Quantum chemical study of mechanisms of dissociation and isomerization reactions in some molecules and radicals of astrophysical significance: Cyanides and related molecules

A theoretical study of the mechanism of photodecomposition in carbonyl cyanide, diethynyl ketone, acetyl cyanide and formyl cyanide has been conducted using density functional and MP2 theories. A complete analysis of the electronic spectra of these molecules in terms of nature, energy and intensity of electronic transitions has been provided by time-dependent density functional theory. Mixing coefficients and main configurations of the electronic states have been used to identify the states leading to the photodecomposition process. While the Rydberg state ¹(n,3s) is involved in the dissociation of formyl cyanide and acetyl cyanide, the π*_CC/π*_CN states are involved in the case of carbonyl cyanide and diethynyl ketone. In all cases, however, stepwise decomposition process is preferred over the concerted reaction process. Based on potential energy curves for bond dissociation and the transition state and IRC studies, it is found that besides the direct dissociation of carbonyl cyanide, a photoisomerization process through a non-planar transition state may also occur resulting in the formation of a stable and planar isomer CNC(O)CN. A complete vibrational analysis of the higher energy isomer has been conducted and several new fundamental bands are predicted. Some of the earlier experimental results on the photodecomposition mechanism and energies of photofragments in carbonyl cyanide and acetyl cyanide have been rationalized.     

Photoproduction of πmesons off protons from the Δ(1232) region to Eγ = 3 GeV

Photoproduction of π⁰-mesons was studied with the Crystal-Barrel detector at ELSA for incident energies from 300MeV to 3GeV. Differential cross-sections dσ/dΩ, dσ/dt, and the total cross-section are presented. For E _γ < 3GeV, the angular distributions agree well with the SAID parametrization. At photon energies above 1.5GeV, a strong forward peaking indicates t-channel exchange to be the dominant process. The rapid variations of the cross-section with energy and angle indicate production of resonances. An interpretation of the data within the Bonn-Gatchina partial-wave analysis is briefly discussed.     

Exciton effects in armchair graphene nanoribbons

We have studied the exciton effects in armchair graphene nanoribbons systematically, using the nonorthogonal tight-binding model supplement by the long-range Coulomb interactions. It is found from our calculations that the excitation energies, the exciton binding energies and the exciton wave function sizes of the E ₁₁ and E ₂₂ excitons all exhibit oscillation as a function of the ribbon width. And there is a phase shift of π between the oscillation of the E ₁₁ and E ₂₂ excitons.     

Combined dynamical and statistical approach to describing induced fission of heavy nuclei

A combined dynamical and statistical approach to describing induced fission of heavy nuclei is proposed. This approach takes into account the nuclear-dissipation phenomenon and the double-humped structure of the fission barrier. A method that is intended for calculating the angular distribution of fission fragments and which is applicable over a broad range of excitation energies is discussed. The potential of the approach is demonstrated by addressing the problems of self-consistently describing experimental data on fission probabilities for plutonium and americium isotopes, the yields of shape isomers in the α+²³⁸U reaction at alpha-particle energies in the range E _α = 20–32 MeV and the d+^242,240Pu reactions at deuteron energies in the range E _d = 20–30 MeV, fission times in the α + ²³⁸U reaction at alpha-particle energies in the range E _α = 20–32 MeV, and angular distributions of fission fragments in the α + ²³⁸U, ²³⁷Np reactions at alpha-particle energies in the range E _α = 20–100 MeV.     

Effect of the deformation of the double bond in chlorinated ethylene on the rate and mechanism of the reaction with ozone

The reactivity of the strained C=C bond of the 1-chloroethylene molecule in the reaction with ozone was studied using ab initio (MP2, CASSCT, MRMP2) and DFT (B3LYP) calculations in conjunction with the 6–31+G** basis set. The mechanisms of concerted and nonconcerted addition were examined. The strain ɛ was introduced into the problem by changing the length of the C=C bond and specifying it as a non-optimized coordinate. It is shown that, at least at ɛ ≤ 2%, the activation energy E _a decreases linearly with increasing strain. The sensitivities of both channels of the reaction to the strain are similar and only slightly dependent on the method of calculation. The results are analyzed within the framework of a previously developed approach, which makes it possible to obtain an analytical dependence of E _a on the force and to relate the change in E _a during deformation to the length and rigidity of the initial and transition states.     

