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We have calculated atomic helium spontaneous decay rates and absorption oscillator strengths for the spin-forbidden transitions from 3′,3P 1,2 and 3′,1P 1 to all lower 1S 0 and 3S 1 states. In particular we found A 10=44.33(4)′, s -1 for the E1 transition 3′,3P 1-1′,1S 0 and 0.1147(1) s -1 for the M2 transition 3′,3P 2-1′,1S 0. © 2011 American Physical Society.

The relativistic corrections to the theoretical oscillator strengths of light elements such as helium are typically less than 0.1% and usually are ignored. However, they can be important for comparisons with the most accurate experiments, and they rapidly increase in magnitude with increasing nuclear charge. We have begun with the spin-forbidden electric-dipole transitions of neutral helium, using calculations consisting of (1) extremely accurate wave functions without relativistic corrections for both infinite and finite nuclear mass, (2) spin-changing matrix elements through the perturbations of the wave functions by the spin-orbit and spin-other-orbit Breit operators, (3) the use of pseudostates in the sums over all the intermediate states including the continuum, and (4) the inclusion as perturbers of the 1S 0 and 3S 1 states the pseudostates corresponding to the doubly excited npn′p 3P 0 e and npn′p 1P 1 e terms, respectively. As examples of these calculations, we present oscillator strengths for the transitions 1 1S 0-2 3P 1, 2 1S 0-2 3P 1, 2 3S 1-2 1P 1, 2 1P 1-3 3D 1,2, and 2 3P 1,2-3 1D 2.

The nuclear charge radii of $^{7,9,10,11}$Be have been determined by high-precision laser spectroscopy. On-line measurements were performed at ISOLDE with collinear laser spectroscopy in the $2s_{1/2} \to 2p_{1/2}$ transition of Be$^{+}$. Simultaneous measurements in collinear and anti-collinear direction combined with absolute frequency determination using a frequency comb yielded accuracy in the isotope shift measurements of better than 1 MHz. Combined with accurate calculations of the mass-dependent isotope shifts the nuclear charge radii along the isotopic chain were extracted. The charge radius decreases from $^7$Be to $^{10}$Be and then increases for the halo nucleus $^{11}$Be. When comparing our results with predictions of {\it ab-initio} nuclear structure calculations we find good agreement. Additionally, the nuclear magnetic moment of $^7$Be was determined to be 1.3998(15) $\mu_{\rm N}$ and that of $^{11}$Be was confirmed with an accuracy similar to previous $\beta$-NMR measurements. The nuclear charge radii of $^{7,9,10,11}$Be have been determined by high-precision laser spectroscopy. On-line measurements were performed at ISOLDE with collinear laser spectroscopy in the $2s_{1/2} \to 2p_{1/2}$ transition of Be$^{+}$. Simultaneous measurements in collinear and anti-collinear direction combined with absolute frequency determination using a frequency comb yielded accuracy in the isotope shift measurements of better than 1 MHz. Combined with accurate calculations of the mass-dependent isotope shifts the nuclear charge radii along the isotopic chain were extracted. The charge radius decreases from $^7$Be to $^{10}$Be and then increases for the halo nucleus $^{11}$Be. When comparing our results with predictions of {\it ab-initio} nuclear structure calculations we find good agreement. Additionally, the nuclear magnetic moment of $^7$Be was determined to be -1.3995(5) $\mu_{\rm N}$ and that of $^{11}$Be was confirmed with an accuracy similar to previous $\beta$-NMR measurements.

Perched precariously on the brink of nuclear stability, 11Li has the lowest two-neutron binding energy of all nuclei. This gives rise to the exotic phenomena of a nuclear halo that has a wavefunction extending beyond the range normally allowed for by the strong interaction. The most accurate mass measurements are achieved using Penning traps. In this letter, we report a new mass for 11Li using the TITAN trapping experiment at TRIUMF's ISAC facility where 'designer atomic nuclei' can be produced and studied in detail. This is by far the shortest-lived nuclide, t1/2 = 8.8 ms, for which a mass measurement has ever been performed with a Penning trap. We derive a new two-neutron separation energy of 369.15(65) keV: seven times more precise than the best previous value. We also report results from state-of-the-art atomic-physics calculations using the new mass and extract a new charge radius for 11Li from recent isotope shift measurements

The root-mean-square (rms) nuclear charge radius of He8, the most neutron-rich of all particle-stable nuclei, has been determined for the first time to be 1.93(3) fm. In addition, the rms charge radius of He6 was measured to be 2.068(11) fm, in excellent agreement with a previous result. The significant reduction in charge radius from He6 to He8 is an indication of the change in the correlations of the excess neutrons and is consistent with the He8 neutron halo structure. The experiment was based on laser spectroscopy of individual helium atoms cooled and confined in a magneto-optical trap. Charge radii were extracted from the measured isotope shifts with the help of precision atomic theory calculations.

Expérience GANIL; International audience; The root-mean-square (rms) nuclear charge radius of ^8He, the most neutron-rich of all particle-stable nuclei, has been determined for the first time to be 1.93(3) fm. In addition, the rms charge radius of ^6He was measured to be 2.068(11) fm, in excellent agreement with a previous result. The significant reduction in charge radius from ^6He to ^8He is an indication of the change in the correlations of the excess neutrons and is consistent with the ^8He neutron halo structure. The experiment was based on laser spectroscopy of individual helium atoms cooled and confined in a magneto-optical trap. Charge radii were extracted from the measured isotope shifts with the help of precision atomic theory calculations.

International audience; The basic formulations of nonreactive scattering are presented in the sections to follow. The quantal approach to this problem is outlined. Specific symmetries, and their closely related conservation laws, which reduce the complexity of computation, are discussed, along with the usual coordinate systems used to express the necessary scattering equations. The derivation of the matrix elements needed for a given calculation is provided in the case of a diatom-diatom collision. Semiclassical and quasi-classical methods are also briefly introduced. The chapter ends with a comparison between theory and experiment for the benchmark CO–H2 system and an outline of future directions in the field. © 2023, Springer Nature Switzerland AG.

Two mechanisms, shake--off and quasi--free absorption, suffice to describe the ratio $R_{P}$ of double to single ionization of Helium in photoeffect at high X--ray energies 3--100 keV, where it is not constant but exhibits a linear risecite{amusia75,drukarev95,drukarev95a}, egin{equation} R_{P}=R_{P}^{s.o}+C_{Z}frac{omegahbar}{mc^{2}} label{petrue}. end{equation} Here $R_{P}^{s.o}$ is the constant shake--off contribution while the second term is the quasi--free contribution at these energies. (In the case of He $R_{P}^{s.o}=0.0168$, $C_{2}=0.03$.) We wish to show that these two mechanisms, shake--off and quasi--free absorption, which suffice to describe the high energy behavior of the total cross-section for double photoionization of helium, can be treated in a similar way. Both mechanisms can be understood in terms of the high momentum behavior of the Fourier transform of the initial state wave function.