Large Impact Parameters Research Articles

A crossed molecular beam investigation of the reaction Cl+propane → HCl+C 3H 7 using VUV synchrotron radiation as a product probe

We have used the crossed molecular beam technique to study the hydrogen atom abstraction from propane by atomic chlorine over a wide range of collision energies. The experiments were carried out using a recently constructed crossed molecular beam apparatus that utilizes tunable VUV synchrotron radiation for product photoionization. We have measured laboratory TOF spectra and angular distributions for E coll=8.0, 11.5, and 31.6 kcal/mol. Center-of-mass flux maps were generated from the measured laboratory distributions. The results demonstrate two distinct reaction mechanisms that depend on the impact parameter of the reactive collision. Large impact parameter collisions proceed via a stripping mechanism resulting in forward scattered products with very little momentum change in going from reactant to product. The stripping reactions are most likely dominated by abstraction of secondary hydrogen atoms. Smaller impact parameter collisions lead to direct reactions with an impulsive recoil and are consistent with a preference for a collinear transition state geometry, C–H–Cl. The larger energy along the line of centers in smaller impact parameter collisions most likely makes the effect of a larger barrier to abstraction of primary hydrogen atoms negligible leaving the ratio of primary to secondary hydrogen abstraction to be dictated by simple statistics.

A mode-selective differential scattering study of the C2H2++methanol reaction: Influence of collision intermediates, collision times, and transition states

We report the vibrational and collision energy dependence of cross sections and product branching in the reaction of C2H2+ with CD3OD, CD3OH, and CH3OD. We also report axial recoil velocity distributions, along with modeling. At low collision energies, reaction is mediated by a picosecond lifetime complex of the [C2H2:methanol]+ form. The bottleneck that controls overall reaction efficiency appears to be formation of the complex, and reactivity is influenced by collision energy and C2H2+ CC stretch excitation, but not by bending vibration. The most energetically favorable exit channel from the complex is isomerization to covalently bound C3H6O+ complexes, but this does not occur. Instead the [C2H2:methanol]+ decays by breakup to C2H2+CH4O+, C2H3+CH2OH+, and C2H+CH3OH2+ channels. Changes in the branching with available energy provide some insight into the nature of the transition states that control decay of the complex. As collision energy is raised above ∼1 eV, the reaction gradually becomes direct, i.e., the collision time drops to well below the rotational period of the collision complex (<∼0.5 ps). In this regime, the dominant charge transfer and hydride abstraction products mostly form in large impact parameter collisions. At high energies there is little dependence of either reaction efficiency or product branching on collision energy or reactant vibrational state, suggesting that both are probably controlled largely by collision geometry.

Collision dynamics of large water clusters

Classical trajectory calculations of (H2O)n+(H2O)n collisions are carried out for n=125 and n=1000. We investigate energy redistribution and fragmentation behavior for relative collision velocities up to 3000 ms−1, impact parameters up to 4 nm, and initial cluster temperatures of 160 and 300 K. Three main scattering channels are identified; coalescence, stretching separation, and shattering collisions. For small impact parameters, low collision velocities produce coalesced clusters while high velocities yield shattering behavior. Large impact parameters combined with high velocities result in stretching separation collisions. A decreased internal temperature influences the dynamics by increasing the stability of the collision complex. The results for (H2O)125 and (H2O)1000 are comparable, although the smaller size allows individual molecules to have a larger influence on the overall behavior. We find good agreement between the cluster simulations and experimental data for water drops in the micrometer range concerning the transition between coalescence and stretching separation, which shows that the clusters in some respects resemble “macroscopic” objects.

Scattering of zero-branes off elementary strings in Matrix theory

We consider the scattering of zero-branes off an elementary string in Matrix theory or equivalently gravitons off a longitudinally wrapped membrane. The leading supergravity result is recovered by a one-loop calculation in zero-brane quantum mechanics. Simple scaling arguments are used to show that there are no further corrections at higher loops, to the leading term in the large impact parameter, low velocity expansion. The mechanism for this agreement is identified in terms of properties of a recently discovered boundary conformal field theory.

