Zong-Chao Yan

Nuclear Charge Radii of 9,11Li: The Influence of Halo Neutrons

The nuclear charge radius of 11Li has been determined for the first time by high-precision laser spectroscopy. On-line measurements at TRIUMF-ISAC yielded a 7Li-11Li isotope shift (IS) of 25 101.23(13) MHz for the Doppler-free [FORMULA: SEE TEXT]transition. IS accuracy for all other bound Li isotopes was also improved. Differences from calculated mass-based IS yield values for change in charge radius along the isotope chain. The charge radius decreases monotonically from 6Li to 9Li, and then increases from 2.217(35) to 2.467(37) fm for 11Li. This is compared to various models, and it is found that a combination of halo neutron correlation and intrinsic core excitation best reproduces the experimental results.

Nuclear Charge Radii of Be-7, Be-9, Be-10 and the one-neutron halo nucleus Be-11

Nuclear charge radii of ;{7,9,10,11}Be have been determined by high-precision laser spectroscopy. On-line measurements were performed with collinear laser spectroscopy in the 2s_{1/2}-->2p_{1/2} transition on a beam of Be+ ions. Collinear and anticollinear laser beams were used simultaneously, and the absolute frequency determination using a frequency comb yielded an accuracy in the isotope-shift measurements of about 1 MHz. Combining this with accurate calculations of the mass-dependent isotope shifts yields nuclear charge radii. The charge radius decreases from 7Be to 10Be and then increases for the halo nucleus 11Be. When comparing our results with predictions of ab initio nuclear-structure calculations we find good agreement. Additionally, the nuclear magnetic moment of 7Be was determined to be -1.3995(5)micro_{N} and that of 11Be was confirmed with an accuracy similar to previous beta-NMR measurements.

Isotope Shift Measurements of Stable and Short-Lived Lithium Isotopes for Nuclear Charge Radii Determination

Changes in the mean square nuclear charge radii along the lithium isotopic chain were determined using a combination of precise isotope shift measurements and theoretical atomic structure calculations. Nuclear charge radii of light elements are of high interest due to the appearance of the nuclear halo phenomenon in this region of the nuclear chart. During the past years we have developed a laser spectroscopic approach to determine the charge radii of lithium isotopes which combines high sensitivity, speed, and accuracy to measure the extremely small field shift of an 8-ms-lifetime isotope with production rates on the order of only 10 000 atoms/s. The method was applied to all bound isotopes of lithium including the two-neutron halo isotope {sup 11}Li at the on-line isotope separators at GSI, Darmstadt, Germany, and at TRIUMF, Vancouver, Canada. We describe the laser spectroscopic method in detail, present updated and improved values from theory and experiment, and discuss the results.

The long-range non-additive three-body dispersion interactions for the rare gases, alkali, and alkaline-earth atoms.

The long-range non-additive three-body dispersion interaction coefficients Z(111), Z(112), Z(113), and Z(122) are computed for many atomic combinations using standard expressions. The atoms considered include hydrogen, the rare gases, the alkali atoms (up to Rb), and the alkaline-earth atoms (up to Sr). The term Z(111) arising from three mutual dipole interactions is known as the Axilrod-Teller-Muto coefficient or the DDD (dipole-dipole-dipole) coefficient. Similarly, the terms Z(112), Z(113), and Z(122) arise from the mutual combinations of dipole (1), quadrupole (2), and octupole (3) interactions between atoms and they are sometimes known, respectively, as dipole-dipole-quadrupole, dipole-dipole-octupole, and dipole-quadrupole-quadrupole coefficients. Results for the four Z coefficients are given for the homonuclear trimers, for the trimers involving two like-rare-gas atoms, and for the trimers with all combinations of the H, He, and Li atoms. An exhaustive compilation of all coefficients between all possible atomic combinations is presented as supplementary data.

Nonrelativistic ab initio calculations for 2 S-2, 2 P-2, and 3 D-2 lithium isotopes: Applications to polarizabilities and dispersion interactions

The electric dipole polarizabilities and hyperpolarizabilities for the lithium isotopes Li-6 and Li-7 in the ground state 2 S-2 and the excited states 2 P-2 and 3 D-2, as well as the leading resonance and dispersion long-range coefficients for the Li(2 S-2)-Li(2 S-2) and Li(2 S-2)-Li(2 P-2) systems, are calculated nonrelativistically using variational wave functions in Hylleraas basis sets. Comparisons are made with published results, where available. We find that the value of the second hyperpolarizability of the 2 S-2 state is sensitive to the isotopic mass due to a near cancellation between two terms. For the 3 D-2 state polarizability tensor, the calculated components disagree with those measured in the sole experiment and with those calculated semiempirically.

Calculations of magnetic moments for lithium-like ions

Fully correlated calculations of the Zeeman gJ-factors for lithium-like ions in the 2 2S and 3 2S states are presented, including relativistic and radiative corrections of orders α2, α2m/M, and α3. The isotope shifts in gJ are determined accurately.

Oscillator strengths between low-lying ro-vibrational states of hydrogen molecular ions

It is important for experimental design to know the transition oscillator strengths in hydrogen molecular ions. In this work, for HD(+), HT(+), and DT(+), we calculate the ro-vibrational energies and oscillator strengths of dipole transitions between two ro-vibrational states with the vibrational quantum number ν = 0-5 and the total angular momentum L = 0-5. The oscillator strengths of HT(+) and DT(+) are presented as supplementary material.

High-order harmonic generation with Rydberg atoms by using an intense few-cycle pulse

We demonstrate that high-order harmonic generation (HHG) with both high cutoff frequency and high conversion efficiency can be realized by using a Rydberg atom in a few-cycle laser pulse. This is because a Rydberg state has a large electron orbital radius and small binding energy; therefore an electron in the Rydberg state can be ionized easily and accelerated directly toward the core under the interaction of a few-cycle laser pulse, leading to emission of harmonic photons. In this case, the tunneling process of the electron is not involved and, hence, the conversion efficiency and the cutoff frequency of harmonic generation can be higher than that predicted by the conventional three-step model.

Determination of resonance energies and widths in Ps–H scattering using the stabilization method

In an effort to explore computational methods to investigate resonances in atomic systems involving positrons, we present a calculation of resonances in positronium–hydrogen scattering, using the stabilization method. The density of resonance states is calculated using extensive Hylleraas-type wavefunctions. Resonance energies and widths for three S, two P, and two D states are determined, from the stabilization plots of the density of resonance states. Our results, obtained by using a real algorithm, are in good agreement with those obtained by using the method of complex-coordinate rotation.

D-wave resonances in e+-H scattering

The method of complex-coordinate rotation is applied to calculate D-wave resonances in e+-H scattering. The wavefunctions of the resonance states are constructed using Hylleraas coordinates. The resonance energies and widths below the positronium n = 4 threshold are reported. Comparisons are made with other available calculations.