Excitation Of Autoionizing States Research Articles

Time dependence and properties of nonstationary states in the continuous spectrum of atoms

The recently measured shape resonance, the Ca KLM 3d5p doubly excited autoionizing state and the long-lived 1s2s2p metastable level were treated as nonstationary states satisfying the time-dependent Schrödinger equation (TDSE). The lifetimes of the first two are short, of the order of s, and the solution of the TDSE well into times where nonexponential decay (NED) is established, is achievable via the state-specific expansion approach (SSEA), according to which the time-dependent solution has the form . is the square-integrable wavefunction of the localized state at t = 0 and X(t) is composed mainly of energy normalized scattering functions with time-dependent coefficients. The coefficient c(t) is related to the survival amplitude, , by , where the overlap matrix element appears when the function spaces are not completely orthonormal. For the diffuse resonance, its analysis as a decaying state has as a prerequisite the calculation of a reliable , with correlation between the two valence electrons. This has been achieved by a special procedure and a related discussion is given. The proximity of the energy E to threshold ( meV), the closeness of the ratio to unity ( is the resonance width) and the energy dependence of the bound - free matrix element, produced the result that NED should appear after only two lifetimes, when the probability of finding the system in the initial state is still non-negligible. From the exponential part of the decay curve, the width was found to be meV, in agreement with the recent width of meV derived from measured cross sections in recent collision experiments (Lee et al 1996). The shortness of the time for which exponential decay (ED) holds and the fact that the survival probability, P(t), is still significant at the beginning of the NED, does not allow the rigorous justification of the definition of the lifetime from , or the equivalence of this with the observed energy width. Thus, we propose that a mean life, , should be obtained from Calculation produces s and s. For the Ca state, whose bound - free interaction is smooth and nearly constant from zero to about 5.5 eV, NED appears after 17 lifetimes. The lifetime of Ca is deduced from the exponential decay (ED) part of P(t) to be s. From our examination of the case of the level by a number of methods based on the use of state-specific wavefunctions, we conclude that for metastable states whose lifetimes are in the range - s, the ab initio calculation of P(t) is, at present, prohibited by the huge requirements for computer time. Finally, having computed the amplitude , we obtain numerically the energy distribution function, , of the two autoionizing states. In the case of Ca it is a perfect Lorentzian.

Multiphoton ionization of magnesium via an autoionizing state.

Multiphoton single and double ionization of magnesium was studied by measuring electron energy spectra and ion mass spectra using 1-ps laser pulses in the 580--595-nm wavelength and ${10}^{12}$--${10}^{13}$-W/${\mathrm{cm}}^{2}$ intensity range. In single ionization the (3p${)}^{2}$ $^{1}$S doubly excited autoionizing state, resonant at the four-photon level, is found to play an important role. Single ionization leaving the ${\mathrm{Mg}}^{+}$ ion in the 3p excited state is strongly enhanced when resonant with the (3p${)}^{2}$ $^{1}$S state. The amount of above threshold ionization observation also varies strongly in the wavelength range under study. At an intensity of 2\ifmmode\times\else\texttimes\fi{}${10}^{13}$ W/${\mathrm{cm}}^{2}$, sequential double ionization is found to occur dominantly via the excited ionic state ${\mathrm{Mg}}^{+}$(3p). The first ionization step populates this state less than the ion ground state ${\mathrm{Mg}}^{+}$(3s), but this is compensated by the lower order of the second ionization step.

Calculation of doubly excited even strontium states

The perturbation theory with zero order model potential has been successfully used for calculating the spectral characteristics (energies, radiative lifetimes, autoionization widths) of highly excited Rydberg and autoionization states of rare earths. Here, the same method is applied for investigating the 4dns, 4dnd and 4dng doubly excited Rydberg states of Sr. The obtained energies of the states are in good agreement with the experimental data of other authors. The calculation aims to give a more complete and reliable information about the experimentally observed spectra, as well as to predict the location of more highly excited states.

