Chemical reactions and fluctuations

Lattice systems with one species diffusion-reaction processes under local complete exclusion rules are studied analytically. We discuss a rigorously derived Fokker-Planck equation for a so-called pseudo-probability. This probability distribution depends on continuous variables in contrast to the original discrete master equation, and their stochastic dynamics may be interpreted as a substitute process which is completely equivalent to the original lattice dynamics. Especially, averages and correlation functions of the continuous variables are connected to corresponding lattice quantities by simple relations. Although the substitute process for diffusion-reaction systems with exclusion rules has some similarities to the well known substitute process for the same system without exclusion rules, their exist a set of remarkable differences. The given approach is not only valid for the discussed single species processes. We give sufficient arguments that arbitrary combinations of uni-molecular and bimolecular lattice reactions under complete local exclusions may be described in terms of our approach.     

Alpha induced reactions

A comprehensive model forα particle induced reactions, recently proposed by two of the authors, has been improved and utilised in the analysis of proton andα particle spectra, angular distributions, excitation functions, spectra of neutrons emitted in the decay of continuum states on nuclei with 50<A<230 atα energies varying from 50 to 160 MeV. A unique set of average parameters has been used in all the calculations. Most of them were the same utilised in previous analyses of nucleon andπ ^? induced reactions. The energy and mass dependence (which turns out to be quite weak) of the additional parameters which are needed to describe theα particle induced reactions has been determined. The comparison with experimental data shows that the model discussed allows one to reproduce successfully the cross sections ofα particle induced reactions with an accuracy comparable to the one already gained in the analysis of nucleon andπ ^? reactions with the Exciton Model.     

Topological reactions of optical correlation vortices

It is now well-appreciated that the correlation functions of partially coherent optical wavefields may possess phase singularities with properties similar to those that appear in monochromatic wavefields. Though much work has been done to investigate the generic properties of such correlation vortices, little effort has gone towards studying topological reactions associated with these vortices. In this paper we investigate three such reactions: (1) The break-up of a second-order optical vortex into first-order correlation vortices. (2) Creation and annihilation of correlation vortices. (3) The behavior of correlation vortices on propagation. These results clearly demonstrate the relationship between optical vortices and correlation vortices, and suggest the possible use of correlation vortices as a probe of the statistical properties of a field or a medium.     

Nucleophilic ring opening reactions of aziridines

Aziridine ring opening reactions have gained tremendous importance in the synthesis of nitrogen containing biologically active molecules. During recent years, a great effort has been put forward by scientists toward unique bond construction methodologies via ring opening of aziridines. In this regard, a wide range of chiral metal- and organo-catalyzed desymmetrization reactions of aziridines have been reported with carbon, sulfur, oxygen, nitrogen, halogen, and other nucleophiles. In this review, an outline of methodologies adopted by a number of scientists during 2013–2017 for aziridine ring opening reactions as well as their synthetic applications is described.     

Temperature effects in ultrasonically activated chemical reactions

Summary The temperature effects on the yield of a sonoactivated model reaction has been investigated, to establish the role played by some physical parameters in determining either the resonant conditions for ultrasonic cavitation as well as the related enhancement of reaction rate in sonochemistry. It has been found that the statical pressure at which the resonant phenomena occur increases straightly with temperature and is related to the ?resonant volume? of the cavitating bubbles, at a given frequency of the ultrasounds. Comparison with our previous results confirms the possibility of driving the liquid-phase reacting system into resonant conditions, by varying the static pressure and/or the temperature, thus increasing the reaction rate and yield of the model chemical reaction chosen,i.e. the KMnO₄ oxidation of indane to indan-1-one.     

Influence functionals in deep inelastic reactions

It is suggested that the concept of influence functionals introduced by Feynman and Vernon could be applied to the study of deep inelastic reactions among heavy ions if the coupling between the relative motion and the internal degrees of freedom has a separable form as suggested by Hofmann and Siemens.     

Stability of membrane bound reactions

We present a novel approach to the dynamics of reactions of diffusing chemical species with species fixed in space, e.g., by binding to a membrane. The nondiffusing reaction partners are clustered in areas with a diameter smaller than the diffusion length of the diffusing partner. The activated fraction of the fixed species determines the size of an active subregion of the cluster. Linear stability analysis reveals that diffusion is one of the major determinants of the stability of the dynamics. We illustrate the model concept with Ca2+ dynamics in living cells, which has release channels as fixed reaction partners. Our results suggest that spatial and temporal structures in intracellular Ca2+ dynamics are caused by fluctuations due to the small number of channels per cluster.     

