Collision Energy Range Research Articles

Ion Momentum Imaging Study of the Ion-Molecule Charge Exchange Reaction of Ar+ + CO2.

Three-dimensional ion momentum imaging is developed in a combination of ion velocity map imaging technique and delay-line anode ion detection, and it is applied for the ion-molecule charge exchange reaction between Ar+ and CO2. In a center-of-mass collision energy range of 7.23-15.96 eV, CO2+ products are primarily populated at the ground state X2Πg and the single-electron excited states A2Πu, B2Σu+, and C2Σg+; the multielectron excited states of CO2+ are also found at the higher collision energies. The production efficiency profiles of CO2+ are distinctly different from the photoionization electron spectrum of CO2, implying that the charge transfer from Ar+ would be not fast as expected. The strong electron correlations in the short-lived intermediate (Ar-CO2)+ should be responsible for the CO2+ yields at the multielectron excited states.

Self-Similarity of Negative Particle Production in Au + Au Collisions at STAR

Results of a new analysis of negative particle yields in Au + Au collisions obtained by the STAR Collaboration in the first phase of the RHIC Beam Energy Scan (BES) program in the framework of $$z$$ -scaling approach are presented. The spectra were measured over a wide range of collision energy $$\sqrt {{{s}_{{NN}}}} = 7.7 - 200$$ GeV and transverse momentum of produced particles for different centralities at $$\left| \eta \right| < 0.5.$$ The concept of the $$z$$ -scaling based on self-similarity, locality, and fractality of hadron interactions at a constituent level is verified. The constituent energy loss as a function of energy and centrality of collision and transverse momentum of inclusive particle was estimated.

The Intricate Dynamics of the Si(3P) + OH(X2Π) Reaction.

The dynamics of the Si(3P) + OH(X2Π) → SiO(X1Σ+,v',j') + H(2S) reaction is investigated by means of the quasi-classical trajectory method on the electronic ground state X2A' potential energy surface in the 10-2-1 eV collision energy range. Although the reaction involves the formation of a long-lived intermediate complex, a high probability for back-dissociation to the reactants is found because of inefficient intravibrational redistribution of energy among the complex modes. At low collision energies, the reactive events are governed by a dynamics with mixed direct/indirect features. As the collision energy increases, the intermediate complex lifetime increases and final state distributions are found to be in reasonable agreement with statistical predictions obtained using the mean potential phase space theory, thus highlighting the indirect character of the process. These rich and puzzling dynamical features are in line with what has been previously observed for the S(3P) + OH(X2Π) reaction.

Quantum wave-packet studies on ion–molecule reaction with and without Coriolis-coupling effect

The time-dependent quantum scattering calculation with Chebyshev wave packet propagation scheme has been carried out based on an accurate electronic potential energy surface of H2O+(X4A″). Due to the influence of the deep potential well, the reaction probability of [Formula: see text] shows resonance structures regardless of the Coriolis-coupling (CC) effect or centrifugal sudden (CS) approximation. In the range of collision energy 0.0–1.0 eV, the integral cross section obtained by the CS approximation calculation is smaller than that by the CC calculation, which indicates that the CC effect plays a significant role in the title reaction.

The effect of collision energy on the stereodynamics of the reaction triatomic NH2 system

The stereodynamics of the reaction H(2S) + NH (v = 0, 1, 2, 3; j = 0) → N(4S) + H2 are studied using the quasi-classical trajectory method on a double many-body expansion potential energy surface to understand the alignment and orientation of the product molecules in the collision energy range of 2–20 kcal·mol−1. The vibrational–rotational quantum number of the NH molecules is specifically investigated for v = 0, 1, 2, and 3 and j = 0. The differential cross section [DCS; ], and average rotational alignment factor are calculated. The stereodynamics results indicate that the reagent vibrational quantum number and initial collision energy significantly affect the distributions of the k–j′, k–k′–j′ and k–k′ vector correlations along with In addition, while DCS is extremely sensitive to the collision energy, it is not significantly affected by the vibrational excitation of the reagents.

Quasi-classical trajectory study of H+LiH (v = 0, 1, 2, j = 0) Li+H2 reaction on a new global potential energy surface

Quasi-classical trajectory (QCT) calculations are reported for the H+LiH (v = 0–2, j = 0) Li+H2 reaction on a new ground electronic state global potential energy surface (PES) of the LiH2 system. Reaction probability and integral cross sections (ICSs) are calculated for collision energies in the range of 0 eV–0.5 eV. Reasonable agreement is found in the comparison between present results and previous available theoretical results. We carried out statistical analyses with all the trajectories and found two main distinct reaction mechanisms in the collision process, in which the stripping mechanism (i.e., without roaming process) is dominated over the collision energy range. The polarization dependent differential cross sections (PDDCSs) indicate that forward scattering dominates the reaction due to the dominated mechanism. Furthermore, the reactant vibration leads to a reduction of the reactivity because of the barrierless and attractive features of PES and mass combination of the system.

