Sharp Forward Peak Research Articles

Reaction dynamics of Cl + butanol isomers by crossed-beam sliced ion imaging

Butanol is now prominent among the prototype renewable biofuels. We have studied oxidation of a variety of butanol isomers under single collision conditions using chlorine atom as the oxidizing agent to gain detailed insight into the energetics and dynamics of these reactions. The interaction of chlorine atom radicals with butanol isomers: n-butanol, iso-butanol, sec-butanol, and tert-butanol have been studied by crossed-beam dc slice ion imaging techniques. The hydroxybutyl radicals generated from the H-abstraction processes were probed by single photon ionization using an F2 excimer laser. After background subtraction and density-to-flux correction of the raw images, translational energy distribution and product angular distributions were generated. At low collision energy, the hydroxyalkyl products are backscattered with respect to the alcohol beam and the scattering shifts to the forward direction as the collision energy is increased. The translational energy distributions are reminiscent to that of Cl + pentane reactions we studied earlier, i.e. a sharp forward peak -80% of the collision energy appears at the high collision energy. Isomer-specific details of the reactions will be discussed.

Multiple-scaling methods for Monte Carlo simulations of radiative transfer in cloudy atmosphere

Two multiple-scaling methods for Monte Carlo simulations were derived from integral radiative transfer equation for calculating radiance in cloudy atmosphere accurately and rapidly. The first one is to truncate sharp forward peaks of phase functions for each order of scattering adaptively. The truncated functions for forward peaks are approximated as quadratic functions; only one prescribed parameter is used to set maximum truncation fraction for various phase functions. The second one is to increase extinction coefficients in optically thin regions for each order scattering adaptively, which could enhance the collision chance adaptively in the regions where samples are rare. Several one-dimensional and three-dimensional cloud fields were selected to validate the methods. The numerical results demonstrate that the bias errors were below 0.2% for almost all directions except for glory direction (less than 0.4%) and the higher numerical efficiency could be achieved when quadratic functions were used. The second method could decrease radiance noise to 0.60% for cumulus and accelerate convergence in optically thin regions. In general, the main advantage of the proposed methods is that we could modify the atmospheric optical quantities adaptively for each order of scattering and sample important contribution according to the specific atmospheric conditions.

Imaging the dynamics of chlorine atom reactions with alkenes

We report a study of chlorine atom reactions with a series of target monounsaturated alkene molecules: 1-pentene, 1-hexene, 2-hexene, and cyclohexene. These reactions were studied using crossed-beam dc slice ion imaging at collision energies of 4 and 7 kcal/mol. Images of the reactively scattered alkenyl radical products were obtained via single photon ionization at 157 nm. The angular distributions at low collision energy are largely isotropic, suggesting the formation of a complex that has a lifetime comparable to or longer than its rotational period, followed by HCl elimination. At high collision energy, the distributions show a sharp forward peak superimposed on the isotropic component accounting for approximately 13% of the product flux. The translational energy distributions peak near zero for the backscattered product, in sharp contrast to the results for alkanes. In the forward direction, the translational energy distributions change dramatically with collision energy. At the high collision energy, a sharp forward peak at approximately 80% of the collision energy appears, quite reminiscent of results of our recent study of Cl+pentane reactions. The scattering distributions for all target molecules are similar, suggesting similarity of the reaction dynamics among these molecules. Ab initio calculations of the energetics and ionization energies for the various product channels were performed at the CBS-QB3 level to aid in interpreting the results.

Crossed-beam dc slice imaging of chlorine atom reactions with pentane isomers

We present the first in a series of detailed investigations of the reaction of Cl atoms with hydrocarbons in crossed beams. This study focuses on reaction with the pentane isomers n-pentane, isopentane, and neopentane. These crossed-beam ion imaging experiments rely on "universal" 157 nm probe of the pentyl radical product, with dc sliced detection providing the scattering distributions directly. The results for the different target reactants were studied at two collision energies: approximately 5 and approximately 9 kcal/mol. The scattering results are quite similar for all reactants, suggesting that the nature of the abstraction site has surprisingly little influence on the dynamics. The angular distributions are broad with a backscattered peak at low collision energy and a sharp forward peak at high collision energy. The translational energy distributions are strongly coupled to the angular distributions and show, for the forward scattered products, a sharp peak at 80% of the collision energy for all reactants at both collision energies. The sideways-scattered products show the lowest fraction of energy in translation, while the backscattered translational energy distribution peaks at the lowest energy but extends to the highest, showing evidence of coupling of the internal degrees of freedom and the reaction exoergicity, into translation. The results are discussed in view of the extensive literature on related systems.