Regge phenomenology of inclusive reactions

This project is devoted to updating the Regge phenomenology of inclusive reaction and correlation, with the help of the low p_T data acquired over the last 5 years or so. The analyses over the fragmentation region are largely based on the Mueller-Regge formalism. For the central cross-section and correlation data, however, the Mueller-Regge formalism is seen to be inadequate; and the analyses are largely based on the multiperipheral cluster model and the Gribov calculus. The effort has been to make the individual sections self-contained, at the cost of some duplication. The important gaps remaining in the data, as also in the theoretical interpretation of the available data, are brought out at the end of the relevant sections.     

Aminoheterocycles as synthons for combinatorial Biginelli reactions

Chlorotrimethylsilane (TMSCl) has been utilized as an efficient promoter and water scavenger in the synthesis of diverse dihydropyrimidines via Biginelli type MCR-heterocyclization using aminoheterocycles. High yields and a simple workup of target compounds enable the facile generation of combinatorial libraries comprising more than 2,000 compounds of high structural and functional diversity. A representative set of 89 compounds is described.     

Role of adsorption complexes in catalytic acceleration of heterogeneous reactions

The universal equation that imposes restrictions on the kinetics of heterogeneous chemical reactions has been derived. A method of studying the participation of chemisorbed or physisorbed gas molecules in heterogeneous chemical reactions has been proposed. It has been shown that the heterogeneous reaction rate is independent of the degree of occupation of the catalyst surface by the reactants as long as the product is formed by chemisorbed complexes and physisorbed molecules. It has been found that the formation of CO₂ in the heterogeneous chemical reactions CO+ O → CO₂ and 2CO + O₂ → 2CO₂ is participated by physisorbed particles and chemisorbed complexes of oxygen atoms and CO molecules.     

Electron-beam-induced surface reactions in Pd/PdO films

Profile imaging observations of clean palladium surfaces suggested that surface diffusion occurred by the slip of surface Shockley partial dislocations resulting in the depletion of material from the surface. The formation of PdO patches under strong electron irradiation and the epitaxial orientation relationships which developed between the oxide and the Pd matrix were studied at the atomic scale. The newly formed PdO crystals were not usually stable against further intense irradiation. Their disintegration as well as depletion has been considered in terms of the mechanical behavior of PdO crystals.     

On the reversibility of stripping reactions

The basis for the equivalence of prior and post forms of the T-matrix in both the Born and the distorted wave Born approximations is here studied in a one-dimensional model that facilitates clarification of some of the underlying mechanism. One sees how important it is that changes in the wave functions bring it about that averages of different perturbing potentials over different regions can still be equal. The post form commonly employed for a (d, p) reaction makes it seem at first sight that the potential stripping the deuteron apart is the potential that holds it together. The prior form DWBA has as a perturbation the intuitively appealing interaction responsible for the classical break-up of the deuteron and contributing to the “surface reaction” nature of stripping.     

Spallation reactions studied with 4π-detector arrays

Recently here has been a renewed interest in the study of spallation reactions in basic nuclear physics as well as in potential applications. Spallation reactions induced by light projectiles (protons, antiprotons, pions, etc.) in the GeV range allow the formation of hot nuclei which do not suffer the collective excitations (compression, rotation, deformation) unavoidable when using massive projectiles. Such nuclei provide an ideal testbench for probing their decay as a function of excitation energy. In these investigations, 4π-detector arrays for charged particles and neutrons play a major role in the event-by-event sorting according to the excitation energy of the nucleus. Spallation reactions induced on heavy nuclei allow the conversion of the incident GeV proton into several tens of evaporated neutrons. The neutron production in thick targets has been investigated in great detail thanks to the use of high efficiency neutron detector arrays. When scattered on samples of inert or biological materials, these neutrons can be used to study details of the material structure. They could also be utilized for the transmutation of long-lived nuclear wastes or for the feeding of sub-critical nuclear reactors. The role of different types of multi-detector arrays is highlighted in this paper. Several references are also given for different uses of high efficiency neutron detectors in other contexts.     

Polarization in heavy-ion reactions

A series of experiments is described in which beta-ray asymmetry has been used to determine polarization of heavy-ion reaction products¹²B and the present status of the studies of polarization phenomena in heavy-ion reactions is reviewed. A large amount of angular momentum sustained by the two colliding nuclei gives rise to polarization phenomena of reaction products. Coupling between the degrees of freedom accompanying the intrinsic and the relative motions is investigated from the systematic behaviour of polarization of reaction products disclosed by the experiments.     

