Near-threshold Resonances Research Articles

Electron–ion recombination and photoionization of Fe XXI

Results for electron–ion recombination and photoionization of ( Fe XXI + h ν ↔ Fe XXII + e ) , with emphasis in high-temperature region, are presented from ab initio unified method. The unified method, based on close coupling (CC) approximation and R-matrix method, (i) subsumes both the radiative recombination (RR) and dielectronic recombination (DR), (ii) enables self-consistent sets of photoionization and recombination cross sections from using an identical wavefunction for both the processes, and (iii) provides state-specific recombination rates of a large number of bound states. A large CC wavefunction expansion, which includes the ground and 28 core excitations of n = 2 and 3 complexes and span a wide energy range, has been used. Compared to Δ n = 2 – 2 , Δ n = 2 – 3 core excitations are found to introduce strong resonant structures and enhance the background photoionization cross sections ( σ PI ) in the high-energy region. These features along with prominent photoexcitation-of-core (PEC) resonances at n = 3 core thresholds have increased the unified total recombination rate coefficients ( α R ( T ) ) at temperatures T > 10 6 K , region of maximum abundance of the ion in collisional equilibrium, by a factor of 1.6 over previous calculations. State-specific recombination rate coefficients α R ( nLS ) , which include both the RR and DR, are presented for the first time for 685 bound states with n ⩽ 10 and l ⩽ 9 . The unified total recombination rate with photoelectron energy α R ( E ) is presented and the role of low-energy near-threshold fine structure resonances is illustrated. The present results should provide a reasonably complete self-consistent set of recombination rates and photoionization cross sections for astrophysical modelings of high-temperature plasmas from optical to far-ultraviolet wavelength regions.

Near-threshold resonances in electron elastic scattering cross sections for Au and Pt atoms: identification of electron affinities

Near-threshold resonances in electron elastic scattering cross sections for Au and Pt atoms: identification of electron affinities

Near-threshold resonances in electron elastic scattering cross sections for Au and Pt atoms: identification of electron affinities

The recent Regge-pole methodology has been employed together with a Thomas–Fermi type potential which incorporates the vital core-polarization interaction to investigate the near-threshold electron attachment in Au and Pt as Regge resonances. The resultant stable negative ion states are found to have the discernible characteristic of very small imaginary parts of the Regge poles, which translate into long-lived resonances. The near-threshold electron elastic total cross sections are characterized by multiple resonances from which we extract the electron affinity (EA) values through the scrutiny of the imaginary part of the relevant complex angular momentum. For the Au− and Pt− negative ions the extracted binding energies of 2.262 eV and 2.163 eV, respectively are in excellent agreement with the most recently measured EA values for Au and Pt. Ramsauer–Townsend minima, shape resonances and the Wigner threshold behaviour are identified in both Au− and Pt− ions.

Three-body resonances Λnn and ΛΛn

Possible bound and resonant states of the hypernuclear systems Λnn and ΛΛn are sought as zeros of the corresponding three-body Jost functions calculated within the framework of the hyperspherical approach with local two-body S-wave potentials describing the nn, Λn, and ΛΛ interactions. Very wide near-threshold resonance is found for the Λnn system. The position of this resonance turned out to be sensitive to the choice of the Λn-potential. Bound Λnn state only appears if the two-body potentials are multiplied by a factor of ∼1.5. The system ΛΛn can only have a near-threshold resonance with the ΛΛ-potential which overbinds the hypernucleus He Λ Λ 6 .

Shape resonances in the absoluteK-shell photodetachment ofB−

$K$-shell photodetachment of ${\mathrm{B}}^{\ensuremath{-}}$ has been measured using the collinear photon-ion beamline at the Advanced Light Source, Lawrence Berkeley National Laboratory, as well as calculated using two separate $R$-matrix methods. The measurement of the absolute photodetachment cross section, as a function of photon energy, exhibits three near-threshold shape resonances due to the $^{3}S$, $^{3}P$, and $^{3}D$ final partial waves. A fit to the measured data using three resonance profiles shows good overall qualitative agreement with the three partial wave cross sections calculated using either $R$-matrix method. However, certain significant and unresolved quantitative discrepancies exist between the experimentally inferred and the calculated resonance profiles.

