Detection of the 1pe series of doubly excited helium states below N=2 via the stark effect

The Stark effect on the doubly excited states of helium below the N=2 threshold has been studied by vacuum ultraviolet fluorescence yield spectroscopy. Two new series of states are observed at moderate fields (<10 kV/cm), and assigned to the previously unobserved even 1pe series, and a group of 1De series. The 1Se states are observed indirectly via their mixing with nearby 1 po states. The observations at moderate field contradict theoretical predictions that field strengths about an order of magnitude greater are necessary to observe the Stark effect on He doubly excited states at low quantum numbers.     

Radiative and relativistic effects in the decay of highly excited states in helium

A recent experimental study [J.-E. Rubensson et al., Phys. Rev. Lett. 83, 947 (1999)] measured a significant fluorescence yield of the He( 2lnl(')) photoexcited resonances, showing major qualitative differences from nonrelativistic predictions. We present a further theoretical study of these states, and perform R-matrix multichannel quantum defect theory calculations to extract fluorescence and ionization cross sections. These theoretical results are in excellent agreement with newer, higher-resolution measurements. Radiative and spin-orbit effects are quantified and shown to play an important role in the overall characterization of highly excited states.     

Radiative decay from doubly to singly excited states of He via generalization of Laguerre-type orbitals： A non-orthogonal formalism

By taking full account of the non-orthogonality of the orbitals between the low-lying doubly excited states ¹P^o and the singly excited states ¹S^e and ¹²D^e of He, the corresponding radiative decay rates have been investigated theoretically via analytic generalized Laguerre-type atomic orbitals at a nearly numerical multi-configuration self-consistent field accuracy in a general non-orthogonal configuration interaction scheme. From these rates, we calculate the VUV photon emission and metastable atom spectra, and both are found to be in good qualitative agreement with recent excellent measurements. We obtain, successfully, the enhancement of the VUV photon spectrum, experimentally observed at the energy of (2s4p-4s2p)/(2p,3d) ¹P^o as compared with other nearby lying states. The mechanism proposed by Odling-Smee et al is verified, implying that taking appropriate account of the overlap existing between orbitals of the low-lying doubly excited and singly excited states (especially important for the compact orbitals) can reveal basic physical dominant mechanism and is crucial in understanding these spectra.     

Properties of doubly excited states of H- and He associated with the manifolds from N = 6 up to N = 25

This paper discusses theory and results on ¹P⁰ doubly excited states (DES) in He and in H^- of very high excitation, up to the N = 25 manifold. Our calculations employed full configuration interaction (CI) with large hydrogenic basis sets and produced correlated wavefunctions for the four lowest roots at each hydrogenic manifold by excluding open channels and the small contribution of series belonging to lower thresholds. The suitability of the hydrogenic basis sets for such calculations is justified, apart from their practicality, by the fact that, by computing from them natural orbitals, the results were shown to be the same with those of earlier multiconfigurational Hartree-Fock (MCHF) calculations on low-lying DES. In total, 160 states were computed, most of them for the first time. Their energy spectrum should be of use to possible future photoabsorption experiments. For certain low-lying DES up to N = 13, for which previous reliable results are available, comparison of the calculated energies shows good agreement. The correlated wavefunctions contain systematically chosen single and double excitations from each hydrogenic manifold of interest. From their analysis, we determined the “goodness" of different quantum numbers and the geometry (average angles and radii) as a function of excitation. For the Sinano lu-Herrick ( K , T ) classification scheme, whose basis is a restricted CI with hydrogenic functions and which has thus far been tested only on low-lying DES, we established that, whereas T remains a good index as energy increases, K does not. Consequently, a more flexible than K quantum number is needed in order to account for most of the additional correlation. This number, represented by F = N - K - 1, where N and K are not good numbers anymore, produces consistently a much higher degree of purity than the ( K , T ) scheme does, especially as N increases and as the relative significance of various virtual excitations due to electron correlation increases. Among the four states of each manifold, in all cases in H^- and in most cases in He, the three are of the intrashell type and one is of the intershell type with ( F , T ) = (0, 0). The lowest intrashell states and the lowest intershell states exhibit a wide angle geometry tending to 180 ^° as N ↦∞. Received 10 September 2001 and Received in final form 12 November 2001     

