Small Impact Parameter Collisions Research Articles

State-to-state quantum dynamics of the H + LiF → Li + HF reaction on an accurate ab initio potential energy surface

Quantum state-to-state dynamics calculations are reported for the H + LiF(vi, ji) → HF(vf, jf) + Li reaction on a recently reported ground state X̃2A′ potential energy surface. It was found that at low collision energies the title reaction is dominated by long-lived resonances, while the lifetimes of resonances become shorter at higher energies. The existence of resonances is also supported by the differential cross section, which has mostly forward and sideways bias. The integral cross section rises with collision energy slowly, and the opacity function suggests that the reaction is dominated by small impact parameter collisions. The HF product has cold vibrational state distributions in range of the collision energy studied here, although its rotational state distributions peak near the highest accessible rotational states. This early-barrier reaction shows some enhancement by translational energy, but internal excitations of the LiF reactant have more complex behaviors.

Direct mapping of the angle-dependent barrier to reaction for Cl + CHD3 using polarized scattering data.

The transition state, which gates and modulates reactive flux, serves as the central concept in our understanding of activated reactions. The barrier height of the transition state can be estimated from the activation energy taken from thermal kinetics data or from the energetic threshold in the measured excitation function (the dependence of reaction cross-sections on initial collision energies). However, another critical and equally important property, the angle-dependent barrier to reaction, has not yet been amenable to experimental determination until now. Here, using the benchmark reaction of Cl + CHD3(v1 = 1) as an example, we show how to map this anisotropic property of the transition state as a function of collision energy from the preferred reactant bond alignment of the backward-scattered products-the imprints of small impact-parameter collisions. The deduced bend potential at the transition state agrees with ab initio calculations. We expect that the method should be applicable to many other direct reactions with a collinear barrier.

State-to-state quantum dynamics of the N(4S) + H2(X1Σ+) → NH(X3Σ−) + H(2S) reaction and its reaction mechanism analysis**Project supported by the National Natural Science Foundation of China (Grant No. 11074151) and the Natural Science Foundation of Shandong Province, China (Grant No. ZR2014AM022).

Quantum state-to-state dynamics of the N(4S) + H2 (X1Σ+) → NH(X3Σ−) + H(2S) reaction is reported in an accurate novel potential energy surface constructed by Zhai et al. (2011 J. Chem. Phys. 135 104314). The time-dependent wave packet method, which is implemented on graphics processing units, is used to calculate the differential cross sections. The influences of the collision energy on the product state-resolved integral cross sections and total differential cross sections are calculated and discussed. It is found that the products NH are predominated by the backward scattering due to the small impact parameter collisions, with only minor components being forward and sideways scattered, and have an inverted rotational distribution and no inversion in vibrational distributions; both rebound and stripping mechanisms exist in the case of high collision energies.

Nuclear Modification ofψ′,χc, andJ/ψProduction ind+AuCollisions atsNN=200 GeV

We present results for three charmonia states (psi^prime, chi_c and J/psi) in d+Au collisions at |y|<0.35 and sqrt(s_NN)=200 GeV. We find that the modification of the psi^prime yield relative to that of the J/psi scales approximately with charged-particle multiplicity at midrapidity across p+A, d+Au, and A+A results from the Super Proton Synchrotron and the Relativistic Heavy Ion Collider. In large impact-parameter collisions we observe a similar suppression for the psi^prime and J/psi, while in small impact-parameter collisions the more weakly bound psi^prime is more strongly suppressed. Owing to the short time spent traversing the Au nucleus, the larger psi^prime suppression in central events is not explained by an increase of the nuclear absorption due to meson formation time effects.

