R J Champeau

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

J-dependent isotope shifts in the ground term of samarium I

+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.

Three-step laser spectroscopy and multichannel quantum defect analysis of odd-parity Rydberg states of neutral ytterbium

Measurements are presented for the isotope shifts 144Sm-152Sm of the seven levels of the ground term 4f66s2 7F of Sm I. It appears that each level has a distinct shift, the shift difference between two levels being of the order of ten times the normal (Bohr) mass-shift difference. This J dependence is interpreted through the use of one parameter with the same angular dependence as the spin-orbit interaction. The relativistic Hartree Fock method yields ab initio field-shift evaluations which agree in sign and magnitude with the measured shifts.

Isotope shift of 20Ne, 21Ne and 22Ne and hyperfine structure of 21Ne in the lines 6096 AA and 6074 AA of neon I

High-lying odd-parity levels of neutral ytterbium are investigated by the method of selective three-step laser spectroscopy. Excitation from the ground state 4f146s2 is performed by means of three dye lasers and the Rydberg levels are detected by using the field ionisation method. Three different levels are used as starting levels of the transitions to Rydberg levels. Thus Rydberg levels pertaining to 6snp J=0-2 and 6snf J=2, 3 series as well as some 4f135d26s levels are observed. The new experimental data are analysed by means of the multichannel quantum defect theory. The theoretical analysis consists of a J by J parametrisation of the observed energy spectra. The perturbations of the Rydberg series 6snp J=1, 6snp 3P0 and 6snf J=3 by levels of the 4f135d26s configuration are clearly interpreted. The limitations of the MQDT models derived solely from energy data are discussed. A revised value of the first ionisation limit is proposed.

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

The odd-even isotope shift in the lambda =6096 AA and lambda =6074 AA lines of Ne I has been remeasured by conventional high-resolution optical spectroscopy; in contradiction with a recently published value, the shift has been found in good agreement with the mass-shift formula. In addition, the hyperfine structure coupling constants of the 1s4 and 2p4 levels of Ne I have been obtained.

Measurement by Fourier spectroscopy of the hyperfine structure of the lines λ=11522 Å (2s2 to 2p4) and λ=15234 Å (2s2 to 2p1) of neon (21Ne)

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.

Spectroscopic determination of the nuclear moments of 169Yb; relative isotope shift between the isotopes 168Yb, 169Yb and 170Yb

The values of the A and B hyperfine structure constants for the 2s2 and 2p4 levels of 21Ne have been measured using Fourier transform spectroscopy. In addition it is found that the centre of gravity for the 21Ne line lies between the 20Ne and 22Ne components in agreement with the isotope shift theory.

Photodetachment of trapped negative gold ions: I. Experimental methods and near-threshold cross sections

The nuclear spin, the nuclear magnetic dipole and electric quadrupole moments of the radioactive 169Yb isotope have been determined by measuring the hyperfine splitting for the line lambda =5556 AA (4f146s6p3P1 to 4f146s21S0) of Yb I. The results are: I=; mu =(-0.63+or-0.02) mu N;Q=(4.1+or-0.06) barns. The relative isotope shift between the isotopes 168Yb, 169Yb and 170Yb is found to be: delta sigma (168Yb-169Yb)/ delta sigma (168Yb-170Yb)=0.44+or-0.01.

Photoionisation laser spectroscopy of ytterbium atoms in the presence of a DC electric field

In this paper and in a subsequent one, we present relative and absolute measurements of photodetachment cross sections of negative gold ions in an RF trap. Several laser devices needed to be used. Absolute measurements are possible only when saturation occurs for a reasonable focusing of the laser and are deduced from the variations of photoelectron yield versus laser pulse energy. We describe the principle behind our measurements and we report on the results obtained in the energy range from threshold up to about above threshold. In this region, we essentially observe a slow departure from Wigners law (always less than 50%).

Photodetachment of trapped negative gold ions: II. Observation of window resonances

The authors report a study of the stepwise photoionisation of ytterbium atoms near the first ionisation threshold in the presence of a DC external electric field. Several spectra corresponding to ionisation from the levels 4f146s7s 1S0 and 3S1, 4f146s6p 1P10 and 4f146s5d 1D2 and to various polarisation schemes of the exciting light beams were recorded by detecting the ion signal. Modulations of the photoionisation cross sections above the field-free ionisation limit were observed and investigated: for the 6s7s 1S0 and 3S1 initial levels, the spectra exhibit modulations only for pi polarisation of the last excitation, spectra obtained via the 6s6p 1P10 level exhibit modulations for the pi pi polarisation scheme only. An experimentally unprecedented result is obtained for the 6s5d 1D2 level: in that case the largest modulation rate is obtained for sigma polarisation of the last excitation. The behaviour of the rate and of the period of modulation as a function of energy were studied. It was observed that the period of modulation as a function of energy were studied. It was observed that the period of modulation at the ionisation limit varies with the electric field intensity F according to a F3/4 law. The structure of the Stark spectra below the field-free ionisation limit was also investigated. Narrow resonances, all having a symmetrical shape, were recorded. They could be identified by calculating their energies using perturbation theory. A number of doubly excited levels were recorded, their influence on the spectra was analysed.