Near-threshold Structure Research Articles

The 4p6-core excited autoionising states in strontium: Classification and excitation dynamics

The ejected electron spectra corresponding to the electron decay of the 4p 5 n1l1n2l2n3l3 low-lying autoionising states in strontium atoms were measured in the impact energy range from the excitation threshold of states up to 200 eV. By using the results of single-conguration HartreenFock (HF) calculations, six ejected electron lines with the lowest excitation thresholds were attributed to the decay of the (4p 5 4d5s 2 ) 3 P0;1;2 and 3 F2;3;4 autoionising states. For the 3 P1;2 and 3 F4 states the excitation functions were obtained at an incident electron energy resolution of 0.15 eV. In all excitation functions the strong near-threshold structure was revealed for the rst time. The origin of the structure was attributed to the decay of short-lived states from the 4p 5 5s 2 4d 2 conguration of Sr i ion. The cascade population of the 3 P0;1;2 and 3 F2;3;4 states was considered by using the HF energies and decay probabilities for the 4p 5 5s 2 5p levels.

Positron impact electronic excitation of N2

We present the results of scattering cross sections for positron impact excitation of electronic states of nitrogen molecule (N2) using the Schwinger multichannel method (SMC). All calculated cross sections took three collision channels into account (X1Σg and degenerate a1Πg states). Present theoretical results for excitation to the a1Πg states failed to reproduce the near-threshold structure observed in the recent and the only available experimental data [Sullivan et al., Phys. Rev. Lett. 87 (2001) 073201-1]. Scattering calculations from the a1Πg states (elastic and superelastic) are also reported. A spurious resonant structure found in the excitation to the a1Πg states was detected in a square integrable basis set calculation designed to reproduce the first Born approximation (FBA). Such spurious structure was removed by taking out the trial configuration state functions in which the positron was weakly coupled to the target. This may be a promising technique to separate unphysical resonances from the physical ones. We also observe that a combination between SMC scattering amplitudes (l⩽2) with FBA ones (l⩾3) significantly improved the cross sections at higher energies.

Control of near-threshold detachment cross sections via laser polarization.

The behavior of near-threshold cross sections for dissociation of a target into a pair of particles, as described by Wigner's threshold law, can depend sensitively on the angular momentum of the particles. In this Letter, we investigate the near-threshold nonresonant two-photon detachment process in the negative ion of gold. The expected s-wave threshold behavior is observed with linearly polarized light. Closure of the s-wave channel is realized by using circular polarization, allowing the first observation of a d-wave threshold. Practical applications are discussed, including extensions which could prove valuable for investigations of negative ions with near-threshold structure.

Effective range analysis of positron-hydrogen collisions

Humberston [Can. J. Phys. 60, 591 (1982)], using the Kohn variational method, showed that the range of validity of Wigner's threshold law for positronium formation in positron-hydrogen collisions is extremely narrow for the S wave. To examine the near-threshold behavior, we analyzed ab initio calculations using the multichannel effective range theory of Watanabe and Greene [Phys. Rev. A 22, 158 (1980)] and the single-channel effective range theory of Fabrikant [Opt. Spectrose 53, 131 (1982)] for the positronium-proton channel. We confirmed the presence of a Ramsauer minimum in the $L=0$ elastic cross section for positronium-proton scattering, and found a similar minimum for $L=1.$ The near-threshold structure is interpreted in terms of a virtual state, resonances, tunneling through and transmission over a barrier, and quantum suppression.

Inner-shell photoabsorption spectra

Based on multi-scattering self-consistent field method, the photoabsorption spectra near C 1s threshold of CH4(near-threshold structure) have been studied with the broken symmetry from To. The most possible geometry of the core excited CH4** is determined as the C3v symmetry; the core excited CH4** molecules have the same bond angle (109.5o) as that of the ground state CH4 and consist of one shorter bond length (1.91 a. u.) and three longer bond lengths (2.04 a. u.). The three longer bond lengths are almost the same as that of the ground state CH4, and the averaged bond length (2.01 a. u.) of core excited CH4** is shorter than that of the ground state CH4(2.06 a. u.).

Electron impact ionization of (1s2s2p)4 P j autoionization states of the lithium atom

Electron spectroscopy using intersecting electron and atomic beams was used for the first time to measure the electron excitation function of the (1s2s2p)4P1/2,3/2 autoionization states of the lithium atom in the range of collision energies between the level excitation threshold and 90 eV with an energy resolution better than 0.4 eV and a magic observation angle θ=54.7°. An analysis of the intense near-threshold structure observed revealed for the first time that autodetachment states of the negative lithium ion having excitation thresholds of 58.2±0.1 and 59.5±0.1 eV exist in the autoionization energy range.

Multiple ionization of lead by electron impact

A pulsed crossed-beam method incorporating time-of-flight spectroscopy, previously developed in this laboratory, has been used to obtain relative cross sections for n = 1- to 6-fold ionization of ground-state Pb atoms by electrons at energies ranging from near threshold to 3000 eV. These results greatly extend the energy range of previous measurements which, in any case, exhibit serious discrepancies. Rather than rely on any previous measurements on Pb, the present data have been normalized to the previously measured cross section for electron impact ionization of Ga at 200 eV. At our high-energy limit of 3000 eV, our measured cross sections decrease by much less than an order of magnitude with increasing n and and are also converging rapidly at high velocities, indicating large contributions from Auger processes. Observed near-threshold structure in the cross section curve is consistent with known autoionization transitions, and while there is also some indication of structure in cross section curves for multiple ionization, the large number of possible levels and experimental uncertainties preclude a detailed analysis. Recent theoretical predictions of multiple ionization based on approximate scaling procedures have been shown to be only partially successful in describing the present data.