Ionization and fragmentation of polycyclic aromatic hydrocarbon clusters in collisions with keV ions

In this thesis a series of experiments on collisions between atomic projectile ions at keV energies and target vapors of either isolated molecules or van der Waals clusters is presented and analyzed. The atomic ions are produced in an Electron Cyclotron Resonance (ECR) ion source, accelerated and guided into the target volume. The charged target collision products are mass-to-charge analyzed in a time-of-flight spectrometer. The Polycyclic Aromatic Hyrdrocarbons (PAHs) Anthracene (C14H10), Coronene (C24H12), two C16H10 isomers, Pyrene and Fluoranthene, and the fullerene C60 are examined.For projectile ions in low charge states, small impact parameter collisions dominate, which leads to internal heating of the target. With isolated molecules as targets, this typically results in ionization and often also in fragmentation. For cluster targets energy and charge are rapidly distributed among the cluster building blocks. This is followed by cluster evaporation and very limited fragmentation of the individual molecules. C119+ and C118+ are observed as products. These are due to the formation of the reactive C58/59+ ions by direct knockout processes, which react with another C60 of the cluster to form dumb-bell shaped molecules.For projectile ions of high charge (Xe20+) larger impact parameters dominate, leading to little internal heating. For isolated molecule targets, intact molecular ions are the main collision products. Charged fragments stem mostly from multifragmentation following ionization to high charge states. For cluster targets, the collision products consist mainly of singly charged monomers. Fragmentation of the individual molecules is comparatively strong. This suggests a quick distribution of charges followed by a Coulomb explosion, which leads to internal heating.The results show that weakly bound clusters do not sustain the impact of keV-ions and that it is possible to form new molecular structures.

High electron stabilization rate in slow collisions of bare Ne10+ with C60

We report cross-section measurements of collisions between bare Ne10+ ions and C60 at two different collision velocities (v=0.3 and 0.64 a.u.). We focus our attention on small impact parameter collisions (b<C60 radius), in which C60 can be considered as a nanometric solid target. The final charge state distribution corresponding to these small impact parameter events is modeled using a multi-cascade charge equilibration model. We show that for bare Ne10+ ions, the initial emptiness of the n=1 shell leads to higher rates for the stabilization on the projectile of the first two electrons.

Energy loss rate for guiding-center antihydrogen atoms

Collisional drag between a bound positron and a background positron plasma is considered as a mechanism for guiding-center antihydrogen atoms to relax to deeply bound states. Contrary to previous assessment, an adiabatic cutoff to the drag is predicted at deep binding, when the bound positron’s E×B drift speed vd exceeds the plasma positron thermal speed. In this regime, small-impact parameter collisions neglected in the drag calculation become the dominant 3-body recombination mechanism. At shallow binding, when ξ=vd/v̄≪1, the atom’s energy loss rate due to drag scales like ξ3/2 log2 ξ. When ξ≫1 the adiabatic cutoff takes over and the rate scales as ξ7/6 exp(−32(2ξ)2/3). The adiabatic cutoff implies that collisional drag can only assist positron–antiproton recombination up to a finite binding energy.

Reaction of acetaldehyde cations with water: The effects of CH3CHO+ vibrational mode and impact parameter on reactivity and product branching

Scattering of mode-selectively excited acetaldehyde cations from D2O was studied in a guided ion beam instrument. The effects of reactant vibrational state and collision energy on reactivity, product branching, and product ion recoil velocity distributions were measured. Ab initio calculations were performed to help understand the reaction coordinate. The dominant reaction is H/D exchange, which occurs in about 40% of low energy collisions, dropping to just a few percent at high energies. H/D exchange is also inhibited by CH3CHO+ vibration, but with a smaller effect than the equivalent amount of collision energy. H/D exchange is mediated by a long-lived complex, and several candidates are identified. The other low energy channel corresponds to methyl elimination from the collision complex. This channel is the most energetically favorable, but is only a few percent efficient, even at low energies, and is negligible at high energies. Methyl elimination is strongly suppressed by both collision energy and vibration, and the vibrational effects are nonmode specific. The most interesting channel is proton transfer (PT), which occurs by a direct mechanism at all collision energies. At low energies, PT occurs only in small impact parameter collisions, while at high energies, PT occurs primarily for large impact parameters, and is suppressed for small impact parameters. PT also shows strongly mode-specific dependence on CH3CHO+ vibrational state.