A comparison of quantum and quasiclassical statistical models for reactions of electronically excited atoms with molecular hydrogen

A detailed comparison of statistical models based on the quasiclassical trajectory (SQCT) and quantum mechanical (SQM) methods is presented in this work for the C((1)D)+H(2), S((1)D)+H(2), O((1)D)+H(2) and N((2)D)+H(2) insertion reactions. Reaction probabilities, integral (ICS) and differential (DCS) cross sections at different levels of product's state resolution are shown and discussed for these reactions. The agreement is in most cases excellent and indicates that the effect of tunneling through the centrifugal barrier is negligible. However, if there exists a dynamical barrier, as in the case of the N((2)D)+H(2) reaction, some of the SQM results can be slightly different than those calculated with the SQCT model. The rationale of the observed similarities and discrepancies can be traced back to the specific topologies of the potential energy surfaces for each of the reactions examined. The SQCT model is sensitive enough to show the relatively small inaccuracies resulting from the decoupling inherent to the centrifugal sudden approximation when used in the SQM calculations. In addition, the effect of ignoring the parity conservation is also examined. This effect is in general minor except in particular cases such as the DCS from initial rotational state j=0, which requires, in order to reproduce the sharp forward and backward peaks, the explicit conservation of parity.

Backscattering of beams by forward-peaked scattering media.

For a beam impinging on a scattering medium the diffusion approximation to the radiative transport equation is not valid for analyzing the radiance near the source, especially if the medium scatters strongly with a sharp forward peak. To analyze the radiance, we use the Fokker-Planck approximation to the radiative transport equation. Numerical results show a backscattered ring appearing around the beam center. It also appears in Monte Carlo simulations of the radiative transport equation. This ring is manifested from successive near-forward scattering events, so it requires a directional description. Therefore the diffusion approximation cannot predict this ring.

On the emission of fast and slow target fragments from [formula omitted]–AgBr interactions at [formula omitted

Multiplicity distributions of secondary charged particles coming out of 84 Kr –AgBr interaction at 0.95 GeV/ A have been reported. Angular distributions of fast and slow target fragments have also been studied. The sharp forward peak in the angular distribution of knocked out protons has further been analyzed in the light of intermittency and scaled factorial moment.

Energy dependence of forward scattering in the differential cross section of the H+D2→HD(v′=3,j′=0)+D reaction

Extensive time-independent quantum mechanical scattering calculations for the H+D2(v=0,j=0) reaction have been performed in the collision energy range 1.39–2.20 eV on the Boothroyd–Keogh–Martin–Peterson potential energy surface. The theoretical differential cross sections (DCS) obtained for the H+D2→HD(v′=3,j′=0)+D channel of the reaction have been compared with recent measurements by Zare and co-workers over the collision energy range 1.39–1.85 eV using the photoloc technique [S. C. Althorpe et al., Nature (London) 416, 67 (2002)]. An excellent agreement between experiment and theory has been found for most of the collision energies studied. In particular, the appearance and evolution of forward scattering with collision energy observed experimentally has been quantitatively reproduced by the theoretical calculations. An analysis of the theoretical results, including a semiclassical complex angular momentum analysis, have been performed in order to ascertain the origin of the sharp forward peaks in the DCS.

Efficient technique to determine backscattered light power for various atmospheric and oceanic sounding and imaging systems

A new technique that efficiently simplifies the estimations of backscattered light power is presented. This technique allows one to consider real inhomogeneous scattering media with an underlying surface and to account for all the features both of media phase functions and of the reflection of underlying surfaces. It is assumed that the phase function has a sharp forward peak. Different sounding, vision, and location systems can be considered. The image transfer equation is derived for any observed object, including three-dimensional and non-Lambertian objects. A few examples of the successful use of the developed approach to solve various problems are given.

