Photodetachment microscopy to an excited spectral term and the electron affinity of phosphorus

A beam of P- ions produced by a cesium sputtering ion source was photodetached in the presence of an electric field, with a single-mode ring dye laser. Neutral P can be produced at one or the other of the fine-structure sub-levels of its 3s23p3 2Do excited term. This is the first atomic photodetachment microscopy experiment with excitation of the parent neutral atom out of the fundamental spectral term. The background electron signal due to ground-state photodetachment notwithstanding, photodetachment microscopy images produced at the excited thresholds could be analysed to provide a measure of these excited-term thresholds with interferometric precision. Starting from the three possible fine-structure sub-levels of P- 3s23p4 3P, the five fine-structure thresholds that may be detected, taking the selection rules into account, have been measured. They are combined with the spectroscopic data available in the literature on neutral P to produce an improved experimental value of the electron affinity eA of phosphorus: 602 179(8) m -1 or 0.746 607(10) eV. Taking all covariances of the optimized energy levels into account, one can merge them with the former measure of the three lowest detachment thresholds of P-, which results in a slightly more precise value of eA(P): 602 181(8) m-1, or 0.746 609(9) eV. The accuracy of eA(P) is now essentially limited by the uncertainty on the 2Do 3/2 and 2Do 5/2 energy levels of the neutral atom. The fine-structure intervals of the 3s23p3 2D o doublet of the neutral atom and of the 3s23p4 3P triplet of the negative ion have their accuracy improved by more than one order of magnitude. © 2011 IOP Publishing Ltd.The authors wish to thank Evelyne Cottereau and Christophe Moreau, at the Laboratoire de mesure du carbone 14 (CEA Saclay), for making their expertise repeatedly available to us during the first months of operation of the SNICS II ion source, and Kevin Glize, for some useful numerical simulations that helped adapting the first decelerator to our low-energy beam requirement.Peer Reviewe

Calculated and measured values (the latter with uncertainty) of the electron affinity of tin ^e<em>A</em>(Sn)

<p><b>Table 1.</b> Calculated and measured values (the latter with uncertainty) of the electron affinity of tin ^e<em>A</em>(Sn). Laser photodetachment electron spectrometry (LPES), limited to a few meV precision, was superseded by direct LPT measurement before electron spectrometry came back in its interferometric form, laser photodetachment microscopy (LPM). The first tabulated measurement (Feldmann <em>et al</em> <a href="http://iopscience.iop.org/0953-4075/46/12/125002/article#jpb466134bib11" target="_blank">1977</a>) had been performed by PT spectroscopy with a conventional light source.</p> <p><strong>Abstract</strong></p> <p>A beam of Sn⁻ ions produced by a caesium sputtering ion source is photodetached in the presence of an electric field, with a single-mode ring Ti:Sa laser. The laser wavelength, about 806 nm, is set just above the excitation threshold of the ³P₂, highest fine-structure sublevel of the ³P ground-term of Sn I. The photoelectron energy is measured by photodetachment microscopy. The measured photodetachment threshold is 1239 711.8 (11) m⁻¹, from which an improved value of the electron affinity of tin can be deduced: 896 944.7 (13) m⁻¹ or 1.112 070 (2) eV.</p

The apparent detachment energy at the ³P₂ threshold, for mean photoelectron kinetic energies ranging from 33.8 to 70.7 m⁻¹

<p><strong>Figure 3.</strong> The apparent detachment energy at the ³P₂ threshold, for mean photoelectron kinetic energies ranging from 33.8 to 70.7 m⁻¹. Open circles, triangles and diamonds are data taken at 183, 267 and 364 V m⁻¹, respectively. Extrapolation down to = 0 produces a measurement of the ³P₂ PT, 1239 711.8(11) m⁻¹. The continuous line is the linear regression of the data, the nearly zero slope of which reveals that the error on the electric field value was probably lower than 0.5%. Larger error bars for lower electric fields are due to the larger perturbation of electron interferograms by spurious electric fields in the vicinity of the electron detector.</p> <p><strong>Abstract</strong></p> <p>A beam of Sn⁻ ions produced by a caesium sputtering ion source is photodetached in the presence of an electric field, with a single-mode ring Ti:Sa laser. The laser wavelength, about 806 nm, is set just above the excitation threshold of the ³P₂, highest fine-structure sublevel of the ³P ground-term of Sn I. The photoelectron energy is measured by photodetachment microscopy. The measured photodetachment threshold is 1239 711.8 (11) m⁻¹, from which an improved value of the electron affinity of tin can be deduced: 896 944.7 (13) m⁻¹ or 1.112 070 (2) eV.</p

