Collision Energy Range Research Articles

Rotational Mode-Specificity in the Cl + C2H6 → HCl + C2H5 Reaction.

We perform rotational mode-specific quasi-classical trajectory simulations using a high-quality ab initio analytical potential energy surface for the Cl(2P3/2) + C2H6 → HCl + C2H5 reaction. As ethane, being a prolate-type symmetric top, can be characterized by the J and K rotational quantum numbers, the excitation of two rotational modes, the tumbling (J, K = 0) and spinning (J, K = J) rotations of the reactant is carried out with J = 10, 20, 30, and 40 at a wide range of collision energies. The impacts of rotational excitation on the reactivity, the mechanism, and the post-reaction distribution of energy are investigated: (1) exciting both rotational modes enhances the reactivity with the spinning rotation being more effective due to its coupling to the C–H stretching vibrational normal modes (C–H bond elongating effect) and larger rotational energies, (2) rotational excitation increases the dominance of direct rebound over the stripping mechanism, while collision energy favors the latter, (3) investing energy in tumbling rotation excites the translational motion of the products, while the excess spinning rotational energy readily flows into the internal degrees of freedom of the ethyl radical or, less significantly, into the HCl vibration, probably due to the pronounced rovibrational coupling in this case. We also study the relative efficiency of vibrational and rotational excitation on the reactivity of the barrierless and thus translationally hindered title reaction.

HeH+ Collisions with H2: Rotationally Inelastic Cross Sections and Rate Coefficients from Quantum Dynamics at Interstellar Temperatures.

We report for the first time an accurate ab initio potential energy surface for the HeH+–H2 system in four dimensions (4D) treating both diatomic species as rigid rotors. The computed ab initio potential energy point values are fitted using an artificial neural network method and used in quantum close coupling calculations for different initial states of both rotors, in their ground electronic states, over a range of collision energies. The state-to-state cross section results are used to compute the rate coefficients over a range of temperatures relevant to interstellar conditions. By comparing the four dimensional quantum results with those obtained by a reduced-dimensions approach that treats the H2 molecule as an averaged, nonrotating target, it is shown that the reduced dimensionality results are in good accord with the four dimensional results as long as the HeH+ molecule is not initially rotationally excited. By further comparing the present rate coefficients with those for HeH+–H and for HeH+–He, we demonstrate that H2 molecules are the most effective collision partners in inducing rotational excitation in HeH+ cation at interstellar temperatures. The rotationally inelastic rates involving o-H2 and p-H2 excitations are also obtained and they turn out to be, as in previous systems, orders of magnitude smaller than those involving the cation. The results for the H2 molecular partner clearly indicate its large energy-transfer efficiency to the HeH+ system, thereby confirming its expected importance within the kinetics networks involving HeH+ in interstellar environments.

Integral cross sections for electron impact excitations of argon and carbon dioxide

Electron-impact excitation integral cross sections play an important role in understanding the energy transfer processes in many applied physics. Practical applications require integral cross sections in a wide collision energy range from the excitation threshold to several keV. The recently developed BE-scaling method is able to meet the demands of integral cross sections for dipole-allowed transitions while the prerequisite relies on the accurate generalized oscillator strengths. Fast electron and x-ray scatterings are the conventional experimental techniques to approach the generalized oscillator strengths, and the joint study by both methods can provide credible cross-checks. The validated generalized oscillator strengths can then be used to extrapolate optical oscillator strengths by fitting the data with the Lassettre formula. The fitted curve also enables the integration of generalized oscillator strengths over the whole momentum transfer region to obtain the BE-scaled integral excitation cross sections. Here, experimental measurements by both fast electron and x-ray scattering of argon and carbon dioxide are reviewed. The integral cross sections for some low-lying states are derived from the cross-checked generalized oscillator strengths for the first time. The integral cross sections presented in this paper are openly available at https://doi.org/10.11922/sciencedb.01466.