Ion sputtering as a probe of liquid surfaces

We have measured the energy and angular distribution of the ions emitted from a perfluoropolyether liquid polymer surface due to an incident 320 eV Ar+ beam. The anion distributions are comprised almost exclusively of F?. Substantial amounts of high-energy F? recoils are produced, which can be attributed to multiple-collision processes. The production of these high-energy recoils indicates a very open surface structure, which allows hard (small impact parameter) collisions between Ar+ and individual functional groups of the polymer. Scattered Ar+ is only observed for small total scattering angles.

Single and double electron removal from helium by protons

The independent event model (IEV) is used to calculate single and double electron removal cross sections for collisions. The single electron removal cross sections are in good agreement with experiment. In contrast to the model of Janev and co-workers, it is found that the single electron removal cross section for is much larger than the double electron removal cross section for , since in small impact parameter collisions of protons with He the probability of removing the first electron is less than unity. Double electron removal cross sections for calculated in the IEV are found to be in poor agreement with experiment, in contrast to where good agreement was obtained. For , the IEV double ionization result is improved somewhat if approximate allowance is made for the screening of the projectile by the electron if it is captured in the first event. The difference in the agreement of the IEV double ionization results with experiment may also indicate a qualitative difference in the role played by electron correlation in these two collision systems.

Impact-parameter dependence of inelastic energy losses in slow atom-atom collisions

A quasiclassical expression has been derived for the inelastic energy losses in atom-atom collisions below the Bohr velocity as a function of the impact parameter. A collision of slow atoms is treated on the basis of the Thomas-Fermi statistical model. The gas of atomic electrons is characterized by the space dependent value of the Fermi energy. The inelastic energy losses are believed to be due to the electron momentum transfer between colliding partners. The excitation of the Fermi gas at each point of the phase space is assumed to be small compared with the Fermi energy. The projectile trajectory shape is accounted for classically by means of the Thomas-Fermi-Firsov potential. It has been found that the Coulomb repulsion between the nuclei of the projectile and the target atom decreases greatly the contribution of small impact-parameter collisions in the limiting case of low reduced energies.

Role of Intersystem Crossing in the Dynamics of the O(3P) + C2H5I Reaction

Reactive scattering of O(3P) atoms with C2H5I molecules has been studied at initial translational energies E ∼ 51 and 16 kJ mol-1 using a supersonic beam of O atoms seeded in He and Ne buffer gas generated from a microwave discharge source. A mildly forward- and backward-peaked angular distribution of IO product observed at lower initial translational energy becomes backward-scattered at higher initial translational energy. An isotropic angular distribution is observed for HOI product at lower initial translational energy which becomes forward- and backward-peaked, slightly favoring the backward direction at higher initial translational energy. Extended phase space calculations indicate that reaction leading to IO product proceeds via intersystem crossing from the lowest triplet 3A‘‘ potential energy surface to a bound OIC2H5 intermediate on the singlet 1A‘ potential energy surface in small impact parameter collisions at low initial translational energy. However, the emergence of backward-scattered IO product indicates the onset of direct reaction over the triplet 3A‘‘ potential energy surface at higher initial translational energy. The formation of HOI reaction product arises from the singlet OIC2H5 complex via a five-membered ring transition state where the H atom is abstracted from the terminal CH3 group at the top of a barrier leading to vibrational excitation of HOI and repulsion between the reaction products in this strongly exoergic exit valley of the potential energy surface.

Autocorrelations and intermediate-mass-fragment multiplicities in central heavy-ion collisions.