Intermolecular energy transfer probabilities from trajectory calculations: A new approach

A new method to calculate intermolecular energy transfer probability density function P(E′,E) from trajectory calculations is proposed. The method distinguishes between effective trajectories that contribute to P(E′,E) and those with very large impact parameter which do not. The P(E′,E) thus found obeys conservation of probability and detailed balance and is independent of the impact parameter. The method is demonstrated for benzene–Ar collisions at various temperatures and internal energies. With this method it is possible to combine ab initio inter and intramolecular potentials with trajectory calculations, obtain P(E′,E) and use that in master equation calculations to obtain rate coefficients and populations distributions without resorting to any a priori assumptions and energy transfer models. In addition, the effects of internal energy, temperature and rotations on the average energy transferred are discussed. Global potentials in center-of-mass and minimal distance coordinates which are obtained by averaging 20 000 and 50 000 trajectories are reported. It is shown that Lennard-Jones or ab initio pairwise potentials yield a Buckingham-type global potentials.

DECOHERENT SCATTERING OF LIGHT PARTICLES IN A D-BRANE BACKGROUND

We discuss the scattering of two light particles in a D-brane background. It is known that, if one light particle strikes the D-brane at small impact parameter, quantum recoil effects induce entanglement entropy in both the excited D-brane and the scattered particle. In this letter we compute the asymptotic "out" state of a second light particle scattering off the D-brane at large impact parameter, showing that it also becomes mixed as a consequence of quantum D-brane recoil effects. We interpret this as a non-factorizing contribution to the superscattering operator $ for the two light particles in a Liouville D-brane background, that appears when quantum D-brane excitations are taken into account.

Experimental and computational comparative study of electron emission of thin carbon foils traversed by MeV protons and hydrogen atoms in frozen charge states

The secondary electron emission has been measured for 2 MeV H + and for H 0 projectiles passing through very thin carbon foils without charge changing. We have studied the statistics of the emission from both entrance and emergence surfaces. Simultaneously, Monte Carlo calculations have been performed for similar experimental conditions (500 keV < E < 2 MeV). Both experimental and theoretical distributions can be fitted adequately by Pólya distributions. The measured and calculated differences of the yields and of the distribution widths between the H + and “transmitted” H 0 cases are interpreted in terms of screening effects on the large impact parameter collisions with target electrons.

Plasma screening effects on eikonal phase for electron-ion elastic collisions in weakly coupled plasmas

Eikonal approximation is applied to investigate elastic electron-ion collisions in weakly coupled plasmas. Plasma screening effects on eikonal phase are investigated for eikonal differential elastic scattering cross sections. The electron-ion interaction potential in weakly coupled plasmas has been obtained by the introduction of the longitudinal plasma dielectric function. The semiclassical straight-line trajectory method is applied to the path of the projectile electron in order to investigate the variation of the eikonal phase as a function of the impact parameter and the plasma parameters. In the first-order eikonal approximation, the dynamic plasma screening effect is identical to the static screening effect obtained by the Debye–Hückel potential. The eikonal differential elastic cross section substantially decreases with an increase in the projectile energy and increases as the plasma screening effect decreases through the Debye length. The plasma screening effects are more significant for large impact parameters.

Charm quark production in noncentral heavy ion collisions

The effect of gluon shadowing on charm quark production in large impact parameter ultrarelativistic heavy ion collisions is investigated. Charm quark production cross sections are calculated for a range of noncentral impact parameters which can be determined from the global transverse energy distribution. We show that charm quark production is a good probe of the local parton density which determines the effectiveness of shadowing. We discuss why shadowing may have a spatial dependence and show that this spatial dependence may be detected in noncentral heavy ion collisions.

Relativistic brane scattering

We calculate relativistic phase shifts resulting from the large impact parameter scattering of 0-branes off p-branes within supergravity. Their full functional dependence on velocity agrees with that obtained by identifying the p-branes with D-branes in string theory. These processes are also described by 0-brane quantum mechanics, but only in the non-relativistic limit. We show that an improved 0-brane quantum mechanics based on a Born-Infeld-type Lagrangian also does not yield the relativistic results. The scattering of 0-branes off bound states of arbitrary numbers of 0-branes and 2-branes is analyzed in detail, and we find agreement between supergravity and string theory at large distances to all orders in velocity. Our careful treatment of this system, which embodies the 11-dimensional kinematics of 2-branes in M(atrix) theory, makes it evident that control of 1 n corrections will be necessary in order to understand our relativistic results within M(atrix) theory.

