbromine ions. Furthermore, strong field detachment experiments of\nF \n2 were conducted and the results from molecular fluorine ions are\ncompared to results from atomic fluorine ions.\nPhotodetachment cross sections and doubly excited states in K and\nCs have been investigated through double detachment. A resonant\nionization scheme was used where the ion beam and laser beams were\nin a collinear configuration allowing for measurements of partial cross\nsection channels individually with nearly 100% detection efficiency.\nA new threshold behavior has been found in photodetachment into\natomic states having a large and negative polarizability. A new model\nwas developed to explain the observed phenomena. Furthermore, two\nnew resonances stemming from doubly excited states were found below\nthe K(7 2P) channel opening in K and a set of new resonances\nwas observed in Cs below the Cs(10 2P) channel.
The negative ion of lanthanum, La–\mathrm{L}{\mathrm{a}}^{\text{--}}La–, has one of the richest bound state spectra observed for an atomic negative ion and has been proposed as a promising candidate for laser-cooling applications. In the present experiments, La–\mathrm{L}{\mathrm{a}}^{\text{--}}La– was investigated using tunable infrared photodetachment spectroscopy. The relative signal for neutral atom production was measured with a crossed ion-beam--laser-beam apparatus over the photon energy range 585--920 meV (2120--1350 nm) to probe the continuum region above the La neutral atom ground state. Eleven prominent peaks were observed in the La–\mathrm{L}{\mathrm{a}}^{\text{--}}La– photodetachment cross section due to resonant excitation of quasibound transient negative ion states in the continuum, which subsequently autodetach. In addition, thresholds were observed for photodetachment from several bound states of La–\mathrm{L}{\mathrm{a}}^{\text{--}}La– to both ground and excited states of La. The present results provide information on the excited state structure and dynamics of La–\mathrm{L}{\mathrm{a}}^{\text{--}}La– that depend crucially on multielectron correlation effects.
The effect of final-state interactions on the photodetachment cross section of negative ions in a magnetic field is discussed. Experiments on the location of the 2P32^{2}P_{\frac{3}{2}}2P23​​ threshold in ${\mathbf{S}}^{\ensuremath{-}}$ as a function of magnetic field show clearly the effect of the zero-point energy of an electron in a magnetic field. The experiments also give a precise value for the electron affinity and provide information about the interaction between the atom and the detached electron.
Photoelectron imaging techniques are used to experimentally investigate photodetachment of negative ions in a strong laser-field as well as the orbital alignment dynamics in the residual neutral.\n\nStrong-field photodetachment in a linearly polarized pulsed laser with 100 fs pulse length is used to create a wave packet in the neutral atoms of carbon, germanium and silicon. The negative ions of these three atomic species are isoelectronic with a half-filled ppp-shell. The photodetachment rate is highly dependent on the magnetic quantum number $m_\\ell$ with the orbital with $m_\\ell = 0$ having a detachment rate of an order of magnitude larger than orbitals with $m_\\ell$ non-zero, with the polarization axis acting as a quantization axis. The residual neutral is thus left in an aligned state after photodetachment. The aligned state constitutes of a coherent superposition of fine-structure states and the population in the different orbitals will accordingly display a time-dependent spatial oscillation. The created wave packet is subsequently probed through photoionization by a second strong-field laser pulse.\n\nSecondly, a tomographic method is implemented to observe the three-dimensional distribution of photoelectrons produced in strong-field photodetachment processes. The method makes no assumptions of the existence of any symmetry axes in the photoelectron distribution and is thus applicable to any laser polarization ellipticity. Traditional imaging methods are dependent on the photoelectron distribution to be cylindrically symmetric by using either linearly or circularly polarized light, a requirement which can be lifted by using a tomographic technique. The method is applied to multiphoton detachment of the negative ion of silver.\n\nFinally, strong-field photodetachment of homonuclear diatomic negative ions is performed. The possibility of the additional electron being ejected from either of the two atomic centers can significantly alter the photoelectron distribution compared to an atomic negative ion with the same detachment energy. Experiments are performed on C2−_2^-2−​, Si2−_2^-2−​ and Ag2−_2^-2−​ and compared to theoretical models.
In a recent experiment, Lindahl et al. ( Lindahl A. O. et al. , Phys. Rev. Lett. , 108 (2012) 033004) observed a new threshold behavior in the photodetachment of the potassium negative ion when the residual atom is left in the state. Unfortunately the resonance close to threshold made it more difficult to apply the semi-classical model that was developed to describe the threshold behavior. In this paper we present a study of the threshold behavior of the partial cross-section in the photodetachment of the sodium negative ion. The experiment was conducted using a collinear beams apparatus where the detection was performed using a resonance ionization scheme. We observe the same threshold behavior as in the previous experiment, but without any obfuscating resonance. The model derived in the previous paper is found to fit the observed cross-section up to 35 meV above threshold. The present experiment clearly demonstrates the threshold behaviour for the fundamental process of a free electron moving in the field of an atom with a large negative polarizability.