Coherent and incoherent π0 photoproduction from the deuteron

Differential cross-sections for the reactions d (γ,π⁰)d and d (γ,π⁰)pn have been measured at MAMI with the TAPS detector setup in the energy range 140 MeV < E _γ < 306 MeV. By use of the Glasgow tagging spectrometer an 0.8 MeV energy resolution for photons incident on the target was achieved. The π⁰ missing energy resolution was sufficient for a reliable separation of coherent and incoherent channels. The data for the break-up channel exhibit very strong final state interaction effects, whereas the observed angular dependence of the inclusive process d (γ,π⁰)X is in quantitative agreement with predictions for a quasi-free process. The observed absolute d (γ,π⁰)X cross-sections, on the other hand, are significantly smaller than predicted by the quasi-free process for E _γ >∼ 250 MeV. Associating this failure with the π⁰ photoproduction on the neutron would suggest that its cross-section is up to 25% below the presently believed value. Received: 13 February 2001 / Accepted: 13 April 2001     

Photoproduction of neutral pion pairs from the proton

Double neutral pion photoproduction from the proton has been measured at MAMI for photon energies between threshold and 820 MeV. The reaction was identified by an invariant mass and missing mass analysis. From threshold up to 370 MeV the total cross-section does not exceed 30 nb. For higher energies it shows a smooth rise until it reaches a maximum of about 10 μb at E _γ = 740 MeV. Dalitz plots of m ²(π⁰π⁰) versus m ²(p,π⁰) for seven bins of incident photon energy have been analysed. For E _γ > 610 MeV, a strong contribution of a sequential decay is observed with the Δ(1232)-resonance as intermediate state. A comparison to model calculations shows that these sequential decays presumably originate from the D ₁₃(1520) and also the P ₁₁(1440)-resonance. Received: 6 June 2000 / Accepted: 18 August 2000     

Parabolic model of the concerted molecular decomposition of chloroalkanes

The intersecting parabolas model (IPM) is used to analyze the measured kinetic parameters for the concerted molecular decomposition of chloroalkanes RCl to olefin and HCl. According to this model, the configuration of the transition state is formed by three atoms: C…H…Cl. The activation energy E and the rate constant k for 12 previously unstudied reactions of concerted molecular decomposition of RCl are calculated based on the enthalpy of reaction by using the IPM algorithms. The factors that influence the activation energy E for RCl decomposition are established: the enthalpy of reaction, energy of stabilization of radical R^?, presence of a π bond adjacent to the reaction center, and dipole–dipole interaction for the decomposition of polychloroalkanes. The values of E and k for reverse reactions of addition of HCl to olefins are evaluated. The energy spectrum of partial activation energies for the concerted molecular decomposition of RCl is constructed.     

np → ηd near threshold and the s -wave ηN amplitude

We calculate the process np → ηd near threshold using a separable potential model of the coupled ηN - πN - ππN subystems, and a relativistic three-body calculation for the ηd scattering amplitude. The ππN channels are represented by an effective σN channel, and we compare the case where the σ and π masses are related by m _σ = 2m _π and no width is considered, to another where the mass and width of the σ -meson are taken from ππ scattering data. The np → ηd cross-section can be well described up to about 60MeV by models where the real part of the ηN scattering length lies between 0.4≤Re(a _ηN)≤0.6 fm which allows us to determine the s -wave ηN scattering amplitude for -60≤E≤60 MeV.     

πNN Dynamics in pp → pp,pp →πd and πd →πd Processes Below 1 GeV

pp and πd scattering and pp →πd reaction processes at incident proton laboratory energies T _L ≤ 1 GeV are studied in the framework of πNN dynamics. For this purpose three-body calculations are performed with the πN interaction in the P ₁₁, P ₃₃, S ₁₁ and S ₃₁ states and NN forces in the ³ S ₁ -³ D ₁, ¹ S ₀, and ³ P ₂ states. In addition, the backward-going pion contribution at the πNΔ vertex, the πN-ρN coupling, the heavy-meson exchanges in the NN → NN driving term, and the effect of the off-shell structure in the πN-ρN P ₃₃ interaction are taken into account. Received March 28, 1994; revised July 29, 1994; accepted for publication August 31, 1994     

Lifetimes and configuration mixing in 110Cd

Lifetimes of excited states in ¹¹⁰Cd have been measured by the Doppler shift attenuation method in the reaction (α,2nγ) at E _α= 25 MeV. Lifetime values for 8 states and lifetime limits for 3 states were obtained. The band structures of ¹¹⁰Cd have been interpreted in terms of a modified version of the interacting boson model (IBM + 2 q.p.). The calculations explain well the excitation energies and electromagnetic transition probabilities up to J ^π= 16⁺, except for the 10⁺ ₁ state. The structural features are discussed in terms of collective and two quasiparticle excitations. Received: 20 March 1999 / Revised version: 28 May 1999     

New double-magic nucleus <Superscript>96</Superscript>Zr and conditions for existence of new magic nuclei