Post-collision interaction in inelastic ion-atom scattering

The ion-atom inelastic scattering processes including (i) excitation of autoionization states of atoms by ion impact, and (ii) ionization of inner atomic shells are considered. The Coulomb interaction of particles in the final state of such resonance processes (the post-collision interaction — PCI) exerts a strong influence on the energy and angular distributions of the particles. The theoretical models of PCI and experiments that make it possible to observe the PCI effects in different kinematic regions are discussed in this survey. The characteristics of PCI are considered by comparing photon and electron impact processes. Coincidence and noncoincidence experiments as well as the dependence of the lineshift on the incoming ion energy are discussed. The PCI effects are considered also in the kinematic region, where the relative velocity of a scattered ion and an autoionization electron is close to zero. In this case PCI results in the so-called Coulomb focusing of the autoionizing electrons and the oscillatory structure of the cross section. The interference between the direct and resonance scattering processes is considered, and possibilities of its experimental observation are discussed.

Excitation of autoionization states of GaI by electronic collisions

Excitation of autoionization states of GaI by electronic collisions

Atomic-structure effects in multiphoton ionization of magnesium.

We present an application of a simple configuration-interaction approach to multiphoton ionization of magnesium, using a finite ${\mathit{L}}^{2}$-basis set constructed from a nearly complete set of one-particle frozen-core Hartree-Fock orbitals. The effects due to multielectron interactions to two- and three-photon processes at energies both near and away from the resonant structures dominated by singly excited intermediate states and doubly excited autoionization states are examined in detail. Our calculation has shown that the multiphoton spectra are mostly dominated by the resonant structures due to intermediate singly excited bound states. Such spectra are qualitatively different from the single photoionization spectrum, which often exhibits a strong presence of doubly excited autoionization structures. Our calculated results are in close agreement with other existing experimental and theoretical results for single- and two-photon ionizations.

LINEAR STARK EFFECT OF THE n′dnl AUTOIONIZATION STATES OF Ca AND Sr ATOMS IN THE LOW-FIELD REGION

The electric field effect on the 3dnl and 4dnl doubly excited autoionization states of Ca and Sr atoms have been observed using a multisiep laser excitation technique. The Stark spectra show that splittings of the Stark levels are linearly dependent on the applied electric field in the lowfield region of F5. We have observed also that the autoionization resonance broadens with increasing electric field because of field-induced mixing of nearly degenerate 1 substates, which can play important role for increase of dielectronic recombination rate in plasmas.

Photoionization of quasi-two-electron atoms dominated by the doubly excited autoionization states.

A theoretical procedure based on the configuration-interaction approach of Fano [Phys. Rev. 124, 1866 (1961)] is presented for the photoionization between states dominated by strong configuration interaction. By correctly identifying the main contributing interaction between the dominating bound configurations, the effectiveness of this procedure is demonstrated by its application to photoionization from the Mg 3s3${\mathit{p}}^{1}$P excited state to the $^{1}\mathrm{S}$ continuum dominated by the 3${p}^{2}$ $^{1}S$ doubly excited autoionization state above the first ionization threshold. In addition to the good agreement with the experimental data, the individual contributions from different physical processes are also examined in detail.

Resonance positions and widths of doubly excitedN5+ions associated withN=3, 4, and 5N6+thresholds

Some doubly excited autoionizing states of heliumlike nitrogen ions converging on the N=3, N=4, and N=5 hydrogenic thresholds are calculated by a method of complex-coordinate rotation. Hylleraas-type wave functions are used for L=0 and L=1 resonances, and products of Slater-orbital-type wave functions with expansion lengths up to 323 terms for L\ensuremath{\ge}2. Results can be used for experimental references in the recently identified doubly excited autoionization states of ${\mathrm{N}}^{5+}$ ions in double-electron charge-transfer collision experiments between ${\mathrm{N}}^{7+}$ ions and helium atoms.