Inelastic processes in oxygen–hydrogen collisions

ABSTRACT New accurate theoretical rate coefficients for (de)-excitation and charge transfer in low-energy O + H, O+ + H− and O− + H+ collisions are reported. The calculations of cross-sections and rate coefficients are performed by means of the quantum probability current method, using full configuration interaction ab initio electronic structure calculations that provide a global description of all 43 lowest molecular states from short to asymptotic internuclear distances. Thus, both long- and short-range non-adiabatic regions are taken into account for the first time. All the doublet, quartet and sextet OH molecular states, with excitation energy asymptotes up to 12.07 eV, as well as the two lowest ionic states with the asymptotes O−H+ and O+H− are treated. Calculations are performed for the collision energy range 0.01–100eV and the temperature range 1 000–10 000 K. The mechanisms underlying the processes are analysed: it is shown that the largest rate coefficients, with values exceeding 10−8 cm3 s−1, are due to ionic–covalent interactions present at large internuclear distances, while short-range interactions play an important role for rates with moderate values involved in (de)-excitation processes. As a consequence, a comparison of the present data with previously published results shows that differences of up to several orders of magnitude exist for rate coefficients with moderate values. It is worth pointing out the relatively large rate coefficients for triplet–quintuplet oxygen transitions, as well as for transitions between the O$(\rm 2p^{3}3s\, ^{5}$So) and O$(\rm 2p^{3}3p\, ^{5}$P) levels of the oxygen triplet and H(n = 2) levels. The calculated data are important for modelling stellar spectra, leading to accurate oxygen abundances.

Relative interfacial cleavage energetics of protein complexes revealed by surface collisions

To fulfill their biological functions, proteins must interact with their specific binding partners and often function as large assemblies composed of multiple proteins or proteins plus other biomolecules. Structural characterization of these complexes, including identification of all binding partners, their relative binding affinities, and complex topology, is integral for understanding function. Understanding how proteins assemble and how subunits in a complex interact is a cornerstone of structural biology. Here we report a native mass spectrometry (MS)-based method to characterize subunit interactions in globular protein complexes. We demonstrate that dissociation of protein complexes by surface collisions, at the lower end of the typical surface-induced dissociation (SID) collision energy range, consistently cleaves the weakest protein:protein interfaces, producing products that are reflective of the known structure. We present here combined results for multiple complexes as a training set, two validation cases, and four computational models. We show that SID appearance energies can be predicted from structures via a computationally derived expression containing three terms (number of residues in a given interface, unsatisfied hydrogen bonds, and a rigidity factor).

A Mixed Quantum/Classical Approach to Associative Detachment in Low-Energy H- + H Collisions.

A theoretical approach, in which the rotational degrees of freedom are treated quantum mechanically while translational degrees of freedom classically or semiclassically, is applied to calculate the total and final-state specific cross sections for the reaction H- + H → H2( v, j) + e- in the range of low collision energies ( E = 1-754 meV). The calculated cross sections agree well with the existing experimental and theoretical results. Results are also presented for the inverse dissociative attachment process: H2( v, j) + e- → H- + H.

Cold temperatures invert product ratios in Penning ionisation reactions with argon

In many chemical reactions with more than one possible outcome, the branching ratio, which is the ratio between the different reaction paths, is nearly constant over a wide range of collision energies. In barrierless systems governed by long-range interactions, however, the branching ratio is more sensitive to collision energy, and its dependence on it can be useful for better understanding the dynamics and reconstructing interaction potentials. Here we present the reaction rates of Penning and associative ionisation of metastable neon and helium with argon atoms. We obtain reaction rates in merge beam experiments, over a wide range of collision energies corresponding to that of room temperature, all the way down to a few millikelvins. We observe a change of two orders of magnitude in the branching ratio in the measured collision energy range and explain these changes using theoretical calculations.

Collision Energy Dependence of the Competing Mechanisms of Reaction of Chlorine Atoms with Propene.

Quasi-classical trajectory simulations examine the reaction of Cl with propene across a range of collision energies, from 7 to 28 kJ mol-1. The majority (70% at 7 kJ mol-1, 86% at 14 kJ mol-1, and 93% at 28 kJ mol-1) of reactive trajectories produce HCl by direct abstraction of a hydrogen atom from the methyl group of propene, but the remainder involve a variety of delayed mechanisms. Among these longer-lived trajectories, transient formation of an energized 1-chloropropyl radical intermediate is predominant, with only a minor contribution from the 2-chloropropyl radical and roaming pathways. The branching ratios between these intermediate states are largely invariant to collision energy, although the overall proportion of indirect trajectories increases at lower collision energies. The greater role for longer-lived trajectories is reflected in the computed product scattering angle distributions, which become more isotropic at lower energies. However, the distributions of population over vibrational and rotational states of the product HCl do not change with collision energy because they are controlled by the dynamics late along the reaction path.