The average multiplicities of intermediate mass fragments (IMFs) for central heavy-ion collisions in the (nearly) symmetric entrance channels $^{20}\mathrm{Ne}$${+}^{27}$Al, $^{40}\mathrm{Ar}$${+}^{45}$Sc, $^{84}\mathrm{Kr}$${+}^{93}$Nb, and $^{129}\mathrm{Xe}$${+}^{139}$La, are systematically studied over a wide range of intermediate beam energies. Cuts on experimental variables commonly assumed to be correlated with the impact parameter are used to select the most central collisions. The results for six different centrality variables are compared, and the extent to which measurements of the multiplicities of IMFs in small impact parameter collisions are affected by the variable used to select the central events is discussed. General methods for locating such ``autocorrelations'' are described. The two centrality observables that are the least autocorrelated with the number of intermediate mass fragments are identified, and these variables are used to select the most central collisions. The entrance channel mass and beam energy dependence of the experimental IMF multiplicities are presented and compared to a variety of model predictions. The models picturing the disassembly as a sequential binary process always underpredict the experimental IMF multiplicities. A generally more accurate reproduction of these multiplicities is provided by several similar chemical equilibrium models commonly assumed to be the theoretical description of multifragmentation.

A beam scattering study of non-dissociative charge transfer between Kr + and CH 4 at collision energies below 1 eV

The charge-transfer process Kr + (CH 4, Kr)CH + 4 was investigated at collision energies of 0.45–1.18 eV in crossed beam scattering experiments. The energy transferred was found generally to be almost (within 0.05 eV) equal to the recombination energy of Kr + (resonant energy transfer). However, in addition a number of internal states of CH + 4 is populated in inelastic and superelastic collisions. The width of this energy enveloped depends on the scattering angle and on the collision energy, and extends to about ±0.5 eV on both sides of the peak value. The inelasticity of small angle scattering, which increases from 1.18 to 0.45 eV, suggests an increasing fraction of “sticky” small impact parameter collisions in formation of an intermediate, Kr·CH + 4.

Dependence on scattering angle of the internal energy distribution of products of charge‐changing collisions

AbstractThe internal energy distributions arising from charge‐changing collisions were measured as a function of scattering angle, θ, for the ‘thermometer’ molecule W(CO)6. The experiments were performed by modifying a reverse‐geometry mass‐analyzed ion kinetic energy (MIKE) spectrometer by adding angle‐resolving slits which allow measurement of the scattering angle in the non‐focusing plane of the instrument. Charge exchange of W(CO) with benzene to give W(CO), and charge stripping of W(CO) on collision with O2 to give W(CO), were studied at 6 keV with the product ions being collected over laboratory scattering angles selected in the range 0–0.60°. The results show that charge‐changing collisions accompanied by scattering have the potential for depositing extremely large internal energies. The observation of the W(CO)2+ ion formed in dissociative charge stripping of W(CO) shows that it is possible to deposit at least 27 eV into the colliding W(CO) ion; of this energy, 15 eV is used for the charge‐stripping process, leaving 12 eV of internal energy in the nascent W(CO). Even greater internal energies (more than 15 eV) can be deposited into scattered W(CO) produced by charge exchange of the doubly charged ion. The availability of such high internal energies has potential use in causing dissociation of refractory ions such as those of biomolecules. The average internal energy, εAVE, deposited increases with the scattering angle at a rate of 10 eV degree−1 for charge exchange, and at approximately 5 eV degree−1 for charge stripping and for simple collision‐induced dissociation (CID). This observation suggests that non‐zero angle charge stripping and CID may occur via similar mechanisms in which direct vibrational activation occurs in small impact parameter collisions which also lead to angular scattering. The higher internal energies and larger εAVE vs. θ dependence observed for charge exchange are consistent with the formation of the products upon a highly repulsive surface associated with coulombic repulsion between the separating products.