Observation of a Large Steric Effect on the XeBr* Formation in the Reaction of Oriented CF3Br with Xe(3P)

A large steric effect on XeBr* formation was observed in the reaction of oriented CF3Br with Xe(3P). The reaction cross-section was found to be largest for the Br-end collisons and smallest for the CF3-end collisions, even though the electron transfer takes place at relatively large internuclear distances in the entrance channel. The large orientational dependence may be attributed to the fraction of the collisions that have small impact parameters which can lead to the formation of XeBr*. The majority of the collisions, having larger impact parameters, are supposed to lead to electron back-transfer in the exit channel and are relatively insensitive to the reactant orientation.

Collisions between metal clusters and multiply charged ions

Metal clusters are electrically charged by collisions with charged ions through electron transfer reactions from the clusters to the ions. The reaction processes are studied by the time-dependent mean field theory with the usual Local Density Approximation for the exchange-correlation potential. Preliminary results show the interesting feature that collisions with large impact parameters make ionized clusters with relatively low excitation, which gives important new information on the stability of metal clusters against fission.

Instantons, scale invariance and Lorentz invariance in matrix theory

In this paper we consider features of graviton scattering in Matrix theory compactified on a 2-torus. The features which interest us can only be determined by nonperturbative effects in the corresponding 2+1 dimensional super Yang Mills theory. We show that the superconformal symmetry of strongly coupled Super Yang Mills Theory in 2+1 dimensions almost determines low energy, large impact parameter ten dimensional graviton scattering at zero longitudinal momentum in the Matrix model of IIB string theory. We then show that amplitudes involving arbitrary transverse momentum transfer are governed by instanton processes similar to the Polchinski Pouliot process. Finally we consider the influence of instantons on a conjectured nonrenormalization theorem. This theorem is violated by instanton processes. Far from being a problem, this fact is seen to be crucial to the consistency of the IIB interpretation. We suggest that the SO(8) invariance of strongly coupled SYM theory may lead to a proof of eleven dimensional Lorentz invariance.

Stabilization of cluster dimers by centrifugal effects

The influence of finite angular momentum on the stabilization of an (Ar13)2 cluster dimer in Ar13+Ar13 collisions is investigated with simulations and simple models. Local optimizations on the effective rovibrational potentialenergy surface show that the centrifugal effects favor the mechanical stability of strongly prolate shapes. Molecular dynamics simulations are performed to study the collisions between two Ar13 subclusters. At low collisional energy and large impact parameter, trapping of the product cluster in its dimer-like configuration is found. For larger values of the angular momentum, the “molecular” cluster is found to remain in this dimer-like structure during the time of the simulation. Intermediate values of the angular momentum allow isomerization into more compact structures, but still dimer-like. Again, the final product appears as dynamically stable. All of these dimer-like products have the thermodynamical properties of a rotating rigid-like body. At high energy, unimolecular evaporation rates from collisional heating are calculated and comparisons with the predictions of the Rice, Ramsperger, Kassel (RRK) and Weisskopf models are discussed. Qualitative agreement with the simulations, in particular concerning the evolution of the rates versus angular momentum is found. The rates decrease when angular momentum is increased, especially when the total energy is low. Thus, the enhancement of the stability of the product cluster by centrifugal effects has also a statistical character.

A three-dimensional quantum mechanical study of the H+H2+→H2+H+ system: Competition between chemical exchange and inelastic processes

In this publication is presented a three-dimensional quantum mechanical study, within the coupled states approximation, of the process H+H2+(v1=0,j1)→H++H2. Both reactive (exchange) and inelastic processes were considered. The main findings are: (a) The charge transfer process takes place at large distances (∼3.5 Å) and so the reagents are essentially on the lower potential energy surface when they approach the close interaction region; (b) The main contributions to the reaction (exchange) are from large impact parameters; (c) The initial rotational states have at most a minor effect on the results (whether being charge transfer or chemical exchange); (d) The deep potential well in the interaction region of the lower surface has only a secondary effect on the results.

High-order interior caustics produced in scattering of a diagonally incident plane wave by a circular cylinder

We examine scattering of a family of initially parallel diagonally incident rays by a dielectric circular cylinder and show that the interior and exterior caustics that occur are qualitatively identical to those produced at normal incidence. We find, however, that (1) varying the plane-wave tilt angle has the same effect on the caustics as varying the refractive index of the cylinder at normal incidence and (2) high-order interior caustics are visible because of larger internal-reflection Fresnel coefficients at diagonal incidence than at normal incidence. We also observe noncaustic ray trajectories produced by the sharp peaking of internal-reflection Fresnel coefficients at large ray impact parameters, as well as another class of internal caustics produced by scattering from inhomogeneities in our glass cylinder.

