Time-dependent quantum wave packet and quasiclassical trajectory studies of the Au(2S) + H2(X1∑+g) → AuH(X1∑+g) + H(2S) reaction

The time-dependent quantum wave packet (TDWP) and quasiclassical trajectory calculations (QCT) are carried out for the Au(²S) + H₂(X¹∑⁺_g) → AuH(X¹∑⁺_g) + H(²S) reaction on a global potential energy surface. The reaction probabilities at a series of J values, integral cross sections (ICSs) and differential cross sections of the title reaction are calculated by the TDWP method. For reaction probabilities, there are a mass of sharp oscillations at low collision energy, which can be attributed to resonances supported by the potential well. Due to the endothermicity of the title reaction, the total ICS shows a threshold about 1.53 eV. In order to further investigate the reactive mechanism, the lifetime of complex is calculated by QCT method. At the low collision energy, most intermediate complexes are long lived, which implies that the reaction is governed by indirect reactive mechanism. With the collision energy increasing, the direct reactive mechanism occupies the dominant position. Due to the change of the reactive mechanism, the angular distribution shifts toward the forward direction with collision energy increasing. The isotopic variant, Au + D₂→AuD + D reaction, is also calculated by TDWP method. The calculated reaction probabilities and ICSs show that the isotope effect reduces the reactivity.     

Evaluated Integral Cross Sections of the <Superscript>3</Superscript>H(<Emphasis Type="Italic">t</Emphasis>,2<Emphasis Type="Italic">n</Emphasis>)<Superscript>4</Superscript>He Reaction in the Low-Energy Region Obtained with Regard to Electron Screening

procedure is considered for analyzing ³H(t,2n)⁴He reaction proceeding in a gas environment with regard to electron screening [1–4]. Results from such an analysis are presented. An electron screening potential of 121 eV is obtained. The magnitude of this potential is three times higher than the one given in [5]. Starting with a 100 eV energy of particle interaction the cross sections of ³H(t,2n)⁴He reaction are calculated using the above potential. The reaction rates are calculated using the evaluated cross sections in the lowenergy region. Enhancement factors for cross sections and reaction rates are defined.     

Continuum shell model calculations for 15C

Microscopic calculations of the discrete and continuous spectrum of ¹⁵C are carried out within the framework of the continuum shell model, using a recently developed generalization of the extended R-matrix formalism. Comparison is made with the predictions of a conventional shell model calculation and also with the presently rather meager experimental data. The calculated total elastic and total reaction cross sections for the ¹⁴C+n reaction exhibit some interesting resonance phenomena which it would be desirable to investigate experimentally. The calculated stripping form factors for the ¹⁴C(d, p) reaction are similar to Woods-Saxon eigenfunctions for those states in ¹⁵C which have strong single-particle character, but differ markedly from this shape when the single-particle component is small. The effect on the calculated results of the addition or removal of closed channels is shown to be small. The present calculational method is found to be convenient, flexible and economical.     

Low-Energy 12C States in Three-α Model

Cross sections for three-α photodisintegration process of the¹²C nucleus are calculated by a Faddeev formalism, from which 0⁺- and 2⁺ low-lying resonant ¹²C states are studied. The triple-alpha reaction rate is evaluated from the calculated cross sections of the disintegration process.     

Reaction cross sections for collisions involving exotic light nuclei within the glauber approach

The reaction cross sections for the interaction of exotic nuclei ⁶He and ¹¹Li with ¹²C nuclei are calculated for energies of about 0.8 GeV per nucleon. The cross sections calculated by the exact Glauber formula are compared with their counterparts found by using the formulas of the optical limit, the rigidtarget approximation, and the few-body approximation. The effect of the structure of the nuclei being considered on the calculated cross sections is examined. The root-mean-square radii of the ⁶He and ¹¹Li nuclei are estimated on the basis of experimental data on the cross sections for the interaction of these exotic nuclei with ¹²C nuclei.     

Cross section and stellar reaction rates for the 42Ca(p, γ) reaction

The cross section for the ⁴²Ca(p, γ)⁴³Sc reaction has been measured over the lab energy range from 0.7 to 5.5 MeV using a positron spectrometer to measure the annihilation radiation from the decay of 3.9 h ⁴³Sc. Stellar reaction rates N_A〈σv〉 have been calculated from the experimental cross section curve for a series of three temperatures of interest for explosive oxygen and silicon burning in stars. The calculated rates are compared with the theoretical predictions of Woosley et al. and found to be in agreement within the experimental errors and the quoted validity of the theoretical calculation     

Proton and alpha evaporation spectra in low energy 12C and 16O induced reactions

Proton and alpha particle spectra have been measured in the ¹²C+⁹³Nb and ¹²C+⁵⁸Ni reactions at E(¹²C)=40 and 50 MeV and in the ¹⁶O + ⁹³Nb reaction at E(¹⁶O) =75 MeV. The spectra are compared with the statistical model calculations. The shapes of the calculated spectra are in agreement with experimental data except for the alpha spectrum in the ¹²C + ⁹³Nb reaction at 40 MeV. The observed evaporation bump is at ∼2 MeV lower energy compared to the calculated one. This discrepancy could imply alpha particle emission from a deformed configuration before compound nucleus formation at this near Coulomb barrier beam energy.     