Transition-state dynamics of F + Cl2 reactive scattering

Reactive scattering of F atoms with Cl2 molecules has been studied at an initial translational energy E≈ 16 kJ mol–1 using a supersonic beam of F atoms seeded in Ne buffer gas generated by a high-pressure microwave discharge source. The centre-of-mass angular distribution peaks sharply in the forward direction with a minor peak of relative height ca. 0.2 in the backward direction. A major fraction f′pk≈ 0.6 of the total available energy is disposed into product translational energy for the forward and backward scattering but only a minor fraction f′pk≈ 0.16 for the sideways scattering. The total flux scattered in the sideways direction has a very low intensity ca. 0.06 relative to the forward scattering. The observed reactive scattering is related to the dynamical motion of a slightly bent FCICI reactant transition state formed at the top of a low barrier in the entrance valley of the potential-energy surface and its dissociation over a weakly attractive surface to form reaction products. Transition states formed in large impact parameter collisions with the Cl2 molecule lying initially in the broadside orientation precess into the forward hemisphere and tend to dissociate adiabatically to form CIF products in the ground and first excited vibrational states. Transition states formed in small impact parameter collisions where the F atom interacts with the Cl atom of the Cl2 molecule lying in the backward hemisphere undergo dissociation from more contracted configurations and may form CIF reaction products in the second excited vibrational state.

Impact-parameter filters for 36Ar+ 197Au collisions at [formula omitted

Collisions between 36Ar projectiles and 197Au target nuclei at E A = 50, 80 and 110 MeV have been studied with the MSU Miniball, a 4 π phoswich array with a low detection threshold. Various impact-parameter filters, based upon total charged-particle multiplicity, transverse energy, midrapidity charge and multiplicity of emitted hydrogen nuclei, are compared and cross-correlated. The relative selectivity of each prescription for small impact-parameter collisions is evaluated by assessing the suppression of fast-particle emission at forward angles.

Collision-induced dissociation of the propane molecular ion

A crossed-beam tandem hybrid mass spectrometer equipped with a supersonic neutral beam source was used to investigate collision-induced dissociation (CID) of the propane molecular ion at a series of relative collision energies from 1.7–449 eV. The competing processes of elimination of methane and loss of methyl radical were studied using beams of helium and argon neutrals. The dynamics of both dissociation processes change relatively little with increasing collision energy, demonstrating that the CID mechanism does not change with collision energy over a very broad range. The most probable energy transfer into the ion increased only slightly with collision energy. The dynamics are consistent with previous conclusions that internal energy is randomized amongst internl degrees of freedom before dissociation. The intensity maxima at all energies occurred at non-zero scattering angles in the center-of-mass reference frame; scattering angles decrease with increasing collision energy but never decreased to zero. This implies that the CID mechanism for propane ion is dominated by small impact parameter collisions with large angular momentum exchange at all energies.

Recoil ions from near-zero-impact-parameter H+-Xe collisions in the range 20-70 keV.

Recoil ions from very small impact-parameter collisions of 20--70-keV protons with xenon atoms were selected by viewing only those ejected at 50\ifmmode^\circ\else\textdegree\fi{} and 70\ifmmode^\circ\else\textdegree\fi{} from the beam direction. These ions were charge-state analyzed and the cross sections determined for the production of charge states up to 6+. In such collisions, multiple ionization accounts for 60--80 % of the ionizing collisions and 80--90 % of the total ionization.

State-selected ion–molecule reactions: N+2(v)+H2→N2+H+2 and Ar+(2P J) +H2→Ar+H+2

Charge transfer cross sections for state-selected N+2 (X,A;v) and Ar+(2PJ) ions colliding with H2 have been measured at laboratory energies of 10.5 and 14.5 eV by means of the TPEPICO (threshold-photoelectron/photoion coincidence) method using synchrotron radiation. The cross sections are put on an absolute basis by normalizing to earlier measurements for N+2+H2. For the purpose of comparison total cross sections have also been measured for state-selected H+2(v)+N2 at 10 eV lab. A simple model, which assumes that small impact parameter collisions lead to chemical reaction, gives a qualitative explanation of the N+2+H2 and Ar++H2 data. The work sheds light on previous problems with the application of the principle of microscopic reversibility to these systems.