Energy Dependence of the Ratio of the Elastic-to-Total Cross Sections for Proton—Proton and Antiproton—Proton Collisions

The paper presents the phenomenological analysis of the energy dependence for the ratio of elastic to total cross section in proton-proton and antiproton-proton scattering. The analytic functions based on the study of low- and high-energy experimental data for various scattering parameters provide the quantitative description of energy dependence of the ratio with statistically acceptable qualities in wide range of collision energy $\sqrt{s} \geq 3$ GeV in the case of the separate datasets for $pp$, $\bar{p}p$ collisions and at $\sqrt{s} \geq 5$ GeV for the joined experimental data ensemble. Based on the fit results the estimations are derived for the ratio of elastic to total cross section in $pp$ scattering at various $\sqrt{s}$ up to energy frontier $\sqrt{s}=10$ PeV which can be useful for present and future hadron colliders as well as for cosmic ray measurements at ultra-high energies. The indication is observed for onset of the asymptotic region at $\sqrt{s} \gtrsim 5-10$ PeV for the ratio of cross sections under consideration.

State-to-state quasi-classical trajectory study of the D + H2 collision for high temperature astrophysical applications.

We report state-to-state quasi-classical trajectory rate constants for the D + H2 reactive collision, using the accurate H3 global potential energy surface of Mielke et al. [J. Chem. Phys. 116, 4142 (2002)]. High relative collision energies (up to ≈56 000 K) and high rovibrational levels of H2 (up to ≈50 000 K), relevant to various astrophysical media, are considered. The HD product molecule is formed in highly excited rovibrational states, over a wide collision energy range. The collision-induced dissociation channel (often overlooked in fully quantum reaction dynamics calculations) is found to be significantly populated, even at collision energies as low as 1500 K.

The electron couplings in the transition states: The stereodynamics of state to state autoionization processes.

Measurements of the kinetic energy distribution of electrons, emitted in collision between Ne*(3P2,0) and Kr(1S0) and Xe(1S0), have been performed in a crossed molecular beam apparatus which employs a mass spectrometer and a hemispherical electron analyzer as detectors. The analysis of the obtained experimental results provides new insights on electronic rearrangements and electronic angular momentum coupling effects that determine relevant properties of the transition state of autoionization processes, and that we have found useful to classify as adiabatic and non-adiabatic effects. In particular, while the adiabatic effects control sequence, energy, and symmetry of quantum states accessible to both reagents and products in the probed collision energy range, the non-adiabatic ones trigger the passage from entrance to exit channels. The obtained results are important not only to compact previous theoretical schemes of autoionization reactions in a unified representation but also to cast light on the role of electronic rearrangements within the transition state of many other types of chemical processes that are more difficult to characterize.

Dilepton radiation in heavy-ion collisions at small transverse momentum

We study the invariant-mass distributions of dileptons produced in ultrarelativistic heavy-ion collisions at very low pair transverse momenta, PT≤0.15GeV. Specifically, we investigate the interplay of thermal radiation with initial photon annihilation processes, γγ→l+l−, triggered by the coherent electromagnetic fields of the incoming nuclei. For the thermal radiation, we employ the emission from the QGP and hadronic phases with in-medium vector spectral functions which describes the inclusive excess radiation observed over a wide range of collision energies. For the coherent photon fusion processes, whose spectrum is much softer than for thermal radiation, we employ initial fluxes from the Fourier transform of charge distributions of the colliding nuclei in the equivalent-photon approximation. We first verify that the combination of photon fusion, thermal radiation and final-state hadron decays gives a fair description of the low-PT invariant-mass as well as PT spectra as recently measured by the STAR collaboration in sNN=200GeV Au+Au collisions for different centrality classes, including experimental acceptance cuts. The coherent contribution dominates in peripheral collisions, while thermal radiation shows a markedly stronger increase with centrality. We extend the calculations to lower collision energies (sNN=17.3GeV) and compare to the acceptance-corrected dimuon excess spectra measured by the NA60 experiment at the CERN SPS; the contribution from photoproduction turns out to be subleading. We also provide predictions for the ALICE experiment at the LHC. The resulting excitation function from SPS to LHC energies reveals a nontrivial interplay of photoproduction and thermal radiation.

Quantum state-to-state study for (H-(D-),HD) collisions on two potential energy surfaces.

