S-wave Capture Research Articles

Disagreement between capture probabilities extracted from capture and quasi-elastic backscattering excitation functions

Experimental quasi-elastic backscattering and capture (fusion) excitation functions are usually used to extract the s-wave capture probabilities for the heavy-ion reactions. We investigated the $^{16}$O+$^{120}$Sn,$^{144}$Sm,$^{208}$Pb systems at energies near and below the corresponding Coulomb barriers and concluded that the probabilities extracted from quasi-elastic data are much larger than the ones extracted from fusion excitation functions at sub and deep-sub barrier energies. This seems to be a reasonable explanation for the known disagreement observed in literature for the nuclear potential diffuseness derived from both methods.

Photon Linear Polarization Coefficient of the Radiative Capture Process 2H(p, γ)3He at Thermal Energies

The photon polarization parameter of the proton-deuteron radiative capture process at thermal proton energies is calculated up to leading order (LO), using pionless effective field theory (EFT). In order to make a comparative study of this model, we compare our results for the photon polarization parameter with the realistic Argonne v14(AV14) and Argonne v18(AV18) modern nucleon-nucleon potentials and Urbana IX or Tucson-Melbourne three-nucleon interactions. The theoretical description of the 2H(p, γ)3 He reaction at thermal energies is complicated by the presence of the Coulomb interaction. Only the s-wave capture contribution to the thermal energy of this reaction is calculated. Three-body currents give small but significant contributions to some of the observables in the proton-deuteron radiative capture cross section at thermal neutron energies. Our result is in good agreement with the available experimental data and potential models' calculation at this order.

Mutual Capture of Dipolar Molecules at Low and Very Low Energies. I. Approximate Analytical Treatment

Approximate analytical expressions are derived for the low-energy rate coefficients of capture of two identical dipolar polarizable rigid rotors in their lowest nonresonant (j(1) = 0 and j(2) = 0) and resonant (j(1) = 0,1 and j(2) = 1,0) states. The considered range extends from the quantum, ultralow energy regime, characterized by s-wave capture, to the classical regime described within fly wheel and adiabatic channel approaches, respectively. This is illustrated by the table of contents graphic (available on the Web) that shows the scaled rate coefficients for the mutual capture of rotors in the resonant state versus the reduced wave vector between the Bethe zero-energy (left arrows) and classical high-energy (right arrow) limits for different ratios δ of the dipole-dipole to dispersion interaction.

Low energy electron attachment to C60

Low energy electron attachment to the fullerene molecule (C60) and its temperature dependence are studied in a crossed electron beam–molecular beam experiment. We observe the strongest relative signal of C60 anion near 0 eV electron energy with respect to higher energy resonant peaks confirming the contribution of s-wave capture to the electron attachment process and hence the absence of threshold behavior or activation barrier near zero electron energy. While we find no temperature dependence for the cross-section near zero energy, we observe a reduction in the cross-sections at higher electron energies as the temperature is increased, indicating a decrease in lifetime of the resonances at higher energies with increase in temperature.

62 Ni( n , γ) 63 Ni Reaction and Overproduction of Ni Isotopes

The neutron capture cross section of 62Ni has been measured in the energy range from 5.5 to 90 keV by employing a prompt discrete γ-ray detection method with the use of an anti-Compton NaI(Tl) spectrometer. The Maxwellian-averaged capture cross section at 30 keV is derived as being 37.0 ± 3.2 mbarn, 3 times larger than the value currently used for the nucleosynthetic yield estimations in massive stars. The result could provide crucial information for the long-standing problem of the largest overproduction of 62Ni. The intense discrete γ-ray feeding from a neutron capture state to the ground state of 63Ni was for the first time observed at stellar neutron energy. This observation and the neutron energy dependence of the total neutron capture cross section of 62Ni indicate clearly that the dominant capture process of 62Ni is a nonresonant direct s-wave capture process and not a p-wave capture process as previously predicted.