Displacement Reaction Dynamics of Fluorine Atoms with Vinyl Bromide Molecules

Reactive scattering of F atoms with C 2 H 3 Br molecules leading to both H and Br atom displacement has been studied at an initial translational energy E∼40 kJ mol -1 using a supersonic beam of F atoms seeded in He buffer gas. The center-of-mass angular distribution of C 2 H 3 F reactive scattering shows a sharp forward peak with a lower backward peak of relative intensity ∼0.5. The product translational energy distribution peaks at a low fraction f' pk ∼0.1 of the total available energy with a tail extending up to higher energy. The branching ratio ∼10 strongly favors Br atom displacement over H atom displacement

Neutron scattering and magnetic susceptibilities of dilute P tCo alloys calculated by Monte Carlo methods

Neutron scattering experimental data for dilute PtCo alloys show sharp forward peaks indicating the inhomogeneous distribution of magnetization in these alloys and the size and shape of magnetization clouds. These characteristics of the clouds, however, do not agree with those obtained by magnetization measurements. The disagreement suggests an important nonelastic contribution to such scattering. We, therefore, calculated the diffuse neutron cross sections for these alloys employing a magnetic environment model for the magnetization clouds, that has been used successfully for other giant moment dilute alloys such as PdFe and PtFe. The calculations were made by Monte Carlo methods for alloys of PtCo in the concentration region 1.0≤c≤6.0 at. %Co. The results show sharp peaks due to quasielastic scattering which reproduce well the experimental neutron data. We also calculated by the same methods the magnetic susceptibilities for alloys of PtCo with c<1.0 at. %Co. The results show a good agreement with experimental data. The validity of the model and of the magnetic correlations used for the calculations is supported by their agreement with experimental results. At high concentrations the model should not apply as the clouds are considered independent in the calculations.

Convergent polynomial expansion, lines of zeros, and slopes of diffraction scattering. II. Oscillation of the slope parameter inπ−pscattering

It is shown that the simple polynomial approximation of transversity amplitudes using complex zeros of Barrelet type cannot account for the slope-parameter data at any of the energy ranges except for providing only a qualitative explanation of the bump structure around $s\ensuremath{\simeq}3$ Ge${\mathrm{V}}^{2}$. At high energies experimentally observed linearly increasing or decreasing trajectories can never account for the slope-parameter data. These results reflect the fact that the sharp forward peak cannot be represented by finite-order polynomials in the conventional scheme of parametrization. Using Mandelstam analyticity and information on the real and complex zeros we propose a new scheme of parametrization to understand the energy dependence of slope-parameter data for all energies such that the problem of a dynamical understanding of diffraction scattering reduces to the problem of providing a dynamical origin of zero trajectories. This leads to a formula that relates the slope parameter with equations to boundaries of spectral functions and real and complex zero trajectories. An excellent description of the data is obtained for all energies with $s>1.44$ Ge${\mathrm{V}}^{2}$ using theoretical elastic boundaries of spectral functions, experimentally determined real and complex zero trajectories in a limited energy range, and their realistic extrapolations for higher and lower energies. The slope-parameter data are found to be very sensitive to small variations of the real trajectory around its extrapolated value at low energies. Using our formula, real zero trajectory in the range $1.33\ensuremath{\le}s\ensuremath{\le}1.62$ Ge${\mathrm{V}}^{2}$ has been determined from the slope-parameter data. From the structures observed in the slope-parameter data we make some qualitative predictions of complex trajectories at high energies. Contrary to the general supposition that the forward peak cannot be represented by true zeros of the amplitude, our new scheme of parametrization strongly suggests that the diffractive parts of transversity amplitudes possess all the complex zeros and the real zero existing away from the forward-peak region.

A study of the reaction [formula omitted] in the resonance region

The reaction p+ d → 3 He + π 0 was studied at incident proton energies of 377, 462 and 576 MeV. A comparison of our data with others suggests the existence of a sharp forward peak in θ He at 377 and 462 MeV, which is unexplained by the theory and disappears at higher energies.

Study of charge-exchange double dissociation of protons at the CERN intersecting storage rings

We have measured the charge-exchange reaction pp → (p π +) (p π −) at s = 23, 31, 45, 53, and 63 GeV at the ISR, using the Split Field Magnet detector. The data are characterized by a sharp forward peak of width 0.02 (GeV/ c) 2, followed by a much gentler slope at higher | t|; sizeable Δ ++, Δ 0, and N 0 production is observed in the (Nπ) mass spectra. Two different components are present in the data following power-law energy dependence of the type p lab − n with n 1 = −1.63 ± 0.13 and n 2 = −0.96 ± 0.07, respectively, indicating the onset of mechanisms in competition with pion exchange at ISR energies.