The photoionization spectra of effusive and supersonic molecular beams of monosilane

Effusive and supersonic molecular beams of monosilane have been ionized by photons from the Berlin synchrotron light source (BESSY). Photoionization efficiency curves have been taken as a function of wavelength (50 to 120 nm) for a number of fragment ions. The main products observed are SiH+3 and SiH+2 . No appreciable SiH+4 could be detected and less than 5% of the signal was due to lower fragments (SiH+, Si+). The threshold energies Ethr, the standard enthalpy of formation ΔH○f,g of the fragment ions SiH+2 and SiH+3 and the ionization potentials Iz of the corresponding radicals have been determined to be SiH+2: Ethr=11.67±0.04 eV ΔH○f,g=276.3±0.9 kcal/mol, Iz(SiH2)=9.47±0.03 eV. SiH+3: Ethr=12.23±0.02 eV, ΔHf,g=237.1±0.6 kcal/mol, Iz(SiH3)=8.32±0.07 eV. Discrete structure in the photoionization curves is observed in the range of Ethr=15.8–17.6 eV and interpreted as vibrational progressions of a highly excited autoionizing state of SiH4 which lies 15.8 eV above the ground state. Vibrational constants have been found to be 1790±80 and 680±50 cm−1 which correspond to the ground state constants 2187 and 711 cm−1 of the ν1 and the ν2 modes of monosilane. The broadening of some lines in the progression is interpreted as being caused by a transition into a dissociative excited neutral state with lifetimes as short as 3×10−14 s for v=3 of the ν2 mode. Additional ions (predominantly Si2H+4, Si2H+6, Si2H+7) have been observed when photoionizing a supersonic molecular beam of monosilane. These are shown to stem from the ionization and subsequent fragmentation of dimers and possibly higher multimeres. Threshold values have been determined to be Si2H+4: 11.58±0.02 eV; Si2H+6: 11.4 eV; and Si2H+7: 11.4 eV.

Many‐electron theory of autoionizing states using complex coordinates: The position and the partial and total widths of the Ne+ 1s hole state

AbstractThis article presents the first results of the application of quantum mechanics with complex coordinates to the calculation of partial widths for the radiationless decay of an inner‐hole excited autoionizing state, the Ne+1s2s22p6 2S. This is succeeded by the reduction of the multi‐electron, multichannel problem in the complex energy plane to five, symmetry adapted, two‐electron problems, in accordance with a published theory of many‐electron resonances. These two‐electron problems are solved independently by using rotated analytic Hartree‐Fock orbitals (expressed in terms of Slater orbitals) for the localized components, and Slater plus Gamow orbitals for the rotated, asymptotic square‐integrable functions carrying the width information. A recently proposed variational principle is employed for the optimization of nonlinear parameters. Within this independent asymptotic pair approximation (IAPA), our results for the partial widths to the five Ne2+ channels are (in 10−2 a.u.): 1s‐2p2 1D: 0.560, 1S: 0.048; 1s‐2s2p, 3P0: 0.029, 1P0: 0.154; 1s‐2s2, 1S: 0.044. The total width is 0.835. These numbers agree reasonably well with those obtained by Kelly [Phys. Rev. A 11, 556 (1975)] from a many‐body perturbation theory (MBPT) calculation, and by Howat et al. [J. Phys. B 11, 1575 (1978)] from a configuration‐interaction in the continuum calculation. The most recent experimental results yield 0.604, 0.089, 0.063, 0.174, and 0.060, respectively, with a total width of 0.99. Previous real‐coordinate many‐electron calculations by Beck and Nicolaides–including relativistic and radiative effects–have predicted the position of the Ne+ 1s hole state at E0 = 870.4 eV above the Ne ground state. It has already been shown that the real energy corresponding to the localized component of the autoionizing state is stable under rotations of the function space describing it. Therefore, the earlier E0 can be incorporated into the present calculation in the complex plane. The shift due to the additive contribution of the IAPA is found to be − 0.09 eV. When this is added to E0, the final E = 870.3 eV is in excellent agreement with experiment [870.3 eV; T. D. Thomas and R. W. Shaw, Jr., J. Electron. Spectrosc. Relat. Phenom. 8, 45 (1976)].