Inner-shell resonant photodetachment of C−: the dramatic role of dynamical relaxation in collective response

The near-threshold strong resonance in 1s inner-shell photodetachment of the carbon negative ion is investigated and the role of collective effects in its formation is evaluated. The shape resonance parameters, the resonance energy of εres = 0.857 eV and width of Γ = 0.122 eV, are determined using the many-body theory (RPAE&DEM) method. The complex mixed (‘shape-Feshbach’) nature of the resonance is revealed. The calculated cross sections are in fair agreement with the recent experiment (Gorczyca T W 2004 Rad. Phys. Chem. 70 407–15). The collective character of the response to the external electromagnetic field in the strongly correlated C− target is clearly demonstrated. The dynamical relaxation of the core is the most pronounced feature in the interplay of collective effects.

Electron-impact excitation of Fe XXII: Comparative study of relativisticR-matrix and distorted-wave approaches

An implementation of the Dirac $R$-matrix theory is presented and applied to the calculation of electron-impact excitation of B-like Fe XXII in the $n=2$ complex. A detailed comparison between the $R$-matrix and relativistic distorted-wave (DW) results is given. Contrary to the previous close-coupling studies of this ion, where significantly different background collision strengths are found for several transitions as compared with the DW results, we obtain excellent agreements between the two methods for most transitions, except for the weak transitions to the higher members of the $2{p}^{3}$ configuration, where significant channel-coupling effects are indeed present. We show that the discrepancies found in the previous Dirac $R$-matrix calculation for dipole transtions at high energies are due to the nonconvergence of partial wave summation, despite the explicit inclusion of partial waves up to $l=40$. The reason for large differences in the threshold energy region for some transitions between the present and previous Dirac $R$-matrix results is not clear. We also show that the independent-process, isolated-resonance approximation within the DW framework can describe the near-threshold resonances reasonably well for this ion.

Resolution and accuracy of resonances in R-matrix cross sections

We investigate the effect of resonances in photoionization and recombination cross sections computed using the R-matrix method. Photoionization and recombination rates derived from high-resolution cross sections for oxygen ions are compared with earlier works with less resolution and accuracy, such as in the widely used Opacity Project data. We find significant differences in photoionization rates for O II metastable states, averaged over Planck functions corresponding to ionizing radiation fields, with respect to the intrinsic accuracy of the calculations and improved resolution. Furthermore, for highly charged ions other physical effects are also important. Recombination rate coefficients, averaged over a Maxwellian distribution, are extremely sensitive to the position and resolution of near-threshold resonances and radiation damping, in (e + O VII) ⟷ O VI + hν. Surprisingly, however, the effect on the monochromatic and the mean Rosseland and Planck bound-free opacities is relatively small, but may be potentially significant.

Is Θ+(1540) a kaon–skyrmion resonance?

We reconsider the relationship between the bound state and the SU(3) rigid rotator approaches to strangeness in the Skyrme model. For non-exotic S=−1 baryons the bound state approach matches for small m K onto the rigid rotator approach, and the bound state mode turns into the rotator zero-mode. However, for small m K , we find no S=+1 kaon bound states or resonances in the spectrum, confirming previous work. This suggests that, at least for large N and small m K , the exotic state may be an artifact of the rigid rotator approach to the Skyrme model. An S=+1 near-threshold state comes into existence only for sufficiently large SU(3) breaking. If such a state exists, then it has the expected quantum numbers of Θ +: I=0, J= 1 2 and positive parity. Other exotic states with (I=1,J P= 3 2 +) , (I=1,J P= 1 2 +) , (I=2,J P= 5 2 +) and (I=2,J P= 3 2 +) appear as its SU(2) rotator excitations. As a test of our methods, we also identify a D-wave S=−1 near-threshold resonance that, upon SU(2) collective coordinate quantization, reproduces the mass splittings of the observed states Λ(1520), Σ(1670) and Σ(1775) with good accuracy.

Electron impact excitation of H2:resonance excitation of B 1Σu+(Jj = 2, vj = 0)and effective excitation function of EF 1Σg+