Dynamics of ion–molecule complex formation at very low energies and temperatures

The dynamics of complex formation in collisions of rotationally excited diatomic molecules with ions at very low collision energies and translational temperatures is discussed. Under these conditions, the locking of the intrinsic angular momentum of the diatomic molecule to the collision axis occurs in the region of centrifugal barriers, and the motion of collision partners across these barriers bears quantum features (tunneling transmission and overbarrier reflection). The capture in this energy range is described by coupled radial wave equations that are solved numerically. In two limits, the respective rate coefficients are expressed analytically: for low collision energies (adiabatic channel approximation with classical relative motion of partners) and at zero collision energy (Bethe limit, s-wave capture). By comparison with accurate numerical quantum results, it is shown that these two limits are satisfactorily bridged by the so-called axially-nonadiabatic channel model that considers uncoupled relative motion of the colliding partners across effective potentials; the latter are generated by adiabatic channel potentials and Coriolis interaction in the perturbed rotor basis. The limits of applicability of the standard adiabatic channel model are discussed. The general approach is illustrated by the capture of H2 and HCl in collisions with ions.

Electron attachment to and anion reactions with SF5Cl: electron-swarm and selected ion flow tube studies

The first investigation of non-thermal electron attachment to SF5Cl in a swarm environment is reported here. An electron swarm mass spectrometer, the drift tube of which operates in an atmospheric pressure of N2 or Ar, has been used to obtain the data. Experimental attachment rate coefficients, ka, and the anion products resulting from electron attachment are reported. ka is observed to decrease as the mean electron energy increases. The estimated thermal (300K) electron attachment rate constant is kth≈(1.3±0.4)×10−8cm3s−1, which is significantly less than the expected s-wave capture rate. The mass spectra obtained show a number of anions resulting from electron attachment, namely F2−, SF4−, SF5− and SF5Cl−, with SF5− and SF5Cl− being the only anions observed as the mean electron energy approaches zero. Anion reactions with SF5Cl made it difficult to accurately assign branching ratios, and therefore only a qualitative description of the results will be presented here. Anion reactions with SF5Cl are of general interest and make interesting comparisons with anion reactions involving SF6. In this paper, we also present the results of a selected ion flow tube study investigating the 300K reactions of SF5Cl with a number of anions; O2−, SF6−, OH−, CF3−, F−, Cl− and SF5−.

Resonance and Threshold Phenomena in Low-Energy Electron Collisions with Molecules and Clusters

A survey of resonance and threshold phenomena in low-energy electron collisions with molecules and clusters is presented. Following an introduction into the role of resonances to promote vibrational excitation and anion formation through electron attachment, we discuss recent progress in achieving high energy resolution, using optimized conventional setups and photoelectron methods (the latter achieving sub-meV energy widths in attachment studies). Discussing selected cases, we highlight threshold and resonance phenomena in electron scattering and attachment channels, as observed in recent high resolution experiments and characterized by improved theoretical models. The threshold behaviour for dissociative electron attachment proceeding through s-wave and p-wave capture is demonstrated for CCl4 and Cl2, respectively. Threshold peaks, vibrational Feshbach and outer-well resonances as well as boomerang-type oscillatory structures are discussed for the polar molecules HF, HCl, and CH3I. Narrow vibrational Feshbach resonances, observed in cluster anion formation due to electron attachment to molecular clusters of CO2 and N2O, and their size-dependent redshifts, illustrating the effects of solvation, are discussed and explained as diffuse weakly-bound electron states. We briefly address the recent observation and the relevance of vibrational resonances in positron-molecule collisions at energies below 0.5 eV, as observed in positron annihilation. We conclude with a brief summary and mention some perspectives for future work.

Astrophysical reaction rate for the8Li(n,γ)9Lireaction

An attempt was made to measure the excitation function of the cross section for the Li-8(n,gamma)Li-9 reaction by performing the inverse reaction Li-9(gamma,n)Li-8, with the equivalent photons in the electric field of nuclei in a Pb target providing the gamma rays for the reaction. The energy spectrum of lithium nuclei in coincidence with neutrons had no discernible peak where any beam-velocity Li-8`s would be located. Statistically, a Gaussian-shaped Li-8 peak could have been present with 30+/-29 counts, which we interpreted as consistent with zero, with a two-standard-deviation upper limit of 87 counts. Using the fact that neutron capture on Li-8 must be dominantly s-wave capture, and applying detailed balance, we obtained, with E in eV, sigma(n,gamma)>930E(-1/2) mu b. The corresponding limit on the astrophysical reaction rate is >790 cm(3) mol(-1) s(-1). Theoretical predictions of the reaction rate have exceeded our upper limit by factors of 3-50.