Pion-induced knock-out reactions

A strong absorption model for pion-induced knock-out reactions is proposed. The distortion of the incoming and outgoing pions has been included by (i) computing the pion wave number in a nuclear medium (dispersive effect) and (ii) excluding the central region of the nucleus where the real pion absorption is dominant (absorption effect). In order to study the dependence of the (π⁺, π⁺p) reaction on the off-shell pion-nucleon t-matrix, different off-shell extrapolations are used. The magnitude of the cross sections seems to be sensitive to the type of off-shell extrapolation; their shapes, however, are similar. The theoretical results are compared with experimental data. The agreement between the theoretical results for separable off-shell extrapolation and the data is good.     

Electronic excitations by chemical reactions on metal surfaces

Dissipation of chemical energy released in exothermic reactions at metal surfaces may happen adiabatically by creation of phonons or non-adiabatically by excitation of the electronic system of the metal or the reactants. In the past decades, the only direct experimental evidence for such non-adiabatic reactions has been exoelectron emission into vacuum and surface chemiluminescence which are observed in a special class of very exothermic reactions. The creation of e–h pairs in the metal has been discussed in many theoretical models but it was only recently that a novel experimental approach using Schottky diodes with ultrathin metal films makes direct measurement of reaction-induced hot electrons and holes possible. The chemical reaction creates hot charge carriers which travel ballistically from the metal film surface toward the Schottky interface and are detected as a chemicurrent in the diode. By now, such currents have been observed during adsorption of atomic hydrogen and deuterium on Ag, Cu and Fe surfaces as well as chemisorption of atomic and molecular oxygen, of NO and NO₂ molecules and of certain hydrocarbons on Ag. This paper reviews briefly exoelectron and chemiluminescence experiments and the concept of the Nørskov–Newns–Lundqvist model. The major part is devoted to the detection of chemically induced e–h pairs with thin metal film Si Schottky diodes by discussing the different influences on the chemicurrent magnitude and presenting experimental results predominantly with hydrogen and deuterium atoms. The experiments introduce a new method to investigate surface reaction kinetics and dynamics by use of an electronic device. In addition, the diodes may be used as selective reactive gas sensors.     

Cluster knockout reactions

Cluster knockout reactions are expected to reveal the amount of clustering (such as that of α, d and even of heavier clusters such as¹²C, ¹⁶O etc.) in the target nucleus. In simple terms, incident medium high-energy nuclear projectile interacts strongly with the cluster (present in the target nucleus) as if it were existing as a free entity. Theoretically, the relatively softer interactions of the two outgoing particles with the residual nucleus lead to optical distortions and are treated in terms of distorted wave (DW) formalism. The long-range projectile–cluster interaction is accounted for, in terms of the finite range (FR) direct reaction formalism, as against the more commonly adopted zero-range (ZR) distorted wave impulse approximation (DWIA) formalism. Comparison of the DWIA calculations with the observed data provide information about the momentum distribution and the clustering spectroscopic factor of the target nucleus. Interesting results and some recent advancements in the area of (α, 2 α) reactions and heavy cluster knockout reactions are discussed. Importance of the finite-range vertex and the final-state interactions are brought out.     

Alpha-particle reactions

Recent work using the concept of alpha-clustering in studies of nuclear structure and nuclear reactions is reviewed. The high symmetry and binding energy of the alpha-particle makes it likely that nucleons inside the nucleus can condense into alpha particles and live long enough to affect many properties of nuclei and also the cross-sections of nuclear reactions, particularly those with alpha-particles as the projectile or ejectile. The alpha-particles inside the nucleus may escape, and the resulting alpha decay is now quite well understood. An incident projectile may collide with a transient alpha-particle in the nucleus and knock it out, thus providing information on the degree of clustering. This model enables the direct part of the cross-sections of (n, α) reactions to be calculated. The alpha-particle mean field unifies many of the characteristics of alpha-particle structure and alpha-particle scattering. Many properties of light nuclei may be simply explained using the concept of alpha-clustering.     

Test of sum rules in nucleon transfer reactions

The quantitative consistency of nucleon transfer reactions as a probe of the occupancy of valence orbits in nuclei is tested. Neutron-adding, neutron-removal, and proton-adding transfer reactions were measured on the four stable even Ni isotopes, with particular attention to the cross section determinations. The data were analyzed consistently in terms of the distorted wave Born approximation to yield spectroscopic factors. Valence-orbit occupancies were extracted, utilizing the Macfarlane-French sum rules. The deduced occupancies are consistent with the changing number of valence neutrons, as are the vacancies for protons, both at the level of <5%. While there has been some debate regarding the true "observability" of spectroscopic factors, the present results indicate that empirically they yield self-consistent results.