Quantum time-dependent wave-packet calculations have been carried out to explore the state-to-state dynamics of the ion-molecule (H-(D-),HD) collisions on two accurate ab initio potential energy surfaces in the collision energy range 0.2-1.2 eV. Total and final state-resolved integral and differential cross sections are elaborated in detail. The differential cross sections vary substantially with the collision energy, turning from predominantly backward-scattering at low collision energies to forward and sideways scattering bias at relatively high collision energies. The rebound, stripping and time-delayed mechanisms are found to be possible in (H-(D-),HD) collisions. A set of quasi-classical trajectory calculations were performed, and the results indicate that the backward-scattering peak is caused by the low impact parameter trajectories, while the trajectories of high impact parameter are responsible for the forward scattering. A set of representative state-to-state differential cross sections at collision energies 0.6 and 1.2 eV are also presented. Different reaction mechanisms are dominant in (H-(D-),HD) collisions at different collision energies, resulting in different product rovibrational state distributions. The differences between the dynamics results based on the two potential energy surfaces are also discussed.

Dynamics studies of O&lt;sup&gt;+&lt;/sup&gt; + H&lt;sub&gt;2&lt;/sub&gt;→ OH&lt;sup&gt;+&lt;/sup&gt; + H reaction

In the present work, the long-range interaction potential part of potential energy surface (PES) of OH&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt; system is revised and the new resulting PES apparently is more reasonable than the old one in the long-range part. Based on the new PES, the dynamics calculations of O&lt;sup&gt;+&lt;/sup&gt; +H&lt;sub&gt;2&lt;/sub&gt;→ OH&lt;sup&gt;+&lt;/sup&gt; + H reaction are carried out at a state-to-state level of theory by using time-dependent quantum wave packet method with second order split operator in a collision energy range from 0.01 to 1.0 eV. The dynamic properties such as reaction probability, ro-vibrational resolved statereaction probability, integral cross section, differential cross section, and state specific rate constant are calculated and compared with available theoretical and experimental results. The results of ro-vibrational resolved state reaction probability reflect some dynamic properties such as resonances which is attributed to the deep well located on the reaction path. The vibrational resolved state reaction probability indicates that the excitation efficiency of the OH&lt;sup&gt;+&lt;/sup&gt; product is relatively low. The results of integral cross sections indicate that the present results are in better agreement with the experimental values than with previous theoretical calculations, especially in the low collision energy region. However, the state specific rate constant results underestimate the experimental values. The comparison betweenour calculations and the experimental results indicates that the contribution of the rotational excitation of H&lt;sub&gt;2&lt;/sub&gt; molecule should be included in the calculations. However, only the initial state &lt;i&gt;v&lt;/i&gt; = 0, &lt;i&gt;j&lt;/i&gt; = 0 is calculated in the present work. We suppose that the deviation of the present results from the experimental data is due to the fact that the rotational excitation of reactant isnot included in the present calculation. The differential cross section signals indicate that the complex-forming reaction mechanism isdominated in the case of low collision energy, but it transforms into abstract reaction mechanism as the collision energy further increases.

Fractality in hadron interactions: a conservation law and quantization of fractal dimensions

A microscopic scenario of constituent interactions in high energy collisions of hadrons and nuclei is studied. The concept is based on the z-scaling of inclusive spectra observed in a wide range of collision energies, multiplicity densities, momenta, and angles of detected particles. We bring arguments that, due to the fractality and self-similarity of the hadron interactions assumed in the z-scaling scheme, there exists a conservation law of a new quantity named “fractal cumulativity.” The conserved quantity is proportional to the corresponding fractal dimensions and is a simple function of the respective momentum fractions. Based on statistical ideas and entropy considerations we demonstrate that the fractal dimensions possess quantum character.

Spectra of negative particles in Au+Au collisions from BES-I at RHIC and z-scaling

Negative particle yields in Au + Au collisions obtained by the STAR Collaboration from BES-I at RHIC are analyzed in the framework of the z-scaling approach. The spectra were measured over a wide range of collision energy $\sqrt {{s_{NN}}} = 7.7 - 200\,{\rm{GeV}}$ and transverse momentum of produced particles for different centralities at $\left| \eta \right| \lt 0.5$. The scaling behavior of the spectra as a function of the collision energy and centrality in z-presentation is verified. The constituent energy loss in dependence on the energy and centrality of collision and transverse momentum of inclusive particle is estimated. New indication on self-similarity of fractal structure of nuclei and fragmentation process is found. A discontinuity of the model parameters - fractal and fragmentation dimensions and specific heat - is considered from the point of view of a phase transition in nuclear matter.

Possible signals of two QCD phase transitions at NICA-FAIR energies

The chemical freeze-out irregularities found with the most advanced hadron resonance gas model and possible signals of two QCD phase transitions are discussed. We have found that the center-of-mass collision energy range of tricritical endpoint of QCD phase diagram is [9; 9.2] GeV which is consistent both with the QCD inspired exactly solvable model and experimental findings.