E1 isobaric analogue resonances of strongly deformed nuclei in (e,e′p) reactions

The cross sections of the (e, e′p) reaction on the deformed nuclei ¹⁵⁹Tb, ¹⁶⁵Ho, ¹⁶⁹Tm, ¹⁷⁵Lu and ¹⁸¹Ta have been measured in the vicinity of the ground isobaric analogue state. Resonances corresponding to the E1 isobaric analogue states are found and displacement energies are calculated from these resonances. These energies are smaller than those obtained from proton scattering, which shows their dependence on the reaction type. The radiative widths of the E1 transitions of the isobaric analogue resonances were obtained. The ratio of these to the single-particle width calculated with the Nilsson model is order of 0.1–1 for non-spin-flip transitions and more than 10² for spin-flip transitions. These results are similar to those of the spherical nuclei ¹³⁹La, ¹⁴¹Pr, ²⁰⁷Pb and ²⁰⁹Bi.     

Vibrational and rotational excitation effects of the N(2D) + D2(X1Σg +) → ND(X3Σ+) + D(2S) reaction

The effects of the rovibrational excitation of reactants in the N(²D) + D₂(X¹Σ_g⁺) → ND(X³Σ⁺) + D(²S) reaction are calculated in a collision energy range from the threshold to 1.0 eV using the time-dependent wave packet approach and a second-order split operator. The reaction probability, integral cross-section, differential cross-section and rate constant of the title reaction are calculated. The integral cross-section and rate constant of the initial states v = 0, j = 0, 1, are in good agreement with experimental data available in the literature. The rotational excitation of the D₂ molecule has little effect on reaction probability, integral cross-section and the rate constant, but it increased the sideways and forward scattering signals. The vibrational excitation of the D₂ molecule reduced the threshold and broke up the forward–backward symmetry of the differential cross-section; it also increased the forward scattering signals. This may be because the vibrational excitation of the D₂ molecule reduced the lifetime of the intermediate complex.     

Spin-parity assignments of excited states in 12C

The reaction ¹²C(α, α')¹²C¹(3α) is observed by using 90 MeV a-particles in ionographic matter. The energies of the three α-particles from the break-up of ¹²C¹ are plotted in Dalitz diagrams. The density distributions for the three α-particles in the diagrams are calculated by assuming the spins and parities for the states in ¹²C¹(3α). The spin-parities for the states are assigned by comparing the observed density distributions with those calculated.     

Partial muon capture rates in 12C from the generalized helm model

Partial capture rates in the reaction μ^? + ¹²C → ν_μ + ¹²B are calculated using the generalized Helm model. The results are in good agreement with the recent experiment data.     

Optical model studies of the 13C (π+, πo) 13N reaction

The pion single-charge-exchange reaction ¹³C(π⁺, π^o) ¹³N is calculated using the pion-nucleus optical-model approach. The two versions of the optical potential, local and non-local, are investigated and the results are compared with the recently measured cross sections in the energy range of 30 to 90 MeV.     

Rate coefficients and kinetic isotope effects of the abstraction reaction of H atoms from methylsilane

ABSTRACT

Thermal rate constants for chemical reactions using improved canonical variational transition state theory (ICVT) with small-curvature tunnelling (SCT) contributions in a temperature range 180–2000 K are reported. The general procedure is used with high-quality ab initio computations and semi-classical reaction probabilities along the minimum energy path (MEP). The approach is based on a vibrational adiabatic reaction path and is applied to the multiple-channel hydrogen abstraction reaction H + SiH₃CH₃ → products and its isotopically substituted variants. All the degrees of freedom are optimised and harmonic vibrational frequencies and zero-point energies are calculated at the MP2 level with the cc-pVTZ basis set. Single-point energies are calculated at a higher level of theory; CCSD(T)-F12a/VTZ-F12. ICVT/SCT rate constants show that the quantum tunnelling contributions at low temperatures are relatively important and the H-abstraction channel from SiH₃ group of SiH₃CH₃ is the major pathway. The total rate constants are given by the following expression: k_tot(ICVT/SCT) = 2.29 10^?18 T^2.42 exp(?350.9/T) cm³ molec^?1 s^?1. These calculated rates are in agreement with the available experiments. The ICVT/SCT method is further exploited to predict primary and secondary kinetic isotope effects, respectively).     

