C Blondel

Pulsed photodetachment microscopy and the 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. The result is 2467 287.4(29) m−1 or 3.059 0463(38) eV.

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 2P1/2 − 2P3/2 energy difference in S−, and 39 605.87(32) m−1 for the 3P1 − 3P2 one in S (with expanded uncertainties), consistent with an independent measurement of the 3P0 − 3P1 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.

Measurement of the three-photon detachment cross sections of the negative ions of iodine, bromine and fluorine at the wavelength 1.0642 mu m

The three-photon detachment cross sections of the ions F-, Br- and I- are measured, using the fundamental 1064.2 nm wavelength of a single-mode Nd:YAG laser. The detachment signal is measured by detecting the neutral atoms produced from an ionic beam of kinetic energy 1.2 keV. The data analysis takes into account the laser pulse time profile, the actual spatial profile of the laser beam in the interaction region, and the fast motion of the ions through the laser beam during the laser pulse. Saturation of the three-photon process is observed and used to normalise the detachment signal. The authors obtain sigma (F-)=6.1

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

+2.6-1.8*10-95 s2 m6, sigma (Br-)=1.6+0.8-0.6*10-94 m6 and sigma (I-)=3.3+16-1.1*10-94 s2 m6. These measured cross sections are compared with theoretical predictions. The result obtained on F- agrees with a very recent measurement performed with a different technique.

Photodetachment microscopy of O

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 2Do3/2 and 2Do5/2 energy levels of the neutral atom. The fine-structure intervals of the 3s23p3 2Do 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.

Three-Photon Detachment of Iodine Negative Ions

Eur. Phys. J. D 5, 207–216 (1999)The formula (9) was erroneous. The exponent of the numerical factor should be 2/3 instead of 3/2, and whole formula should read

Excess-photon absorption in a negative ion

I- ions formed in an ionic beam from a hot cathode discharge source are submitted to the intense light pulse of a 1.064 μm Nd:YAG laser. The neutral atoms produced by three-photon detachment are detected. Varying the laser pulse energy and recording the corresponding variation of the detachment signal makes obvious the saturation of the three-photon process. This observation gives an approximate value of the generalized cross-section, σ = 1.5 10-37 s-1 W-3 m6.

Two-step high-resolution laser spectroscopy of the Stark substates of the n=44 level in atomic hydrogen

A time-of-flight and angular selection of the electrons ejected by multiphoton detachment of F- at the wavelength 1064 nm yields the first experimental evidence for excess-photon absorption in a short-range potential. This makes the coherent character of excess-photon ionization or detachment conspicuous, for no photons could be absorbed by an electron which has untied its bonds to the atomic core. The experimental angular distributions are compared to theoretical angular distributions which were obtained within the free-electron approximation. The time-of-flight spectra of the ejected electrons show that slow detached electrons can receive a super-acceleration of more than 1 eV from ponderomotive effects.

Electron affinity of tin measured by photodetachment microscopy

Two synchronised pulsed single-mode tunable laser systems are used to excite atomic hydrogen in an atomic beam. A vacuum ultraviolet (VUV) laser populates first the 2p 2P3/2 sublevel from the ground 1s 2S1/2 state. A second-step UV laser selects substates of the n=33 manifold in the presence of a weak electric field (23.6 V cm-1). Stark patterns of this manifold are recorded for different polarisation configurations of the exciting light beams. The n=44 substates are detected by the standard field ionisation technique. The measured relative intensities of the Stark components agree with the theoretical excitation probabilities in the two-step process, calculated to the first order of perturbation theory.

Images of a photoelectron interfering with itself: the photodetachment microscope

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.