New information about energies and occupation probabilities of neutron and proton single-particle (hole) subshells in even—even nuclei was obtained in previous studies applying the method of putting into correspondence all available data on one-nucleon pickup and stripping reactions. The most important and interesting resultwas identification of several nuclei as new magic ones. Namely, itwas found that a filling of the neutron 2d _5/2 subshell in ⁹⁶Zr makes this nuclide a magic one. Moreover, changes in proton subshell energies with increasing neutron number N are accompanied by an increasing energy gap between closed 2p _1/2 and empty 1g _9/2 subshells. Thus, the neutron number N = 56 appears to be a magic one if the proton number Z is equal to 40. The proton number Z = 40 manifests properties of the magic number in ⁹⁶Zr. Therefore, ⁹⁶Zr was identified as a new double-magic nucleus. Further investigations revealed that the energy of the first 2⁺ state E(2 ₁ ⁺ ) in ⁹⁶Zr is much higher than that in the neighboring isotopes and isotones, whereas the ratio E(4 ₁ ⁺ )/E(2 ₁ ⁺ ) and the quadrupole deformation parameter β2 are, vice versa, clearly lower. Moreover, the A dependence of the neutron separation energy B(n) in Zr isotopes has an irregularity at N = 56 which is typical of magic nuclei. As a result of these investigations, it was found that, near the Fermi energy, there are two closed subshells with the same (and large) angular momentum j = 5/2 (viz. π1f _5/2 and ν2d _5/2). We call this situation the jj connection. The magic numbers under discussion (Z = 40 and N = 56) are achieved at the points where both subshells are closed, and in addition, the closed subshell with j = 1/2, π2p _1/2, occurs above the proton π1f _5/2 subshell. This looks like a result of some additional attractive proton-neutron interaction. It was found that application of this scheme (jj connection) to other subshells reveals several other new magic nuclei: ⁵⁴Ca (closed π1d _3/2 and ν2p _3/2 together with closed ν2p _1/2), ³⁰S and ³⁰Si (closed π1d _5/2 and ν1d _5/2 together with closed (π/ν)2s _1/2), and ¹⁴O and ¹⁴C (closed π1p _3/2 and ν1p _3/2 together with closed ν2p _1/2). The text was submitted by the authors in English.     

Correlation and polarization characteristics of the nuclear quasi-free (p,dπ<Superscript>+</Superscript>) reaction

An extended set of observables of the nuclear quasi-free (p, dπ⁺) reaction including the triple differential cross-section for coincidence measurements, its analyzing power in case of polarized proton beams and, also, the parameters of the polarization of the excited recoil nucleus and the produced deuteron are considered in the framework of the distorted-wave impulse approximation using the reaction ¹⁶O(p, dπ⁺)¹⁵N at a proton energy of 650 MeV as an example. The calculations show a high sensitivity of the differential cross-section and, especially, of the polarization transfer characteristics of the reaction to the spin-multipole decomposition of the amplitude of the basic two-body pp → dπ⁺ process.     

Production of a0 +-mesons in the reaction pp↦da0 +

We investigate the reaction pp↦da ₀ ⁺ at COSY and SIS energies together with accompanying background reactions and inclusive particle yields. The a ₀ ⁺ is considered as a usual u quark model state with two decay channels a ₀ ⁺↦K ⁺ and a ₀ ⁺↦π⁺η. Calculated cross-sections for a ₀ ⁺ production as well as for the corresponding non-resonant channels pp↦dK ⁺ and pp↦dπ⁺η are compared. Especially, in case of the final channel dπ⁺η, high statistics measurements are necessary to extract the a ₀ ⁺ signal from the high non-resonant background. Received: 20 December 2000 / Accepted: 7 May 2001     

π−-meson and nucleon yields from light targets irradiated by deuterium and tritium nuclei beams at energies of 1 GeV/nucleon

In this paper, we present a model of calculation with respect to the interactions of high-energy nuclei with matter. Based on this model, we obtain results on energy and angular spectra of the n- and π⁻-particles produced in collisions of deuterium and tritium nuclei at energiesT _d=1 GeV/nucleon with light targets such as Li, Be. We have also estimated the production yields of neutrons and π⁻-mesons in targets of various radii, as well as mean energies of these these particles. Summarizing, we find that the lithium target of radiusR=10−12 cm for which the energy cost ε_π to produce one π⁻-meson is estimated as 6.7 GeV/π for a d-beam and 5.3 GeV/π for a t-beam is the most preferred pion-production target.     

