Electron affinity of selenium measured by photodetachment microscopy 

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Photodetachement 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 3s²3p^{3 2}D⁰ 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 ofe A(P): 602 181(8) m⁻¹, or 0.746 609(9) eV. The accuracy of e A(P) is now essentially limited by the uncertainty on the ²D⁰_3/2 and ²D⁰_5/2 energy levels of the neutral atom. The fine-structure intervals of the 3s²3p^{3 2}D⁰ doublet of the neutral atom and of the 3s²3p⁴ 3P triplet of thenegative ion have their accuracy improved by more than one order of magnitude.

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Effect of magnetic field on 

 photodetachement microscopy

The effect of an external static magnetic field of arbitrary orientation with respect to the electric field, on the electron interference ring patterns observed by the photodetachment microscope is studied both experimentally and theoretically. The design of the interaction chamber has been modified to superimpose a controlled uniform magnetic field on the whole volume accessible to the interfering electron. Contrary to a previous study in weaker fields, where the overall dimension of the interferogram was not modified, the effect of the magnetic field here encompasses a regime of magnetic refocusing. A quantitative analysis is carried out using a closed-orbit perturbative calculation of the interference phase at the centre of the ring pattern. The essential result of this work is still the invariance of the extreme interference phase whatever the direction and magnitude of the applied magnetic field, up to values 100 times larger than in the previous experimental study. This property can be applied to revise former electron affinity measurements. Partly due to the previously unsuspected robustness of the electron interferograms vs. magnetic fields, partly thanks to the 2006 CODATA revision of the energy conversion factors, one can update the values of the electron affinities of  ¹⁶O, ²⁸Si and ³²S to 1.4611134(9), 1.3895210(7) and 2.0771040(6) eV respectively.

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Isotope shift in the electron 

affinity of sulfur

The sulfur electron affinities ^eA(S) are measured by photodetachment microscopy for the two isotopes ³²S and ³⁴S (16,752.975 3(41) and 16,752.977 6(85) cm^-1, respectively). The isotope shift in the electron affinity is found to be more probably positive, ^eA(³⁴S) − ^eA(³²S) = +0.0023(70) cm^-1, but the uncertainty allows for the possibility that it may be either “normal” [^eA(³⁴S) > ^eA(³²S)] or “anomalous”[^eA(³⁴S) < ^eA(³²S)]. The isotope shift is estimated theoretically using elaborate correlation models, monitoring the electron affinity and the mass polarization term expectation value. The theoretical analysis predicts a very large specific mass shift (SMS) that counterbalances the normal mass shift (NMS) and produces an anomalous isotope shift ^eA(³⁴S) − ^eA(³²S) = −0.0053(24) cm^-1, field shift corrections included. The total isotope shift can always be written as the sum ofthe NMS (here +0.0169 cm−1) and a residual isotope shift (RIS). Since the NMS has nearly no uncertainty, the comparison between experimental and theoretical RIS is more fair. With respective values of −0.0146(70) cm^-1 and −0.0222(24) cm^-1, these residual isotope shifts are found to agree within the estimated uncertainties.

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Pulse photodetachment microscopy and 

electron affinity of Iodine

Photodetachment microscopy is carried out on a beam of ¹²⁷I⁻ ions with a nanosecond pulsed laser. The photoelectron interferograms are recorded by means of a digital camera that images the light spots produced by the amplified photoelectrons on a phosphor screen. This is the first implementation of such an optical imaging technique in photodetachment microscopy. Due to their sensitivity to the photoelectron energy, the recorded electron interferograms can be quantitatively analysed to produce a measure of the electron affinity of iodine ^eA(¹²⁷I) with an accuracy improved by more than a factor of 2 with respect to the best previous measurement.

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Do fringes and trajectories shift equally in matter-wave interferometers? The example of photodetachment microscopy in a magnetic field

Matter-wave interferometers, either with electrons, atoms or molecules, owe their sensitivity to the accuracy of phase-shift measurements. One may, however, raise the question whether the shift undergone by interference fringes, when external forces are applied, actually differs from the shift of classical trajectories. For the case of a magnetic perturbation, we provide experimental evidence and a vector demonstration that interference patterns only undergo a global shift. The experiment is performed with the photodetachment microscope, with a uniform magnetic field superimposed over the whole volume accessible to the interfering electron. Identity of the fringe and trajectory shifts is established for any two-wave interferometer using charged particles, submitted to a magnetic field.

