1s2s 1S State Research Articles

Charge Exchange Dominates Long-Range Interatomic Coulombic Decay of Excited Metal-Doped Helium Nanodroplets.

Atoms and molecules attached to rare-gas clusters are ionized by an interatomic autoionization process traditionally termed "Penning ionization" when the host cluster is resonantly excited. Here we analyze this process in the light of the interatomic Coulombic decay (ICD) mechanism, which usually contains a contribution from charge exchange at a short interatomic distance and one from virtual photon transfer at a large interatomic distance. For helium (He) nanodroplets doped with alkali metal atoms (Li, Rb), we show that long-range and short-range contributions to the interatomic autoionization can be clearly distinguished by detecting electrons and ions in coincidence. Surprisingly, ab initio calculations show that even for alkali metal atoms floating in dimples at a large distance from the nanodroplet surface, autoionization is largely dominated by charge-exchange ICD. Furthermore, the measured electron spectra manifest the ultrafast internal relaxation of the droplet mainly into the 1s2s1S state and partially into the metastable 1s2s3S state.

An R-matrix with pseudo-state approach to the single photon double ionization and excitation of the He-like Li+ ion

The success of the R-matrix with pseudo-state (RMPS) method to model single photoionization processes, benchmarked against dedicated synchrotron light source measurements, is exploited and extended to investigate, single photon double ionization cross-sections for the He-like, Li+ ion. We investigate these processes from both the ground state and the excited 1s2s 1, 3S metastable levels of this He-like system. Comparisons of the results from the RMPS method are made with other state-of-the-art theoretical approaches such as time-dependent close-coupling (TDCC), B-splines and the convergent close-coupling (CCC). Excellent agreement with various other theoretical approaches are achieved but differences occur which are highlighted and discussed. For the ground state the peak of the cross-section is ∼2 kilo-barns (Kb), that for the 1s2s 1S state is ∼6 and ∼1 Kb for the corresponding 1s2s 3S state. All the cross-sections for single photon double ionization are extremely small, being in the region of 2–10 Kb, or less, rendering their experimental determination extremely challenging.

Angular distribution and polarization of photons during resonant above-threshold scattering on an atom

The effect of the resonant structure in the continuous spectrum induced by an external laser field due to the coupling of continuum states with the 1s4s1S level in helium on cross sections of elastic and inelastic scattering of probe radiation on an atom in the metastable 1s2s1S state at an energy above the ionization threshold is theoretically studied. Explicit expressions for the angular distribution and the degree of polarization of scattered photons as functions of probe and laser radiation characteristics are derived in the rotating wave approximation. Frequency, angular, and polarization dependences of the cross sections of scattering into final 1s21S, 1s2s1S, and 1s4s1S atomic states are calculated.

Photoionization of the excited He atom in Debye plasmas

We present theoretical photoionization cross sections for He 1s2s 1S and He 1s2p 1P states in a Debye plasma environment by the complex coordinate rotation method, using a finite L 2 basis set constructed from one electron Laguerre orbitals. The plasma environment is found to appreciably influence the photoionization cross sections near the ionization threshold. In this regard, the photoionization cross sections of isolated He are compared with other theoretical and experimental results. Our results are in good agreement with the previous results. A new minimum appears in the photoionization cross section curve for the metastable 1s2s 1S state. Results are given for the S- and D-wave partial photoionization for the excited 1s2p 1P state.

Resonance effects on the competition between multiphoton emission of inner-shell and outer-shell electrons

Using the R-matrix Floquet approach, we have investigated ionization of the 1s2s 1S and 1s3s 1S states of He subjected to VUV radiation, following the absorption of two, three and four photons, with a focus on the influence of resonances on the state that the residual He+ ion is left in. For the 1s2s 1S state, we find that final-state 3ℓnℓ′ resonances enhance the relative probability that He+ is left in the n = 2 states. Intermediate 2ℓnℓ′ resonances strongly enhance ionization following three-photon and four-photon absorption. The 2ℓ2ℓ′ resonances enhance in particular the probability that He+ ion is left in n = 2, while higher 2ℓnℓ′ enhance in particular the probability that He+ is left in n = 3, since photoionization of the 2ℓnℓ′ resonances is dominated by emission of the inner 2ℓ electron. For the 1s3s 1S state, we find a stronger influence of the final-state 3ℓnℓ′ resonances on the relative probability that after two-photon absorption He+ is left in n = 2.

Bound states of helium atom in dense plasmas

AbstractWe have obtained the bound 1s2 1S, 1s2s 1,3S, and 1s2p 1,3P states energies of helium atom in dense plasma environments in accurate variation calculations. A screened Coulomb potential to represent the Debye model is used for the interaction between the charged particles. A correlated wave function consisting of a generalized exponential expansion has been used to take care of the correlation effect. The 1s2 1S, 1s2s 1,3S, and 1s2p 1,3P states energies along with the ionization potential, the energy splitting between the 1s2s 3S, and 1s2s 1S states, transition energies between the ground state and low‐excited states of He estimated for various Debye lengths, are reported. The results show high degree of accuracy even under strong plasma conditions. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

Competition between multi-photon emission of the 1s and the 2s electron from He 1s2s 1S

Using the R-matrix Floquet approach, we have investigated multiphoton ionization of the 1s2s 1S state of He. We compare emission rates for the 1s and 2s electrons after absorption of N photons, where N is the minimum number of photons needed to emit the 1s electron. For N = 1 and 2, we find that emission of the 1s electron dominates, while for N = 4 emission of the 2s electron dominates. For N = 3, both processes are of comparable magnitude. In the absence of resonances, emission of the 1s electron typically becomes less likely by a factor of 5 for each additional photon absorbed. However, for N = 4, emission of the 1s electron is strongly enhanced due to the intermediate 2s2p 1P resonance.