Model of the photostimulated fragmentation of PETN molecules in selective photoinitiation

A model of the photostimulated fragmentation of the PETN molecule irradiated by the first harmonic of a neodymium laser (1060 nm) is proposed. Photoexcitation at 1060 nm leads to the n → π* transition of the 2p nonbinding electron of the oxygen to the π* antibonding orbital, a transition that causes the rupture of the O-N bond and the formation of NO₂^* radicals, thereby ensuring a further development of explosive decomposition. In the case of a free molecule or a molecule located on the surface of a microcrystal, the process does not require any thermal activation. In the case of a molecule at a regular lattice site, an additional activation energy (0.4 eV) is needed to overcome the potential barrier associated with the passage of through a bottleneck between the nearest-neighbor molecules.     

The effect of base thickness on the reverse volt–ampere characteristic of an S-diode

The effect of base thickness (π-layer thickness d _π) on the reverse current-voltage characteristic and the switching voltage U _sw in a diffusion avalanche S-diode is studied. It is shown that the current-voltage characteristic shape is independent of d _π, whereas the switching voltage U _sw weakly decreases (to 40%) with a significant (4–5-fold) decrease in d _π. It is assumed that the results obtained can be explained, taking into account electron injection from the forward-biased contact to the π-layer. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 48–53, February, 2009.     

Two-pion decay of the P 11(1440) resonance excited in αp Scattering at 1 GeV/nucleon

Semiexclusive measurements of the two-pion production reaction p(α, α′)pππ have been carried out at an energy of E _α = 4.2 GeV at the Saturne-II (Saclay) accelerator with the SPES4-π installation. The two-pion production reaction was investigated by simultaneous registration of the scattered α particle and the secondary proton. The obtained results show that the two-pion decay of the P ₁₁(1440) resonance excited in the target proton in this reaction proceeds predominantly with emission of two pions in the isospin I = 0, S-wave state. The text was submitted by the authors in English. the SPES4-π Collaboration     

The Birth of the Infinite Cluster:¶Finite-Size Scaling in Percolation

We address the question of finite-size scaling in percolation by studying bond percolation in a finite box of side length n, both in two and in higher dimensions. In dimension d= 2, we obtain a complete characterization of finite-size scaling. In dimensions d>2, we establish the same results under a set of hypotheses related to so-called scaling and hyperscaling postulates which are widely believed to hold up to d= 6. As a function of the size of the box, we determine the scaling window in which the system behaves critically. We characterize criticality in terms of the scaling of the sizes of the largest clusters in the box: incipient infinite clusters which give rise to the infinite cluster. Within the scaling window, we show that the size of the largest cluster behaves like n ^d π _n , where π _n is the probability at criticality that the origin is connected to the boundary of a box of radius n. We also show that, inside the window, there are typically many clusters of scale n ^d π _n , and hence that “the” incipient infinite cluster is not unique. Below the window, we show that the size of the largest cluster scales like ξ ^d π_ξ log(n/ξ), where ξ is the correlation length, and again, there are many clusters of this scale. Above the window, we show that the size of the largest cluster scales like n ^d P _∞, where P _∞ is the infinite cluster density, and that there is only one cluster of this scale. Our results are finite-dimensional analogues of results on the dominant component of the Erdős–Rényi mean-field random graph model. Received: 6 December 2000 / Accepted: 25 May 2001     

d-symmetry superconductivity due to valence bond correlations

It is shown that d-symmetry superconductivity due to valence bond correlations is possible. Valence bond correlations are compatible with antiferromagnetic spin order. In order to explictly construct a homogeneous state with the valence bond structure in the two-dimensional Hubbard model for an arbitrary doping, we have used the variational method based on unitary local transformation. Attraction between holes in the d-channel is due to modulation of hopping by the site population in course of the valence bond formation, and corresponding parameters have been calculated variationally. An important factor for the gap width is the increase in the density of states on the Fermi level due to antiferromagnetic splitting of the band. The gap width and its ratio to the T _c are 2Δ≃0.1t and 2Δ/kT _c≃4.5−4 for U/t≃8. The correspondence between the theoretical phase diagram and experimental data is discussed. The dependence of T _c on the doping δ=|n−1| and the Fermi surface shape are highly sensitive to the weak interaction t′ leading to diagonal hoppings. In the case of t′>0 and p-doping, the peak on the curve of T _c(δ) occurs at the doping δ _opt, when the energy of the flattest part of the lower Hubbard subband crosses the Fermi level at k∼(π,0). In underdoped samples with δ<δ _opt, the anisotropic pseudogap in the normal state corresponds to the energy difference |E(π,0)−μ| between this part of the spectrum and the Fermi level. Zh. éksp. Teor. Fiz. 114, 985–1005 (September 1998)