Experimental evidence of (2)He decay from (18)Ne excited states

Two-proton decay from (18)Ne excited states has been studied by complete kinematical reconstruction of the decay products. The (18)Ne nucleus has been produced as a radioactive beam by (20)Ne primary projectile fragmentation at 45 AMeV incident energy on a Be target. The (18)Ne at 33 AMeV incident energy has been excited via Coulomb excitation on a (nat)Pb target. The obtained results unambiguously show that the 6.15 MeV (18)Ne state two-proton decay proceeds through a (2)He diproton resonance (31%) and democratic or virtual sequential decay (69%). The quoted branching ratio has been deduced from relative angle and momentum correlations of the emitted proton pairs.     

Attosecond two-photon interferometry for doubly excited states of helium

We show that the correlation dynamics in coherently excited doubly excited resonances of helium can be followed in real time by two-photon interferometry. This approach promises to map the evolution of the two-electron wave packet onto experimentally easily accessible noncoincident single-electron spectra. We analyze the interferometric signal in terms of a semianalytical model which is validated by a numerical solution of the time-dependent two-electron Schr?dinger equation in its full dimensionality.     

The effects of doubly excited states on ionization balance

The effects of highly doubly excited states on ionization balance are investigated. In the calculation, A Collisional-Radiative model in Detailed-Configuration-Accounting (DCA) is applied to population calculations for NLTE plasmas. Configuration-averaged rate coefficients that needed in the rate equations are obtained based on the first order perturbation theory. The Hatree-Fock-Slater self-consistent-field method is used to calculate the electron wave functions. The mean ionization stage of high-Z plasma Lu is presented. The comparison shows that the mean ionization stage increases more than 3 stages when doubly excited states 5l6l' and 5l5l' are not included in the population calculations.     

The effects of doubly excited states on ionization balance

The effects of highly doubly excited states on ionization balance are investigated. In the calculation, A Collisional-Radiative model in Detailed-Conflguration-Accounting （DCA） is applied to population calculations for NLTE pl＆smas. Configuration-averaged rate coefficients that needed in the rate equations are obtained based onthe first order perturbation theory. The Hatree-Fock-Slater self-consistent-field method is used to calculate theelectron wave functions. The mean ionization stage of high-Z plasma Lu is presented. The comparison shows thatthe mean ionization stage increases more than 3 stages when doubly excited states 5l6l＇ and 5l5l＇ are not included in the population calculations.     

Geometric quantities of lower doubly excited bound states of helium

Expectation values of single electron and interelectronic geometric quantities such as $\langle r\rangle$, $\langle r_{12}\rangle$, $\langle r_<\rangle$, $\langle r_>\rangle$, $\langle \cos\theta_{12}\rangle$ and $\langle \theta_{12}\rangle$ are calculated for doubly excited $2{\rm p}n{\rm p}\,{}^1P^{\,\rm e}\,(3\leq n\leq5),\, 2{\rm p}n{\rm p}\,{}^3\!P^{\,\rm e}\,(2\leq n\leq5)$ and $2{\rm p}n{\rm d}\,{}^{1,3}D^{\,\rm o}\,(3\leq n\leq5)$ states of helium using Hylleraas-$B$-spline basis set. The energy levels converge to at least 10 significant digits in our calculations. The extrapolated values of geometric quantities except for $\langle \theta_{12}\rangle$ reach 10 significant digits as well; $\langle \theta_{12}\rangle$ reaches at least 7 significant digits using a multipole expansion approach. Our results provide a precise reference for future research.     

Radiative lifetimes of some excited states of Hg+ and Hg2+

A delayed-coincidence method with pulsed electron excitation was used to measure the radiative lifetimes of some 5d⁹6snl states of Hg⁺. Radiative lifetimes of five long lived 5d⁸6s² states of Hg²⁺ and some Hg²⁺ spectral line transition probabilities are also reported. The data are obtained for the first time.     