Two-Photon Excitation of Autoionizing States through Zeros in the Continuum

Description de la methode qui utilise 2 photons de frequences differentes, le 1er atteignant une energie proche d'un maximum de la section efficace a 1 photon, mais dont la section efficace est presque nulle. Application a l'etude des transitions 6snd-7snd de Ba en detectant l'ionisation totale

Comment on the Existence of Doubly Excited Autoionization States With Exceptionally Low Decay Rates

The only measurements for super-narrow two-electron (2, 3c)3P0 and (2, 4c)3P0 states in doubly ionized beryllium are those recently reported by Andersen et al. There they measured the optical decays (2, 2a)3P-(2, 3c)3P0 and (2, 2a)3P-(2, 4c)3P0 in the extreme UV following beam-foil excitation. In this work we present an analysis of additional decay channels of the (2, 3c)3P0 and (2, 4c)3P0 levels in Be III as well as a discussion of their fundamental properties.

Coherent Excitation and Decay of Autoionizing States in Ion-Atom-Collisions

Excitation and decay of autoionizing states in slow ion atom collisions is studied by two methods: (i) by energy analysis of electrons, ejected in He+/He, He+/Ne and Ne+/Ne collisions at an ejection angle of ?= 180° with respect to the beam direction, and (ii) by measurement of angular correlations between autoionization electrons and scattered ions in 2 keV He+/He collisions. It is shown that with a semiclassical description, which assumes coherent excitation of the autoionizing states and their magnetic sublevels the experimental data can be described quantitatively.

Electron impact studies of resonances and autoionizing states of neon

Measurements have been made of the energy spectrum of electrons ejected from autoionizing states of neon with excitation energies in the region 41-46 eV; in particular, the lowest doubly excited autoionizing state of neon, the 2s22p43s2(3P) state, has been observed positively. These studies have given detailed information on post-collision interaction effects. In addition, it has been shown that resonant excitation of autoionizing states via negative-ion states can be an important process and measurements have been made on a number of excited negative-ion states in the region 41-48 eV.

Calculation of doubly excited resonances in neon

Close coupling calculations of doubly excited autoionization states of three low-lying 1P0 series in neutral neon are reported and compared with the results of photoionization experiments. Certain anomalies appearing in both the experiments and the calculations are discussed.

Autoionization states in the photoionization of neon

Low energy photoionization in the vicinity of the singly excited autoionization states of neon is considered using a multiconfiguration close coupling method for the wavefunctions. The positions and widths of the first ten 1s22s2p6 nu np resonances, the shape of the lowest resonance, and the shape of the nonresonant partial cross sections up to an electron energy of 2.8 Ryd are calculated and compared with experiment where possible.

Electron Energy Distributions from Ionizing Collisions of Helium and Neon Ions with Helium

The electron-energy distributions arising from ionizing collisions of He and Ne ions with a He target have been examined under conditions such that the instrumental resolutions does not affect the distribution. Incident ions were in the range of 1-4 keV. The distributions consist of a sharp maximum near the zero of the electron energy with broader peaks occurring in the 20-40-eV region. Two broad peaks are detectable in the ${\mathrm{He}}^{+}$-He spectrum and six are found in the ${\mathrm{Ne}}^{+}$-He spectrum. The energy of the maxima of the prominent broad peaks have been found to fit a linear dependence on the inverse ion velocity over all or part of the ion range investigated. This dependence, when extrapolated to infinite ion energy, has yielded peak energies which correspond to previously identified auto-ionizing states. Where measurement is possible, these same peaks show a decrease in width at half-maximum also linear with the inverse ion velocity. A model is presented which attributes the observed behavior to a collision-induced excitation of auto-ionizing states followed by autoionization transitions which occur in the field of the ion.