Low energy electron impact on C 76 and C 84: excitation, metastable anion formation, and lifetime

Low energy (0–15 eV) electron impact on gas phase fullerenes C 76 and C 84 has been investigated in an e − beam experiment (electron energy resolution 0.050 eV). Electron energy loss spectroscopy results are presented for C 76 at intermediate and low incident energy and are compared with the UV–visible absorption spectrum. The lowest triplet state 3 B 3 has not been detected; however, two bands at 1.70 and 2.58 eV, clearly of triplet character are observed. The band at 1.70 eV is assigned to 3 B 1 and 3 B 2 states. Ion yields of C 76 − and C 84 − versus e − energy are presented in the range 0–10 eV. The capture threshold is shown to appear at zero energy (within 0.030 eV), indicating the occurrence of an s-wave capture. The nature of other features due to resonances appearing on the spectrum is discussed. Lifetimes τ of C 76 − and C 84 − are observed to be strongly dependent of the electron energy. They are reported in the range 10–15 eV. For the capture of a 14 eV electron we have measured τ(C 76 −) = 100 μs and τ(C 84 −) = 180 μs. Comparison between the lifetimes of C 60 −, C 70 −, C 76 −, and C 84 − shows that τ behaves exponentially with the number of carbon atoms involved in the fullerene anion.

Low-energy behavior of exothermic dissociative electron attachment

We discuss two models for electron attachment to molecules: the Vogt-Wannier model for capture into a polarization well and the resonance model for dissociative attachment. The Vogt-Wannier model is generalized for the case of a target with a permanent dipole moment, and results are presented for dissociative attachment to ${\mathrm{CH}}_{3}\mathrm{I}.$ It is shown that the resonance theory should incorporate in this case a weakly bound dipole-supported state of ${\mathrm{CH}}_{3}{\mathrm{I}}^{\ensuremath{-}},$ whereas the generalized Vogt-Wannier theory gives a reasonable estimate for the cross section in the meV and sub-meV region. The Vogt-Wannier model is also applied to the process of attachment to ${\mathrm{SF}}_{6},$ ${\mathrm{CCl}}_{4},$ and ${\mathrm{C}}_{60}.$ In the first case the s-wave capture model provides a satisfactory description of the experimental data for energies below the first vibrational excitation threshold, whereas for ${\mathrm{CCl}}_{4}$ it underestimates the attachment cross section by a factor of 2 in the sub-meV region. For ${\mathrm{C}}_{60}$ we suggest that electron attachment is dominated by s-wave capture in the region below 2 meV and by p-wave capture in the energy range above 4 meV. Our model reproduces data for Rydberg electron and free-electron attachment observed in beam experiments. It is, however, at variance with the strong rise of the attachment rate coefficients with electron temperature observed in flowing afterglow--Langmuir probe measurements.

Formation of NO− following electron attachment to NO clusters

Low-energy (0–1 eV) electron attachment to NO clusters is studied with high-energy resolution (15 meV, FWHM). The dominant signal is NO− which has not been observed in previous Rydberg electron transfer studies to NO clusters. The relative cross-section for NO− formation shows a sharp peak close to 0 eV and a series of further weak peaks with a spacing of about 190 meV. The `0 eV' peak is ascribed to `s-wave capture' by the entire target cluster. Due to the low adiabatic electron affinity of NO (26 meV), NO− cannot be the result of an evaporative attachment reaction. It is suggested that neutral products like N2, via the strongly bound (ONNO)− intermediate, are involved. The weak structures are tentatively assigned as arising from transitions to vibrational excited states of solvated NO−.