The electron impact emission function of the P(3) branchfor the (0, 4) band of the H2 B 1Σu+–X 1Σg+band system has been measured from threshold to 1800 eV. The emission functionexhibits structure indicating strong contributions from both resonance andnon-resonance excitation. The non-resonance component contains direct andcascade contributions. A combination of experimental and theoreticalconsiderations permits separation of resonance, dipole-allowed direct,dipole-allowed indirect, and dipole-forbidden excitation components for the Jj = 2,vj = 0 level of the B 1Σu+state. An effective excitation function for the EF 1Σg+–X 1Σg+band system has been obtained from a nonlinear least-squares analysisof the dipole-forbidden component of the B 1Σu+state emission function. The absolute value of EF 1Σg+–X 1Σg+cross section is established on the basis of earlier experimental results of Liu etal 1995 Astrophys. J. Suppl. 101 375–99 and 2002 Astrophys J. Suppl. 138 229–45and Abgrall et al 1997 Astrophys. J. 481 557–66 and 1999 J. Phys. B: At. Mol.Opt. Phys. 32 3813–38. A near-threshold apparent resonance excitation crosssection of (8.1 ± 3.2) × 10−18 cm2 is obtained for the B 1Σu+ (Jj = 2 andvj = 0). An EF 1Σg+–X 1Σg+Born cross section has been calculated from the electronic form factor ofKolos et al 1982a J. Chem. Phys. 77 1335–44. Analysis shows thatthe Born asymptotic shape function of the EF 1Σg+–X 1Σg+band system starts at ∼400 eV, a significantly higher energy than previouslyexpected. The excitation function is especially important for interpreting outerplanet atmospheric dayglow and auroral activity and can be used to infer energydeposition and heating rates.

Photodissociation spectroscopy of stored CH+ ions: Detection, assignment, and close-coupled modeling of near-threshold Feshbach resonances

We have measured and theoretically analyzed a photodissociation spectrum of the CH+ molecular ion in which most observed energy levels lie within the fine-structure splitting of the C+ fragment and predissociate, and where the observed irregular line shapes and dipole-forbidden transitions indicate that nonadiabatic interactions lead to multichannel dynamics. The molecules were prepared in low rotational levels J″=0–9 of the vibrational ground state X 1Σ+ (v″=0) by storing a CH+ beam at 7.1 MeV in the heavy-ion storage ring TSR for up to 30 s, which was sufficient for the ions to rovibrationally thermalize to room temperature by spontaneous infrared emission. The internally cold molecules were irradiated with a dye laser at photon energies between 31 600–33 400 cm−1, and the resulting C+ fragments were counted with a particle detector. The photodissociation cross section displays the numerous Feshbach resonances between the two C+ fine-structure states predicted by theory for low rotation. The data are analyzed in two steps. First, from the overall structure of the spectrum, by identifying branches, and by a Le Roy–Bernstein analysis of level spacings we determine the dissociation energy D0=(32 946.7±1.1) cm−1 (with respect to the lower fine-structure limit) and assign the strongest features to the vibrational levels v′=11–14 of the dipole-allowed A 1Π state. The majority of the 66 observed resonances cannot be assigned in this way. Therefore, in a second step, the complete spectrum is simulated with a close-coupling model, starting from recent ab initio Born–Oppenheimer potentials. For the long-range induction, dispersion and exchange energies, we propose an analytical expression and derive the C6 coefficients. After a systematic variation of just the vibrational defects of the four Born–Oppenheimer potentials involved, the close-coupling model yields a quantitative fit to the measured cross section in all detail, and is used to assign most of the remaining features to the dipole-forbidden a 3Π state (v′=17–20), and some to the weakly bound c 3Σ+ state (v′=0–2). The model potentials, which reproduce the spectrum and compactly represent the spectroscopic data, should help to predict more accurately C++H scattering in the interstellar medium.

Interaction Energy of Two Trapped Bosons with Long Scattering Lengths

I study two interacting bosons confined to a harmonic trap. The interaction is assumed to be of zero range and is expressed in terms of the s-wave scattering length. The energy shifts of the harmonic-oscillator states due to the atom-atom interaction are calculated analytically. It is found that for an ordinary potential the interaction energy depends only on the scattering length, while if the scattering length is modified by a near-threshold Feshbach resonance there is also a dependence on the width of the resonance.

Coalescence Production of Broad Near-Threshold Resonances

Several cases are discussed when an amplitude of resonance production is a sharply varying function of the resonance mass, leading to considerable differences between true and visible resonance parameters. The case of production is considered in more details. As all non-trivial features are caused by the peculiarities of the triangle graphs (threshold and proper-moving complex singularities near physical region), the possible methods of their visualization in inclusive cross sections and polarization effects are also studied.