Level scheme of the Sn/Sn⁻ system

<p><strong>Figure 1.</strong> Level scheme of the Sn/Sn⁻ system. ^e<em>A</em> labels the ground-to-ground detachment transition that defines the electron affinity; 'measured' labels the transition measured in this study. The ²D levels of Sn⁻, which do not play any role in this work, were observed by Scheer <em>et al</em> (<a href="http://iopscience.iop.org/0953-4075/46/12/125002/article#jpb466134bib20" target="_blank">1998</a>).</p> <p><strong>Abstract</strong></p> <p>A beam of Sn⁻ ions produced by a caesium sputtering ion source is photodetached in the presence of an electric field, with a single-mode ring Ti:Sa laser. The laser wavelength, about 806 nm, is set just above the excitation threshold of the ³P₂, highest fine-structure sublevel of the ³P ground-term of Sn I. The photoelectron energy is measured by photodetachment microscopy. The measured photodetachment threshold is 1239 711.8 (11) m⁻¹, from which an improved value of the electron affinity of tin can be deduced: 896 944.7 (13) m⁻¹ or 1.112 070 (2) eV.</p

Example of an experimental photodetachment interferogram recorded in an electric field of 364.1 V m⁻¹ (left) and the best-fitting theoretical electron interferogram (right), calculated for an initial electron kinetic energy of 59.4(4) m⁻¹

<p><strong>Figure 2.</strong> Example of an experimental photodetachment interferogram recorded in an electric field of 364.1 V m⁻¹ (left) and the best-fitting theoretical electron interferogram (right), calculated for an initial electron kinetic energy of 59.4(4) m⁻¹. Subtracting this energy from the laser wavenumber 1239 762.1 m⁻¹ would directly yield the detachment threshold (here threshold ³P₂), were it not for the Doppler shift of the laser frequency in the ion frame. Comparison of the raw difference of 1239 702.7 with the result of the final adjustment of the ³P₂ threshold at 1239 711.8 m⁻¹ reveals that in this very case, the photon energy, as seen by the ions, was positive-shifted by 9.2 m⁻¹. This corresponds exactly to the 3° deviation from orthogonality that we have set between the laser and ion beam, in order not to illuminate the electron detector with the laser. The other spot produced by the reflected laser beam in the same experiment, 3 mm downstream on the ion beam, is energy-lowered symmetrically. Alternative photodetachment to the lower fine-structure ³P₀ and ³P₁ levels produces a photoelectron background, but the higher energy of the corresponding electrons causes their interference patterns to be completely smoothed out, the only signature of these lower photodetachment channel being a uniform background.</p> <p><strong>Abstract</strong></p> <p>A beam of Sn⁻ ions produced by a caesium sputtering ion source is photodetached in the presence of an electric field, with a single-mode ring Ti:Sa laser. The laser wavelength, about 806 nm, is set just above the excitation threshold of the ³P₂, highest fine-structure sublevel of the ³P ground-term of Sn I. The photoelectron energy is measured by photodetachment microscopy. The measured photodetachment threshold is 1239 711.8 (11) m⁻¹, from which an improved value of the electron affinity of tin can be deduced: 896 944.7 (13) m⁻¹ or 1.112 070 (2) eV.</p

SIPHORE: Conceptual Study of a High Efficiency Neutral Beam Injector Based on Photo-detachment for Future Fusion Reactors

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