Nonresonant directp- andd-wave neutron capture by12C

Discrete gamma-rays from the neutron capture state of 13C to its low-lying bound states have been measured using pulsed neutrons at En = 550 keV. The partial capture cross sections have been determined to be 1.7+/-0.5, 24.2+/-1.0, 2.0+/-0.4 and 1.0+/-0.4 microb for the ground (1/2-), first (1/2+), second (3/2-) and third (5/2+) excited states, respectively. From a comparison with theoretical predictions based on the non-resonant direct radiative capture mechanism, we could determine the spectroscopic factor for the 1/2+ state to be 0.80 +/- 0.04, free from neutron-nucleus interaction ambiguities in the continuum. In addition we have detected the contribution of the non-resonant d-wave capture component in the partial cross sections for transitions leading to the 1/2- and 3/2- states. While the s-wave capture dominates at En < 100 keV, the d-wave component turns out to be very important at higher energies. From the present investigation the 12C(n,gamma)13C reaction rate is obtained for temperatures in the range 10E+7 - 10E+10 K.

Dissociative electron attachment cross section to CHCl3 using a high resolution crossed beams technique

Using a crossed electron-molecule beam ion source in combination with a quadrupole mass spectrometer we have studied the electron energy dependence of the dissociative attachment process CHCl3+e→Cl− at electron energies from about 0 to 2 eV and in a target gas temperature range of about 300–430 K. The energy resolution and working conditions of this newly constructed crossed beams machine have been characterized using CCl4 as a test and calbrant gas. Utilizing the improved energy resolution of the present experimental setup (which allows measurements with FWHM energy spreads down to below 5 meV) it was possible to determine the accurate shape and magnitude of the cross section function in the low-energy range. This leads to the conclusion that between an electron energy of about 20 and 130 meV the reaction proceeds via deBroglie s-wave capture, whereas at higher energy (above about 0.4 eV) autodetachment plays a significant role. Moreover, the present measurements allow us to clarify previously reported differences in the absolute cross section, the number of peaks and in the energy position of these peaks. Finally, by analyzing the measured strong temperature dependence of the cross section close to zero electron energy the activation barrier for this dissociative attachment was determined to be 110±20 meV in good agreement with thermochemical data from swarm experiments performed under thermal equilibrium. Taking into account the present results it is also possible to discuss the mechanism for the existence of the second peak.

A coordinated flowing afterglow and crossed beam study of electron attachment to CCl 3Br

Dissociative electron attachment to CCl 3Br has been studied using a flowing afterglow/Langmuir probe (FALP) and a crossed beam technique. In the FALP experiment the overall attachment rate coefficients and the branching ratio into the Cl − and the Br − product channels, R = Cl − ( Cl − + Br − ), were measured as a function of the gas temperature, T g, in the range of 300–540K and the electron temperatures, T e, from T g to 4000K indicating that R approached the statistical value of 0.75 at the highest T g. At T g = 540K both Cl 2 −1 and ClBr − molecular ions were observed at about the 2% level. An apparent activation energy of 55 meV for the overall attachment reaction was derived using a model developed previously to describe the dependence of dissociative electron attachment rates on T g and T e. The crossed beam experiment provided relative attachment cross-sections for the production of Cl − and Br − as a function of electron energy, E, from near zero up to ∼2 eV at several T g within the range 311–423 K. The absolute cross-sections at T g = 311K were obtained from the FALP value using a calibration procedure. At low E the overall attachment cross-section varies as E −1 in accordance with s-wave capture theory. In accordance with the FALP data R increases from 0.2 at low E and the lowest T g to the statistical value of 0.75 at high E and high T g. A peak observed in the cross-section at an E of about 0.7 eV is tentatively attributed to p-wave electron attachment. The rapid decrease of this peak cross-section with T g is ascribed to autodetachment.

Low-energy electron attachment to fullerenes and in the gas phase

We have investigated the low-energy (0 - 10 eV) electron attachment on and , using an improved energy resolution close to zero incident energy (0.030 - 0.060 eV). We have found evidence of a zero-energy attachment process (within 0.030 eV), for both and , indicating that a s-wave capture process is present at low energy for these two molecules. For , although in contrast with symmetry arguments, and with previous free-electron attachment experiments, this result is in full agreement with the conclusions presented by several groups, using the Rydberg atom energy transfer technique to investigate the low-energy attachment in these systems. Energy positions of several resonant states, appearing between 0.5 and 5 eV in the ion yield versus electron energy spectrum, are clearly established in and , and assignments are discussed. Lifetime measurements are reported for the fullerene anions. In the incident energy range 7 - 11 eV the lifetime of was found to vary from 500 to , respectively, in agreement with previous measurements. The thermionic model for detachment does not seem to describe the values obtained, and their observed variation in this energy range. The lifetime of is longer and could not be measured precisely with our time-of-flight conditions. An estimation of has been made at . Below the lifetime of was larger than and could not be measured precisely.