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Two-colour laser excitation as a way to set an absolute energy scale in photodetachment-microscopy based electron spectrometry

A two-colour laser technique is developed for photodetachment microscopy, by means of microwave modulation of a CW single-mode dye laser. A phase modulation regime is achieved through an electro-optical LiNbO₃ crystal excited at the frequency 1.95 GHz. The two first sidebands created are selected by rejection of the other orders through a plane Fabry–Perot interferometer. With the resulting two-colour radiation, the photodetachment microscopy technique is applied to a beam of ³²S⁻ ions. It is shown that the superposition of the two resulting interference patterns can be used as a ‘spectral vernier’ to remove the uncertainty on the electric field and absolute energy scale. Without any initial assumption on the value F of the electric field in the laser-ion interaction region, a measure of F and of the electron affinity ^eA of Sulfur can be obtained. Putting 16 recordings of two-colour photodetachment interferograms together, with the only condition that F be the same for all experiments, one gets ^eA(³²S) = 16752.978(11) cm^-1, which is quite compatible, even though not as accurate, with the most recently recommended value ^eA(³²S) = 16752.9760(42) cm^-1. A proposal is made for going from an incoherent to a coherent two-colour photodetachment scheme, which would make photodetachment interferograms sensitive to a new degree of freedom.

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The fine structure of S and S⁻ measured with the
photodetachment microscope

Photodetachment microscopy of a beam of ³²S⁻ ions makes it possible to measure the detachment thresholds corresponding to different fine-structure levels of the negative ion S⁻ and the neutral atom S. The electron affinity of sulfur, at 2.077 eV, is well suited for detachment by a tunable dye laser, which provides a third way of measuring neutral S fine structure, besides VUV spectroscopy of S I lines and direct fine-structure resonance spectroscopy. The fine-structure intervals are found to be 48 353.52(34)  m^-1 for the ² P_1/2−² P_3/2 energy difference in S⁻, and 39 605.87(32)  m^-1 for the ³ P₁−³ P₂ one in S (with expanded uncertainties), consistent with an independent measurement of the ³ P₀−³ P₁ interval. The new recommended electron affinity for isotope 32 of sulfur is 1675 297.60(42)  m^-1, or 2.077 104 18(71) eV.

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High precision measurement of the ³² SH electron affinity by laser detachment microscopy

The photodetachment microscopy technique, which was previously used with the OH molecular anion, is applied successfully to the SH ion with a single-mode dye laser. The interferograms of two rotational thresholds corresponding to particular detachment transitions of the SH^- (X¹S⁺; v =0) to SH (² P_{3/2, 1/2};v = 0) band have been recorded. With a double-pass scheme of the laser excitation on the ion beam, pairs of interference patterns are obtained, the 2D fitting of which provides us with a new recommendable value of the electron affinity of ³²SH, ^eA = 18669.543(12) cm^-1, i.e., 2.3147282(17) eV. The precision on the determination of ^eA has been increased by three orders of magnitude in comparison with the previous 1981 determination retained by the most recent review on molecular electron affinities.

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Photodetachment microscopy of the P, Q, and R  branches of the OH⁻ (v=0)to OH (v=0) 

detachment threshold

A photodetachment experiment is performed on the OH⁻ (v=0)to OH (v=0) detachment threshold. The weak O and S branches provide a signal strong enough to make amplitude measurements on all five O, P, Q, R, and S branches possible, which are used to fix the formulas for their relative intensities. Photodetachment microscopy is applied to 15 different thresholds of the P, Q, and R branches. The quantitative analysis of the interference patterns obtained does not show any effect of the dipole moment of OH, but yields a new measurement of the rotational parameters of OH^-(v=0) and of the electron affinity of the molecule. The new recommended value for the electron affinity of ¹⁶0¹H is 14 740.982(7) cm^-1 or 1.827 648 7(11) eV. 

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