Low-transverse-momentum distribution and cluster model

The low-transverse-momentum region of pions in nucleonnucleon interactions is investigated at 22.8 GeV/c and at cosmic-ray energies (∼1012eV). The existence of a sharp forward peak in the transverse-momentum distribution is explained on the basis ofa) resonance formation in the former events andb) cluster production in the latter interactions.

N-pCharge-Exchange Scattering from 60 to 300 GeV/c

We have measured the neutron-proton charge-exchange differential cross section in the momentum interval 60 to 300 GeV/c, with squared four-momentum transfers 0.002 to 0.8 ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{2}$. Independent of incident momentum, the data are characterized by a sharp forward peak of width 0.02 ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{2}$, followed by a shoulder and gentler falloff at higher momentum transfers.

Neutron-proton charge-exchange scattering from 22 to 65 GeV/ c

The differential cross sections for neutron-proton elastic charge-exchange scattering have been measured with a two-arm technique for incident neutron momenta between 22 and 65 GeV/ c and for values of the momentum transfer squared between 0.002 and 0.8 (GeV/ c) 2. The sharp forward peak observed previously at lower energies is also present at momenta up to 65 GeV/ c; however the s dependence of the cross section is slowing down.

ρ±Photoproduction at 9.6 GeV

rho/sup + -/ photoproduction from hydrogen and deuterium at 9.6 GeV is studied. The reactions ..gamma..p..-->..p/sup +/n and ..gamma..p..-->..p/sup -/..delta../sup + +/(1236) have cross sections much larger than expected from pion exchange alone. t distributions do not show the sharp forward peak characteristic of one-pion exchange contributions. Density matrices indicate an isotropic decay distribution. The ratio of rho/sup +/ to rho/sup -/ production on deuterium differs from unity and by the amount expected from interference between rho and A/sub 2/ exchanges. We conclude the ..pi.. exchange is unimportant for rho/sup + -/ photoproduction, and infer that rho exchange dominates. (AIP)

Diffraction dissociation and Coulomb dissociation of high-energy neutrons by nuclei

We present the results of a spark chamber experiment in which we studied the reaction n + A → (p + π −) + A′, where the (p π −) pair is coherently produced off carbon, copper, and lead nuclei. Between 5 000 and 55 000 good events were collected with each target. Incident neutrons in the momentum range 10 to 28 GeV/ c were used. Forward-going (p π −) pairs from the above reaction were detected in a wire spark chamber spectrometer which measured the momentum and angles of each particle. The observation of a sharp forward peak (of width appropriate to the nuclear radius) indicated that a large fraction of the events were coherently produced. There is no evidence for any appreciable production of the well-established I = 1 2 nucleon isobars; our limits appear to be inconsistent with estimates based on a straightforward extrapolation of pp data. The lead data show evidence for coulomb production of the Δ (1236). The carbon data indicate a spin J ⩾ 3 2 for the coherently produced (p π −) states with pure J = 3 2 possible. We cannot draw any firm conclusions about helicity conservation. The Drell-Hiida-Deck model does not adequately fit the data, nor is the double-Regge-pole model completely satisfactory.

Coherent effects of a weak neutral current

If there is a weak neutral current, then the elastic scattering process $\ensuremath{\nu}+A\ensuremath{\rightarrow}\ensuremath{\nu}+A$ should have a sharp coherent forward peak just as $e+A\ensuremath{\rightarrow}e+A$ does. Experiments to observe this peak can give important information on the isospin structure of the neutral current. The experiments are very difficult, although the estimated cross sections (about ${10}^{\ensuremath{-}38}$ ${\mathrm{cm}}^{2}$ on carbon) are favorable. The coherent cross sections (in contrast to incoherent) are almost energy-independent. Therefore, energies as low as 100 MeV may be suitable. Quasi-coherent nuclear excitation processes $\ensuremath{\nu}+A\ensuremath{\rightarrow}\ensuremath{\nu}+{A}^{*}$ provide possible tests of the conservation of the weak neutral current. Because of strong coherent effects at very low energies, the nuclear elastic scattering process may be important in inhibiting cooling by neutrino emission in stellar collapse and neutron stars.