Direct neutron capture in light nuclei

The consistent direct-semidirect capture model (CDSD) is applied to the keV energy region where the direct process is dominant. We study the influence of the correction of the form of the direct term due to a difference ΔV in the potentials used to describe the scattering and the bound single-particle state of the captured neutron. The sensitivity of the cross sections on the semidirect capture mechanism in this energy regiion is examined as well. We consider the 12C(n,γ) and 16O(n,γ) processes which are of astrophysical interest in the energy region of the incoming neutrons between 10 keV to 0.6 MeV where good experimental data exist. It is shown that both corrections are negligible for capture to the states where s-waves are not involved. For s-wave capture to the p-orbits the ΔV correction of the direct term is important. The data do not support the contribution of the semidirect term as described by the model in the energy region studied.

Dissociative electron attachment to using a high-resolution crossed-beams technique

Using a crossed electron - molecule beam ion source in combination with a quadrupole mass spectrometer we have studied the electron energy dependence of the dissociative attachment process (at electron energies from about 0 - 2 eV and with an energy resolution of about 60 meV) in a target gas range of about 300 - 420 K. Utilizing the improved energy resolution of the present experimental set-up it was possible to determine the accurate shape and magnitude of the cross section function in this low-energy range. This leads to the conclusion that close to 0 eV energy (below about 0.15 eV) the reaction proceeds via s-wave capture, whereas at higher energy (above 1 eV) autodetachment significantly determines the shape of the cross section. Moreover, the present measurements allow us to clarify previously reported differences in the absolute cross section, in the number of peaks and in the energy positions of these peaks. Finally, by analysing the measured strong temperature dependence of the cross section peak at about zero electron energy the activation barrier for this dissociative attachment was determined to be in good agreement with thermochemical data deduced from swarm experiments.

Electron attachment to and in the gas phase

The fullerene molecule is known to attach several electrons in the solid phase, but only recently has it been recognized that it can also attach electrons in the gas phase. The first electron - molecular beam results showed that and molecules non-dissociatively attached a single electron over the unusually wide electron energy range from near thermal to about 10 eV, but these studies were not able to provide either the magnitude of the cross sections or describe the low-energy attachment behaviour. But using our flowing afterglow - Langmuir probe (FALP) apparatus we have been able to determine the absolute attachment rate coefficients for both and over the electron temperature range 300 - 4500 K. These FALP experiments have shown that attachment to at low electron energies (<1 eV) proceeds predominantly by p-wave electron capture, and that a centrifugal barrier of 0.26 eV is evident which was corroborated by subsequent theory. A similar situation is observed for attachment to except that for this fullerene molecule there is evidence that at very low electron energies (< 0.05 eV) inefficient s-wave capture may also occur. These FALP data further indicate that at energies above about 0.3 eV extraordinarily efficient electron attachment occurs to both and , and when the mean thermal cross sections derived from these FALP data are used to normalize the crossed electron - molecular beam data at the common low energies accessible in both experiments, it is clear that electron attachment to these fullerene molecules is very efficient over a wide electron energy range from about 0.3 - 10 eV above which electron thermionic emission occurs from the hot and nascent negative ions.

Neutron resonances in 166Ho.

We report the results of a high-resolution study of neutron resonances below 500 eV in $^{166}\mathrm{Ho}$. We find nine previously unreported weak resonances at energies 24.79, 75.08, 119.98, 187.94, 210.86, 227.87, 235.94, 248.60, and 264.98 eV. Bayesian probability analysis of the widths indicates they are formed primarily by s-wave capture and are therefore negative-parity resonances. If they have measurable d-wave neutron widths, they are candidates for the fivefold correlation test of time reversal. \textcopyright{} 1996 The American Physical Society.