Double-beta decay to the excited states (Experiment)

The brief review on the decay to excited states of daughter nuclei is performed. New positive result of experiments with ¹⁰⁰Mo for transition to 0⁺ excited state in ¹⁰⁰Ru are presented.     

Radiative lifetimes for excited states in doubly ionized elements (21 ≤ Z ≤ 30) of astrophysical interest

Beam-foil technique has been applied to measurements of radiative lifetimes of 23 terms or levels, all belonging to the 3dⁿ4p (n = 0 to 9) configurations, in Sc(III), Ti(III), V(III), Cr(III), Mn(III), Fe(III), Ni(III) and Zn(III). The experimental lifetimes are in good agreement with lifetimes derived from oscillator-strength calculations, based upon the scaled Thomas-Fermi method, for ultraviolet lines in the spectra of the doubly ionized transition elements.     

Ultra-long-range states in excited 3He2

Long-range potentials have been calculated for ³He₂ molecules dissociating to ³He 2 ³S + ³He 2 ³P, including the retarded dipole and the van der Waals interactions. Ultra-long-range wells with depths of up to about 2.4 GHz have been found in many of the adiabatic body-fixed potentials and rovibrational levels have been calculated for some of these wells, which have been found to support up to 4 rotationless vibrational levels.     

Energies and electron correlation effects in two-electron doubly excited (nl)2 states

Energies of singlet doubly excited states 2p^{2 1}D, 3d^{2 1}G, 4f^{2 1}I of He isoelectronic series are calculated in the framework of the variational method by using Hylleraas-type wave functions and a real Hamiltonian. The results obtained show quantitatively the importance of electron correlation effects in the doubly excited states and they are in good agreement with some experimental data and other theoretical results. Received 30 March 2000 and Received in final form 4 July 2000     

Observation of triplet doubly excited states in single photon excitation from ground state helium

We have observed, for the first time, LS-forbidden triplet doubly excited states, in single photon excitation of ground state helium, below the second ionization threshold. These states are identified as (3)D(o) and (3)P(o) and their excitation is due to spin-orbit interaction that mixes them with the optically allowed (1)P(o) states. This observation is possible due to the very high efficiency in detecting metastable atoms created after the fluorescence decay of the doubly excited states, and the new capabilities of third generation synchrotron vacuum ultraviolet sources with high resolution beam lines.     

Radiative decays of doubly excited states of oxygen,carbon, and nitrogen ions

Systematic calculations of the probabilities and energies of radiative transitions in doubly excited 3l3l′, 3l4l′, and 4l4l′ states of oxygen, carbon, and nitrogen ions have been carried out within the Hartree-Fock approximation. The emission spectra are obtained, and analysis of the changes in the spectral characteristics with a change in the charge and the number of additional electrons in the ground state of the initial ion is performed. The data obtained are used to interpret the X-ray spectra of the atmospheres of planets and comets.     

Population densities of He(I) and He(II) excited states in plasmas

Reduced population coefficients for He(I) and He(II) levels, both for optically thin and partially optically thick conditions, have been calculated from a collisional-radiative model using recent experimental and theoretical cross sections. The calculated population densities are compared with experimental results on helium plasmas. Agreement with experimental results is better at low electron temperatures.     

Charge distribution of He states, excited by electron capture in He+ (26 keV)-He collisions

Using anticrossing spectroscopy we investigated one-electron capture in He⁺ (26 keV) – He collisions. The intensity of the triplet line λ (1s4l ³D → 1s2p ³P) ≈ 447.2 nm emitted by the fast atoms was measured as a function of an external electric field in the collision volume varied from ?30 kV/cm to +30 kV/cm. The radiative decay of excited He atoms in an almost field-free region outside the collision volume was observed. The highly asymmetric intensity function could be reproduced theoretically using the density-matrix formalism. It indicates that the excited He atoms are in a state with a highly forward-backward asymmetric charge distribution, immediately after the charge-exchange collision.