Excited States Of He Research Articles

Expectation values of p1

We examine expectation values of p/sub 1/xp/sub 2/ in ground states of He and H/sup -/, the singly and doubly excited states of He, and the ground and low-lying valence states of the alkaline-earth-metal atoms. A simple commutator relation derived by Froese Fischer and Smentek-Mielczarek (J. Phys. B 16, 3479 (1983)) can be used to explain the sign and relative magnitudes of in the ground states of He and H/sup -/ and singly excited states of He. In the doubly excited states of He and in the ground and low-lying valence states of the alkaline-earth metals, the molecular model, which treats the electron--core-electron system as a floppy, linear triatomic ''molecule,'' correctly predicts signs and general trends for . These results represent an important test of the molecular model. The ability of this model to make intuitive predictions about quantities like is a strong confirmation of its validity and utility.

Transition Wavelengths Among Doubly Excited States of He I, Li II, and Be III

We report theoretical calculations of doubly excited states for He I, Li II, and Be III below the N = 2 thresholds. Energy levels for autoionizing states are determined by a method of complex-coordinate rotation. Energy levels for non-autoionizing states are determined by the standard variational method, or taken from the best variational results in the literature. The theoretical transition wavelengths among various doubly excited states are compared with those observed in beam-foil experiments.

Electronic structure and the mechanism of autoionization for doubly excited states

Apart from pure phenomenology, the rigorous and quantitative study of many-electron autoionizing states presents intriguing questions as regards their structure and dynamics. In this paper we present an analysis of such states within astate specific theory with application to five low-lying doubly excited states (DES) of He. The zeroth order description is multiconfigurational and is obtained numerically at the MCHF level. In this way, major radial and angular correlations are accounted for accurately, and reliable predictions can be made without the requirement of large computations. The additionallocalized correlation is obtained by optimizing variationally analytic virtual orbitals. Core orbitals corresponding to the open channels are projected out of the MCHF solutions, as well as out of the correlation functions. Theasymptotic correlation, which gives rise to autoionization, is computed from a multichannel reaction matrix approach. It is shown that when calculating the width, the Rydberg series should not be part of the localized correlation if the frozen-core Hartree-Fock scheme is used for the generation of bound and scattering orbitals of each channel. Our calculations of the width of the first three1 S DES of He include the effects of coupling of resonances via the continuum. The wavefunctions for the 2s 2 and 2p 2 1 S DES, together with a Hartree-Fock scattering orbital in the field of He+ 1s are employed for the calculation of two quantities introduced by Rehmus, Ezra and Berry to the study of doubly excited states of He. The “conditional probability density”, ρ, and the “differential transition amplitude”,D. As regards ρ, our approach yields results which agree with those of Rehmus et al. However, as regardsD our results contradict their conclusions. In particular, we are forced to disagree with their proposal about the “generalizedS N − 2 mechanism” for atomic autoionization. Instead, we propose a mechanism which is based on the interelectronic repulsion and the heavy overlap of bound and scattering orbitals in small regions of space. This mechanism is in harmony with the generally observed slower rates of autoionization for triplets as compared to those of singlet states of the same orbital angular momentum.

Distortion effects in the (e,2e) spectroscopy of helium at high momentum

The effect of distortions on the (e,2e) spectroscopy of helium is examined. In particular, it is found that distortions contribute significantly to the (e,2e) cross section for transitions to both the n=1 ground state and n=2 first excited states of He+ at incident energies as high as 1200 eV. This raises questions about the use of uncorrected (e,2e) cross sections at high momenta to test the accuracy of correlated wavefunctions for the ground state of helium. By numerically correcting new experimental data for distortions, it is possible to obtain improved agreement between experiment and calculated values of the momentum space overlap integral (f mod exp(iq.r)g). The helium wavefunction used in the calculations is a recent state-of-the-art CI wavefunction which gives better than 98% of the correlation energy.

Lyman-? excitation spectra in the photodissociation of the doubly excited states of H2

A Lyman-α excitation spectrum has been observed using synchrotron radiation in the energy region corresponding to the double electron excitation of H2. There exist in the spectrum three thresholds at 26.6±0.5 eV, 29.2±0.7 eV and 30.9±0.6 eV, and a dip at 34.1±0.5 eV. A Lyman-α excitation spectrum in the energy region corresponding to the single electron excitation has been also observed using a detection system which works as a band pass filter for detecting of Lyman-α fluorescence. The cross section of Lyman-α fluorescence in the photodissociation of the doubly excited states is very small, e.g., in the order of 10−20 cm2 at 30 eV, in comparison with that from the single electron excitation.