Calculation of electron-impact excitation from metastable states of the Sr atom

Electron-impact excitation of Sr from metastable states is studied in the framework of an elaborate close-coupling approximation, using accurate target wave functions. An interpretation is given of measured cross sections, including their near-threshold resonance structure. The cross sections for transitions induced from the 5s5p{sup 3}P metastable level to the 5s5d{sup 3}D, 5p{sup 23}P, 5s6s{sup 3}S, 4d5p{sup 1}D, 5s4f{sup 3}F, 4d5p{sup 3}F, and 5s6p{sup 3}P levels are calculated in Born-Ochkur and two-state and six-state close-coupling approximations. We also consider transitions from the 5s{sup 21}S ground state to these levels. The calculations are based on a nonrelativistic Hamiltonian. In the configuration-interaction approximation, accurate wave functions are used for the outer electrons, with a frozen core. The results are found to be in good agreement with the most recent experimental data. {copyright} {ital 1997} {ital The American Physical Society}

A possible quasinuclear state near the Λ [formula omitted] threshold

Within the frame of an hermitian coupled-channels model we provide a natural explanation for the main properties of p p → Λ \\ ̄ gL experimental data. In particular, the nearthreshold structure is shown to be produced by a narrow Λ \\ ̄ gL subthreshold state of quasinuclear nature. This resonance, produced in the 3SD 1 partial wave, has quantum numbers J PC = 1 −−. The reason for the smallness of its width (few MeV) is explainend. Some experimental possibilities to find this resonance are proposed.

Inner-shell excitation of NO2-near-threshold structure

The authors present a theoretical study on the near-threshold structure due to inner-shell excitation of the NO2 molecule, based on the multiple scattering self-consistent-field method, which agrees with recent experimental measurements. More specifically, the authors elucidate the following three points. (i) The dynamics of the interactions between the excited electron and the residual molecular ion, (ii) the physical origin of the shape resonance as an antibonding molecular orbital, (iii) the near-threshold structure due to antibonding molecular orbitals which can serve as a probe to study the orientations of the absorbed molecules.

THEORETICAL STUDY ON THE MOLECULAR NEAR-THRESHOLD STRUCTURE

Excited molecular structure in gas phase usually consists of two types of energy states: Rydberg states with diffused molecular orbitals and valencetype excited states with rather localized molecular orbitals. Excitation from molecular inner-shell orbitals into the above-men-rioned excited orbitals leads to near-threshold structures. Based on the multiple-scattering self-consistent field theory, we have studied the molecular near-threshold structure. Our theoretical results are in good agreement with experimental measurements. We have also elucidated:(1) The dynaimcal interaction between the excited electron with the residual molecular ion.(2) Physical orgin of the shaperesonance, namely, as an anti-bonding state embedded in the continuum.

Near-threshold structure in the photoionization of krypton 3p.

Near-threshold structure in the photoionization of krypton 3p.

Relative intensities in X-ray photoelectron spectra.: Part X. Near-threshold structure of photoionization cross-sections of molecular valence shells

A method is described for the calculation of photoionization cross-sections of molecular orbitals using atomic cross-sections. The method is an extension of the additive scheme, and allows description of the resonance structure of the cross-sections, taking into account multiple scattering of photoelectrons by the molecular potential. Calculated photoionization cross-sections for the valence levels of N 2, CO and SF 6 are presented for excitation energies ranging from the ionization threshold to 50 eV. The results are compared with the near-threshold structure in the corresponding X-ray absorption spectra.

Near-threshold resonance structure in the photoionisation cross section of Ca I

The photoionisation cross section of Ca I has been calculated in the threshold region by many-body perturbation theory. Correlation effects among 3p64s2 electrons were found to be very important. Double-electron resonances dominate the cross section and were calculated using the interacting resonance formalism. Comparison is made with several experiments.

Near-threshold structure in the atomic K-shell spectra for ionisation by photons or fast charged particles

The differential optical oscillator strength df/de for ionization, which results in an ejected electron of kinetic energy e, is proportional to the photoionization cross section.1 It has been known for some time2–4 that df/de does not necessarily decrease monotonically with e from the threshold for outer atomic subshells. This is also true for the generalized oscillator strength df(K,e)/de for ionization at momentum transfer ℏK, when K is small. The quantity df(K,e)/de is the essential factor in the Born cross section for secondary-electron ejection by charged particles,5 and it tends to df/de in the optical limit K → 0. The non-monotonic e-dependence, or structure in the spectral strength, is related primarily to angular momentum barriers to the escape of electrons near the threshold.1,6 These barriers are known to be especially important for continuum waves with angular momentum l ≥ 2 near the outer edge of the atom. Thus, it has been thought that the K-shell ionization should be unaffected by the barrier effect because the K-electron wave functions are quite compact and because the final state is limited to p waves only, by the dipole selection rule.

Surface spacings from the secondary electron yield

Small inflections in plots of the total yield of secondary electrons versus primary electron energy were reported more than fifty years ago and attributed variously to thresholds for the excitation of core states, and to diffraction of the incident electrons. Although the existence of these inflections was discounted by later researchers, recent studies of the derivative of the yield leave no doubt as to their reality or their origin. The spectrum of core excitation thresholds provides information on the composition of the surface region. Near-threshold structure is related to the electronic state of the atoms. Extended fine structure above the threshold can be inverted to obtain interatomic spacings even from relatively disordered surfaces. The principal limitation in these measurements results from diffraction of the incident electron beam, which produces variations in electron reflectivity. The extreme simplicity of the equipment required makes this technique available to essentially every surface science laboratory.