(3+1) -D viscous hydrodynamics at finite net baryon density: Identified particle spectra, anisotropic flows, and flow fluctuations across energies relevant to the beam-energy scan at RHIC

To study the bulk properties of the quark-gluon-plasma (QGP) produced at the beam energy scan (BES) energies at the Relativistic Heavy Ion Collider (RHIC), we extend the (3+1)-dimensional viscous hydrodynamics CLVisc to include net baryon number conservation and Israel-Stewart-like equation for baryon diffusion with the NEOS-BQS equation of state, fluctuating initial conditions from Monte-Carlo Glauber model, and the afterburner SMASH. This integrated framework is shown to provide a good description of identified particle spectra, mean transverse momenta and anisotropic flows for different centralities and over a wide range of collision energies (7.7-62.4 GeV). It is found that the mean momenta of identified particles and anisotropic flows increases mildly with the collision energy due to larger radial flow. We further compute the multiple-particle cumulant ratio $v_2\{4\}/v_2\{2\}$ of elliptic flow across BES energies, and find that the relative fluctuations of elliptic flow are insensitive to the collision energy, consistent with the preliminary STAR data. Our model provides a benchmark for understanding the RHIC-BES data and studying the critical properties and phase structure of hot and dense QCD matter.

Experimental dielectronic recombination rate coefficients for lithium-like 40Ca17+

The rate coefficients for dielectronic recombination (DR) of lithium-like 40Ca17+ ions with ∆n = 0 core excitations are derived from electron–ion recombination spectra measured with merged-beams method at the heavy-ion storage ring CSRm. The experimental DR spectrum, in the electron–ion collision energy range of 0 to 42 eV in the center-of-mass frame, comprises of all DR resonance peaks belong to the 2s 2 S 1/2 → 2p 2 P 1/2, 3/2 core excitations. The resonant energies and strengths for the resolved resonances in 2pjnl series are determined by fitting of the measured DR peaks. The further interpretation of the measured DR rate coefficients has been performed by calculating the DR rate coefficients with relativistic configuration-interaction method implemented in flexible atomic code (FAC) and compared with the experimental results. The experimental results and FAC calculations are found to be in a good agreement within the experimental uncertainties. Moreover, temperature dependent plasma rate coefficients were constructed from 4 × 103 to 1 × 107 K energy region by convoluting experimental and theoretical DR rate coefficients with the Maxwellian energy distribution function and then compared with previously available data. The plasma DR rate coefficient is found to be significantly underestimated by the early theoretical data calculated by Jacobs et al, and Mazotta et al in the low temperature. In contrast, a very good agreement has been found between the theoretical DR data of Gu and Colgan et al and the presently measured results at the low temperature region. Therefore, the results in this work composed of a bench-mark data set for plasma modeling at the photoionized temperature range. We have also provided a fit to our measured and theoretical plasma rate coefficients for low temperature plasma modeling.

Inelastic Processes in Nickel–Hydrogen Collisions

Abstract The cross sections and rate coefficients for inelastic processes in low-energy collisions of nickel atoms and positive ions with hydrogen atoms and negative ions are calculated for the collisional energy range 10−4–100 eV and for the temperature range 1000–10,000 K. 74 covalent and three ionic states correlated to 11 molecular symmetries are considered. 3380 partial inelastic processes are treated in total. The study of nickel–hydrogen collisions is performed by the quantum model methods within the Born–Oppenheimer formalism. The electronic structure of the collisional quasimolecule is calculated by the semiempirical asymptotic method for each considered molecular symmetry. For nuclear dynamic calculations the simplified method in combination with the Landau–Zener model is used. Nuclear dynamics within each considered symmetry is treated separately, and the total rate coefficients for each inelastic process have been summed over all symmetries. The largest values of the rate coefficients (exceeding 10−8 cm3 s−1) correspond to the mutual neutralization processes in collisions Ni+(3d 9 2 D) + H−(1s 2 1 S) (the ground ionic state being the initial state), as well as in Ni+(3d 84s 4,2 F) + H−(1s 2 1 S) (the first excited and the second excited ionic states being the initial states) collisions. At the temperature of 6000 K, the rate coefficients with large magnitudes have the values from the ranges (1.35−5.87) × 10−8 cm3 s−1 and (1.02−6.77) × 10−8 cm3 s−1, respectively. The calculated rate coefficients with large and moderate values are important for non–local thermodynamic equilibrium stellar atmosphere modeling.