Autoionisation widths of the3P0and3Dedoubly excited states of He

Scattering of electrons by the He+ ion has been studied in the vicinity of the 2s2p 3P0, 2p3p 3De and 2p4p 3De autoionising states of the helium atom using a 14-state close-coupling approximation scheme. The resonance parameters (energy, width and background phaseshift) are calculated for these states. The autoionisation widths for these states are obtained to be 8.19 meV, 0.873*10-6eV and 0.313*10-6eV respectively. The present autoionisation width for the 2s2p 3P0 is in good agreement with a recent calculation of Ho (8.14 meV) and with experimental line width of Cederquist et al (1983) for the transitions 2s2p 3P0-2p3p3De (8.36+or-0.08 meV) and 2s2p 3P0-2p4p 3De (8.32+or-0.15 meV). However, the smaller experimental linewidth (7.12+or-0.12 meV) from 2s2p 3P0-2s3p 3Pe transition remains unexplained. The few available theoretical widths for the other resonance states do not agree with each other.

Lifetime measurements of VUV emitting levels in He, Ne, Ar and CO using the inelastic electron-photon delayed coincidence technique

The electron-photon delayed coincidence technique has been used to measure the lifetimes of excited states in He, Ne, Ar and CO that decay with the emission of VUV photons. The states with their measured lifetimes are: He, 51P 7.4+or-0.4 ns, 61P 14.0+or-0.3 ns, 71P 21.6+or-1.2 ns; Ne, 3s(3/2)1 20.5+or-0.7 ns; Ar, 4s(3/2)1 7.9+or-0.6 ns; CO,C 1 Sigma +v=0<or=1.5+or-0.3 ns.

Electron correlation in doubly excited states of helium and extensions to beryllium and magnesium

Abstract Connections are established between the treatment of doubly excited states in He, using hyperspherical coordinates and the model of two particles on a sphere related to the use of dynamical group theory. The relevance of this for model wavefunctions of the type antisymmetrized geminal power (AGP) or separated pair approximation (SPA) is shown through the analysis and numerical applications of an effective equation for the valence geminal in Be and Mg.

Electron correlation and relativistic effects on the energies and widths of doubly excited states of He

By employing a many-electron approach and the Breit-Pauli Hamiltonian, the authors have computed the energy levels and autoionisation widths of the 2p2, 2p3p, 2s3d 3D and 1D states and the fine structure of the 2p2, 2p3p 3P, and 2p3p, 2s3d 3D states of He. The fine structure of 2p3p 3D (3.9 cm-1) is apparently not large enough to explain the recent puzzling findings on the width of 2s2p 3P0 by Cederquist et al. (1983). Comparison with the previous large scale calculations shows general agreement for the energies and widths, except for the 2p3p 3D state.

Decay Modes and Lifetimes of Doubly Excited States in He

We present results of many-electron calculations of the distribution of transition probability from doubly excited states of He over the radiative and radiationless decay channels. For He 2p3p 3D, the calculated lifetime is τth = 0.107 × 10-9 s compared with the beam-foil experimental value of τ = 0.115 ± 0.006 × 10-9 s. Our value for the autoionization width (Γ = 0.61 × 10-6 eV) differs significantly from those of three previously many-electron theories of resonances: 1.44 × 10-6 eV, 2.72 × 10-6 eV and 214 × 10-6 eV.

He- Photodetachment: Effects of the 1s2p2p'4Pe Resonance

He- exists in a metastable state, (1s2s2p)4Po, lying 19.74 eV above He(l 1S), with substate lifetimes of 10-500 μs. Following a preliminary survey of the photodetachment cross section over a range of energies, we have concentrated on a large shape resonance associated with the first excited state of He-, 1s2p2p' 4pe which recent calculations have found to lie just above He(23P). The resonance has a profound effect on the overall photodetachment spectrum.

Use of the atomic Fues potential for the excited states of He: energies and oscillator strengths

Use of the atomic Fues potential for the excited states of He: energies and oscillator strengths

Use of the atomic fues potential for the excited states of He: energies and oscillator strengths

The atomic Fues potential has been applied to the description of the excited state in helium. The accuracy in the energies is better than 4.5 × 10 −4 au. the oscillator strengths are well described in the model potential formulation, with an accuracy generally better than 12π.