Folding model analysis of pion elastic and inelastic scattering from 6Li and 12C

π ^±-Nucleus scattering cross sections are calculated applying the Watanabe superposition model with a phenomenological Woods-Saxon potential. The phenomenological potential parameters are searched for π ^± scattering from ⁶Li and ¹²C to reproduce not only differential elastic cross sections but also inelastic and total and reaction cross sections at pion kinetic energies from 50 to 672 MeV. The optical potentials of ⁶Li and ¹²C are calculated in terms of the alpha particle and deuteron optical potentials. Inelastic scattering has been analyzed using the distorted waves from elastic-scattering data. The values of deformation lengths thus obtained compare very well with the ones reported earlier.     

The Deck model in the reaction dp → d(Nπ)+

A Deck-like model is proposed for the reaction pd → (Nπ)⁺d. The narrow bump Nπ (1150 MeV) is well explained. The different hypothesis and approximations are widely reviewed. In particular other mechanisms which can contribute to the same reaction are calculated and off-shell effects are discussed.     

Faddeev Three-body Calculation of Triple-Alpha Reaction

The triple-alpha (3α) process, in which three ⁴He nuclei are transformed into a ¹²C nucleus, is studied in terms of a three-alpha (3-α) model. The reaction rate of the process is calculated via an inverse process, 3-α photodisintegration of a ¹²C nucleus. Both of 3-α bound and -continuum states are calculated by a Faddeev method with accommodating the long range Coulomb interaction. Results of the 3α reaction rate are about 10³ times larger than a standard rate from the Nuclear Astrophysics Compilation of Reaction Rates (NACRE) at a low temperature (T = 10⁷ K), which means our results are remarkably smaller than recent three-body calculations by the method of continuum-discretized coupled-channel.     

Tensor polarization of 6Li(2.186 MeV, 3+) states produced in the reaction 9Be(p, α)6Li* at 40 MeV

A consistent theoretical analysis of polarization moments t _kq is performed for the ⁶Li(2.186 MeV, 3⁺) states produced in the reaction ⁹Be(p, α)⁶Li*. The analysis is based on the distorted-wave Born approximation implemented for finite-range forces with allowance for spin-orbit interaction. The calculated tensor moments are compared with relevant experimental data. Particle-particle angular-correlation functions and tensor moments are shown to provide radically new information about reaction mechanisms and about nuclear interactions.     

Broadening of ρ(→ e+e−) meson produced coherently in the photonuclear reaction

The e ⁺ e ^? invariant mass distribution spectra are calculated to estimate the hadron parameters of the ρ meson produced coherently in the photonuclear reaction. The elementary reaction occurring in the nucleus is considered to proceed as γN → ρ ⁰ N; ρ ⁰ → e ⁺ e ^?. We describe the elementary ρ meson photoproduction by the experimentally determined reaction amplitude $f_{\gamma {\rm N} \to \rho ^0 {\rm N}}$ . The ρ meson propagator is presented by the eikonal form, and the ρ meson nucleus optical potential V _Oρ appearing in it is estimated using the “t?” approximation. The ρ meson nucleon scattering amplitude f _ρN extracted from the measurements is used to generate this potential. The calculated e ⁺ e ^? invariant mass distribution spectra are compared with those measured at Jefferson Laboratory. The calculated results for the transparency ratio are also presented.     

Total cross sections for some (α, n) and (α, p) reactions in medium-weight nuclei

Total cross sections have been measured for the ⁴⁵Sc(α, n), ⁴⁶Ti(α, n), ⁵⁰Cr(α, n), ⁵¹V(α, n), ⁵⁴Fe(α, n) and ⁵⁸Ni(α, p) reactions, and stellar reaction rates have been calculated from them. These have been compared to recent theoretical calculations which used compound nuclear theory. The calculated values are generally higher than the experimental values by factors ranging from 2 to 10.     

ω(→ π+π?π0) meson photoproduction on proton

The cross-section for the π ⁺ π ⁻ π ⁰ invariant mass distribution in the γp reaction in the GeV region is calculated. This reaction is assumed to proceed through the formation of the ω-meson in the intermediate state, because the production cross-section for this meson in the γp reaction in the GeV region is significant and it has a large branching ratio (88.8%) in the π ⁺ π ⁻ π ⁰ channel. The cross-sections for this reaction are calculated using the energy-dependent reaction amplitude, f _γp→ωp(0), extracted from the latest ω-meson photoproduction data. We use established procedure to calculate other factors, like width and propagator of the ω-meson, so that our calculation can provide reliable cross-section. The calculated results reproduce the measured π ⁺ π ⁻ π ⁰ invariant mass distribution spectra in the γp reaction.