Electron-impact detachment and dissociation of C4− ions

CRYRING was used to study collision processes between an electron and a negative ion cluster C4−. The total detachment cross sections for the production of the neutral 4C, 3C, 2C, and C fragments were measured. The cross sections for pure detachment, and for detachment plus dissociation leading to the production of C3+C, 2C2, and C2+2C were extracted using a grid. It was found that the pure detachment process overwhelmingly dominates all other fragmentation processes. The threshold location for the detachment channel is found to be around 6.0 eV. Although the doubly charged negative ion C42− has received little previous attention, a defined near-threshold resonance observed in the detachment cross section curve, has been associated with the short-lived state C42− (0.7 fs lifetime).

Perturbation of a lattice spectral band by a nearby resonance

A soluble model of weakly coupled “molecular” and “nuclear” Hamiltonians is studied in order to exhibit explicitly the mechanism leading to the enhancement of fusion probability in case of a narrow near-threshold nuclear resonance. We, further, consider molecular cells of this type being arranged in lattice structures. It is shown that if the real part of the narrow nuclear resonance lies within the molecular band generated by the intercellular interaction, an enhancement, proportional to the inverse width of the nuclear resonance, is to be expected.

Near-threshold ηd resonance

Near-threshold ηd resonance

High-statistics measurements of thep¯p→Λ¯Λandp¯p→Λ¯Σ0+c.c.reactions at threshold

We report on precision measurements of the cross sections for the reactions p¯p→Λ¯Λ, p¯p→Λ¯Σ0, and p¯p→Σ¯0Λ in the region below 5 MeV excess kinetic energy above reaction threshold. For the p¯p→Λ¯Λ measurement, the data represent a third and definitive scan through the threshold region by this experiment. Improvements in the detector system and analysis techniques, coupled with greatly increased statistics, have negated the question of the existence of a near-threshold resonance as had been indicated in our earlier, lower-statistics studies. The high statistics permit the first extraction of the polarization information at these low energies. We also report the first measurement of the cross section for the complementary reactions p¯p→Λ¯Σ0+c.c. at excess energies up to 2.5 MeV. This allows a comparison of the onset of Λ¯Λ production to that of Λ¯Σ0 and Σ¯0Λ. The data show no evidence for near-threshold resonance structures in either reaction channel. Further, the p¯p→Λ¯Σ0+c.c. reactions exhibit a larger P-wave fraction close to threshold compared to p¯p→Λ¯Λ. Received 5 June 2000DOI:https://doi.org/10.1103/PhysRevC.62.055203©2000 American Physical Society

Vector properties of O(3P) and O(1D) in OH photodissociation: Near-threshold resonance, neighboring resonance, and above-threshold behavior

Theoretical analysis is presented for the vector properties of the oxygen atom produced from the photodissociation of OH at energies from near the threshold to O(3P) to above the threshold to O(1D). The vector properties of O(3Pj, j=0,1,2) (angular distributions and alignment parameters) change rapidly across multichannel resonances near the dissociation threshold to O(3P). The vector properties near the close-lying Lorentzian resonances depend highly on the geometrical factors for the electronic transitions, corresponding to different final total angular momentum quantum numbers, and change rapidly between the resonances. At energies above the dissociation threshold to O(1D), quantum interference between the two direct dissociation pathways gives rise to highly oscillatory pattern of the vector properties of O(1D).

Alignment of the Na* 2p53s2 2P3/2 autoionizing state excited by electron impact: experiment and theory

The alignment of the 2p53s2 2P3/2 autoionizing state in sodium after electron-impact excitation was determined via the angular anisotropy of the intensity I() of the autoionization electrons. For emission angles 90°, the intensities I() were measured by rotating the incident electron gun, whereas magnetic-field deflection was applied for larger angles. Near-threshold negative-ion resonance structures in the alignment of an autoionizing state were observed for the first time and are discussed in detail. R-matrix calculations for incident electron energies from threshold to 80 eV successfully reproduced the broad-scale energy dependence of the alignment. The cross section ratio for the fine-structure components 2p53s2 2P3/2 and 2P1/2 were also measured and reproduced satisfactorily by theory.

Precision measurements of the Q-value for ( p, n) reactions on 37Cl and 59Co

Ground-state Q-values have been measured for ( p, n) reactions on 37Cl and 59Co by determination of the neutron and proton energies associated with near-threshold resonances. The neutron energies were measured by time-of-flight techniques; the energy scale for the protons was calibrated via the well-known Al( p, γ) resonances between 0.6 MeV and 2.2 MeV. By a full covariance analysis the following Q -values were obtained: 37Cl( p, n) 37Ar: −1596.22±0.20 keV, 59Co( p, n) 59Ni: −1855.33±0.20 keV.