Collision Dynamics of Large Argon Clusters

Classical trajectory calculations of collisions between Ar1000 clusters are carried out to investigate the effects of relative velocity and impact parameter on energy transfer and dynamical behavior. In the velocity range 100−1000 m/s, we conclude that the outcome of the collisions can be classified into a few characteristic scattering channels. We observe coalescence, stretching separation, and shattering collisions, and each collision process dominates within distinguishable parameter ranges. A major part of the available energy ends up in vibrational degrees of freedom, and internally hot systems cool by evaporation as well as more severe fragmentation for high-energy collisions. High rotational excitation is observed for large impact parameter values, and the maximum rotational energy of the collision complex is concluded to mainly determine whether a collision will result in coalescence or stretching separation. The results are related to experimental data for collisions between large liquid droplets, and we conclude that argon clusters in the nanometer range partly resemble the larger systems. The boundary between coalescence and stretching separation is surprisingly well predicted by macroscopic models.

Classical Electron‐Ion Coulomb Bremsstrahlung in Weakly Coupled Plasmas

In weakly coupled plasmas the classical electron-ion Coulomb bremsstrahlung process are investigated. The cross section for the electron-ion Coulomb bremsstrahlung in weakly-coupled plasmas is obtained by the Debye-Huckel model with the modified hyperbolic-orbit trajectory method. The results show that the plasma-screening effects on the scaled doubly differential bremsstrahlung radiation (SDDBR) cross section increase as the radiation frequency increases. For a given projectile electron energy, the SDDBR cross section obtained by the modified hyperbolic-orbit trajectory method and by the straight-line trajectory method tend to the same value with increasing radiation photon energy. The SDDBR cross sections using the modified hyperbolic-orbit trajectory method are increased by the plasma screening in the small impact parameter domain and decreased for large and intermediate impact parameters with increasing radiation frequency.

A quasiclassical trajectory study of product state distributions from the CN+H2→HCN+H reaction

An extensive quasiclassical trajectory study of the dynamics of the CN+H2→HCN+H reaction has been undertaken on two of the potential energy surfaces reported by ter Horst, Schatz, and Harding [J. Chem. Phys. 105, 558 (1996)] with the goal of converging product state distributions. The effect of zero-point energy violations on the behavior of the reactive cross section near threshold has been examined leading to an improved estimate of the thermal rate constant on ter Horst–Schatz–Harding potential energy surface 3 (3.01±0.24×10−14 cm3/s at 300 K). The calculated HCN vibrational product state distribution is not statistical and exhibits a systematic over population in the stretching vibrations of the ground state bend manifold indicating that the –C–N does not behave like a “spectator bond” in this reaction. There is also significant population in modes with bending excitation, but these vibrations are under populated relative to prior statistical expectations. The sensitivity of the distribution on the size of the barrier and its location in the entrance channel has been undertaken by comparing results on the ter Horst–Schatz–Harding potential energy surfaces 2 and 3. Similar to the case of exoergic atom-diatom reactions, it is found that the earlier barrier on ter Horst–Schatz–Harding potential energy surface 3 gives rise to more excitation in the –C–H stretching vibration. The rotational distributions of the HCN product appear similar to the thermal distribution of CN reagents from which they are born indicating that the abstraction of the light H atom perturbs the rotational motion of the cyano radical very little. The dependence of the average HCN rotational quantum number, 〈J〉, on the bending quantum number, v2, exhibits an interesting alternation such that the points for even values of v2 are larger than those for odd. There is a corresponding alternation in the dependence of the average scattering angle, 〈θ〉, on v2 in the opposite sense. These observations suggest that for the odd bending states (which are primarily l=1) the energy diverted into exciting motion perpendicular to the reaction path at the transition state is not available to excite product rotation or to produce reactive trajectories with large impact parameters which lead to small scattering angles.

Measurement of R b with the L3 detector

We present a new measurement of R b = г( Z → bb )/г( Z → hadrons) with the L3 detector using a double tag method. The analysis uses 540 k hadronic events collected in 1994 and exploits the capabilities of the Silicon Microvertex Detector. The lifetime tagging of b-events is performed by using tracks with large impact parameter with respect to the interaction point. We obtain a value of R b = 0.2185 ± 0.0028 ± 0.0033 − 0.146( R c − 0.171) where the first error comes from statistics, the second is the total systematic uncertainty and the fourth term gives the dependence of the measurement on the fraction of c-events in the hadronic Z decays.