Multi-Photon Ionization of Excited Atoms in Gaseous Discharge by Ruby Laser Light

Multi-photon ionization of excited states of He or Hg atoms in glow discharge tube by ruby laser light were detected by using the induced variation of discharge current. The probabilities of one-photon, two-photon and three-photon ionizations with an intermediate resonance were also calculated. Laser induced current variation was explained by the calculated value of multi-photon emitted electrons. It was found that two-photon resonant three-photon ionization is dominant in He and three-photon ionization in Hg in case of rather high intense laer light.

Energy levels of doubly excited states of He

The doubly excited states of He, having both electrons in the same $n$ shell, are investigated by constructing a diagonal approximation for ${R}_{12}^{2}$ in terms of SO(4) operators. By inverting this operator and taking the square root, an approximate operator expression for $\frac{1}{{R}_{12}}$ is obtained. Energies of states in the ${(n=3)}^{2}$ shell of He are calculated using this expression and compared with results of more elaborate calculations. Good agreement is found in most cases.

The α∗ and the neutron scattering lengths of 3He

We have investigated the excitation spectrum of 4He around E x = 20 MeV by inelastic scattering of 64 MeV α-particles from 4He. From an R-matrix representation of our data we have extracted level parameters for the α ∗, the first excited state of 4He: E λ = 20.29(2) MeV, Γ 0 = 0.89(4) MeV and ( θ 2 N) 0 = 0.4(1). We are able to predict reliable low-energy p+ 3H and n+ 3He s-wave phase shifts. Our results help to resolve a long standing ambiguity in the neutron scattering lengths of 3He, for which we obtain a s = 7.0(4) + i4.4448(9) fm and a t = 3.4(2) fm. We find the α ∗ to be an instructive example for the influence of thresholds on level parameters and line shapes.

Charge exchange in He ++Li collisions

A two-state model, based on atomic wave functions and allowing for momentum transfer factors, is employed to calculate cross sections for the formation of excited states of He + in collisions of He ++ ions with Li in the ground state, for energies in the range 5–64 keV. It is shown that the selective capture into the n=3 levels of He +, predicted at low energies by Shipsey et al., persists up to energies of ≈30 keV.

Search for highly excited states of high spatial symmetry in 8Be

The 7Li(p, αt)p and 6Li(d, αt)p three-particle reactions are investigated in a kinematically complete experiment for E p = 4.5–12.0 MeV and E d = 1.2–8.0 MeV, respectively, in order to extract excitation functions and angular distributions belonging to the sequential decay via 4He∗(0 +) = α∗, the first excited state of 4He. A prominent bump observed in both excitation functions is discussed in terms of compound levels in 8Be and in connection with theoretical predictions of a rotational band in 8Be having an αα∗ structure. Furthermore the experimental results obtained are compared with those for (π −, 2n) reactions.

A study of interaction in the lowest singlet and triplet states of H2

The potential energy curves of the ground state and the first excited state of H2 are examined in terms of the electronic force acting on each nucleus. The results reveal the detailed course of events that occur when two hydrogen atoms with parallel and antiparallel electron spins approach one another from a large internuclear separation.

Helium abundance anomalies and radiative forces in stellar envelopes

view Abstract Citations (70) References (41) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Helium abundance anomalies and radiative forces in stellar envelopes. Michaud, G. ; Montmerle, T. ; Cox, A. N. ; Magee, N. H., Jr. ; Hodson, S. W. ; Martel, A. Abstract To determine at what effective temperature the largest amount of helium can be supported, detailed radiative acceleration calculations are made for helium in stellar envelopes with log g between 3.5 and 4.5 and effective temperatures between 10,000 and 25,000 K. It is found that as the effective temperature increases above 18,000 K, the radiative acceleration does not increase substantially in the envelope; thus, diffusion cannot lead to (He-4)-rich stellar envelopes. A calculation is made of the contribution of the lines of He I to the effective radiative acceleration, showing strong dependence on the collisional excitation and ionization rates of the excited states of He I, after absorption of a photon. Publication: The Astrophysical Journal Pub Date: November 1979 DOI: 10.1086/157489 Bibcode: 1979ApJ...234..206M Keywords: Abundance; Helium; Radiative Transfer; Stellar Atmospheres; Stellar Envelopes; Gas Ionization; Line Spectra; Particle Acceleration; Photoionization; Stellar Temperature; Stellar Winds; Astrophysics; Helium:Stellar Atmospheres; Radiative Transfer:Stellar Atmospheres; Stellar Atmospheres:Models full text sources ADS |