Correlated radiative capture of two electrons with formation of heliumlike ion in the 1s2s 1S state

We investigate the correlated double-electron capture by a bare nucleus followed by the emission of a single photon and the formation of heliumlike ion in the excited 2 1 S state. The total cross section of the process is calculated taking into account the leading orders of QED perturbation theory. The present evaluation is focused on the domain of collision energies, which allows for the dipole approximation in treatment of the electron–photon interaction. The universal scaling is found for the ratio of cross sections for capture into the 2 1 S and 1 1 S states. In the low-energy regime, the probability for the population of excited states is shown to exceed significantly that of the ground state.

High precision calculations and QED effects for two- and three-electron atoms

Essentially exact results can now be obtained for the nonrelativistic energy and lowest-order relativistic corrections of helium. A combination of variational calculations and asymptotic expansion methods covers the entire singly-excited spectrum. Comparisons with recent high precision experiments allow the Lamb shift for the 1s2s1S state to be extracted with an accuracy approaching that in one-electron He+(2s). Techniques for calculating the two-electron Lamb shift are discussed, including recent asymptotic expansion methods for the Bethe logarithm. Extensions to lithium and other three-electron ions up to U89+ are briefly mentioned.

High precision variational calculations for the 1s 21S state of H − and the 1s 21s, 1s2s 1s and 1s2s 3s states of helium

High precision variational eigenvalues are presented for the 1s 21S state of H −, and the 1s 2 1S, 1s2s 1S and 1s2s 3S helium, together with relativistic, mass polarization and relativistic recoil corrections. In addition to the nonrelativistic energy, all corrections of order α 2, μ M ( μ M ) 2 and α 2 M are calculated to an accuracy of better than 10 −6 cm −1. Previous results of similar accuracy are available for the 1 sn d 1 D and 3 D states up to n = 8. A comparison with the high precision 1 s2 s 3 S 1−1 sn d 3 D 1 transition frequency measurements of Hlousek et al. indicates that the known quantum electrodynamic (Lamb shift) contribution of − 0.13488 cm −1 to the ionization energy of the 1s2s 3S 1 state is too small in magnitude by 0.00048(6) cm −1. A discrepancy of this size can reasonably be accounted for by as yet uncalculated contributions to the two-electron Lamb shift.

Three-parameter wavefunction for the 21S state of helium

A generalised simple Vinti wavefunction for the 1s2s1S state of He, depending on three parameters, is chosen in such a way that it is orthogonal to the ground-state function of Morse et al. (1935). The author corrects some errors in the numerical values of Eckart (1930) and Vinti (1931). The results obtained by the author's modification of Vinti's function are better than those obtained from the function of Marriott and Seaton (1957) and from the function of Das et al. (1879).

Self‐consistent calculation of 1s2s1S state wave function of helium

AbstractA simple self consistent variation perturbation method in the coupled Hartree–Fock scheme has been proposed to calculate 1s2s 1S state of the He atom. The present paper deals with an 1s2s 1S wave function in which all the relevant orthogonality conditions are imposed in successive stages. The resulting wave functions together with some interesting features are discussed.

Multi-Configuration Hartree–Fock Correlation Study of 1s2s1S

A correlation study is performed for the 1s2s1S state of helium to neon. It is shown that the Hartree–Fock approximation is poor, and that the two-configuration approximation [Formula: see text] is equivalent to a nonorthogonal Hartree–Fock approximation for which Koopmans' theorem holds. Additional configurations are introduced which account for 90% of the correlation energy.

A theorem on the Hartree-Fock equations for the nsms1S states of helium with orthogonal orbitals

A theorem is proved which shows that the Hartree-Fock orbitals for the nsms 1S states of helium can be assumed orthogonal if, and only if, the zero order total two electron wavefunction in a perturbation expansion in powers of 1/Z, where Z is the nuclear charge, is a certain linear combination of ns2 1S, nsms 1S and ms2 1S zero order wavefunctions. This provides some insight into the nature of constrained Hartree-Fock wavefunctions. The weights of 1s2 1S and 2s2 1S states in the total zero order wavefunction for obtaining orthogonal orbitals for the particular case of the 1s2s 1S state are computed numerically.

On the Hartree-Fock equations for the excited singlet S states of helium-like ions

Hartree-Fock integro-differential equations for the excited singlet S states and in particular for the 1s2s 1S state of helium-like ions are studied by the examination of the first-order equations in a perturbation expansion in powers of 1/Z; the complete set of hydrogenic eigenfunctions for s orbitals is used as the basis for the solution of these equations. The statement of a theorem due to Froese is shown to be in error and is corrected. Considerable insight into the roles of various zero-order states in the first-order function and the effect of various constraints on these is gained. It is shown that, for the nsms 1S state, the zero-order ns2 1S and ms2 1S states play a very important part in determining the effect of the constraints on the first-order Hartree-Fock functions.