Kinetics and dynamics of the H(2S) + NO(X2Π) → N(4S) + OH(X2Π) reaction: A quasi-classical trajectory study

A quasi-classical trajectory study of the H(2S) + NO(X2Π) → N(4S) + OH(X2Π) reaction kinetics and dynamics is reported on an accurate potential energy surface. The total integral cross sections of the reaction were calculated at the collision energy ranging from 2.00 eV to 2.80 eV. It was found that the total reaction integral cross section increases monotonically with the collision energy. Specifically at the collision energy range of 2.40–2.57 eV, our calculated results are in reasonably good agreement with the experimental data. The calculated thermal rate constants are in fairly good agreement with available experimental results. Through the trajectory analysis at the collision energy of 2.57 eV, it was found that the title reaction is dominated by the indirect trajectories (1.4 times more compared to the direct trajectories), which sheds light on the reaction dynamics of the title reaction in the high collision energy range.

System size and energy dependence of proton rapidity spectra from NA61/SHINE at the CERN SPS

NA61/SHINE is an experiment at the CERN Super Proton Synchrotron. The main goals of the experiment are the search for the critical point of strongly interacting matter and the study of the properties of the onset of deconfinement. To reach these goals, the two-dimensional scan in beam momentum (13A − 150A GeV/c) and system size (p+p, Be+Be, Ar+Sc, Xe+La, Pb+Pb) was performed. In the final stage of the collision, the spectra of protons are only weakly affected by the effects of resonance decays and rescattering due to their large mass. Thus, proton rapidity distribution is particularly sensitive to the onset of deconfinement. This article presents experimental results on proton production in the collision energy range, which is most relevant to the onset of deconfinement. The procedure of measuring the proton rapidity spectra by NA61/SHINE is described, as well as Collaboration’s recent results from reactions of p+p, Be+Be and Ar+Sc. Presented experimental results are confronted with existing data and models.

Ten-dimensional quantum dynamics study of H+CH3D → H2+CH2D reaction

The mode selectivity of the H+CH3D→H2+CH2D reaction was studied using a recently developed ten-dimensional time-dependent wave packet method. The reaction dynamics are studied for the reactant CH3D initially from the ground state, the CH3 symmetry and asymmetry stretching excitation, the CD stretching excitation and the fundamental and the first overtone of the CH3 bending mode. The calculated reaction probabilities show that exciting either of the CH3 stretching modes enhances the reactivity in the collision energy range below 1.0 eV, while the CD stretching excitation does not obviously prompt the reaction. Fundamental CH3 bending excitation has nearly no effect on promoting reactivity. However, a significant enhancement is observed for the first overtone excitation of the CH3 bending mode, resulting from the Fermi resonance between the fundamental state of the CH3 symmetry stretching mode and the first overtone state of the CH3 bending mode.

Updates on Flavor Production from STAR

In these proceedings, some of the latest results from the STAR experiment on heavy, strange and light flavor hadron production and their flow are summarized. Over the years STAR has collected heavy ion collision data over a wide range of collision energies from √SNN = 200 GeV down to √SNN = 3 GeV. In high energy heavy ion collisions, heavy flavor hadrons are a valuable probe to study properties of the Quark Gluon Plasma (QGP) created in these collisions. Measurements of strange and light flavor hadrons at lower collision energies help understand the properties of the QCD matter at finite baryon densities and the phase structure of the QCD matter. Furthermore, the high statistics data collected at √SNN = 3 GeV allows measurements of hypernuclei production and flow, which can provide insights into the hyperon - nucleon interactions and hyperon contribution to the nuclear equation of state.

Time-dependent wave packet dynamics study of the resonances in the H + LiH+(v = 0, j = 0) → Li+ + H2 reaction at low collision energies.

The depletion process of LiH+ by H collision plays an important role in the evolution of the early universe and astrophysical processes, including the eventual charge-states, abundances of atomic and molecular species and ensuing astrochemistry. Here, a quantum dynamics study on the H + LiH+(v = 0, j = 0) → Li+ + H2 reaction is performed at the low collision energy range from 0.1 meV to 10 meV using the time-dependent wave packet method. A Feshbach resonance peak is observed near 0.8 meV collision energy on the total reaction probability curves. This resonance originates from the coupling with the v = 0, j = 1 energy level of the reactant LiH+, and it is dominated by the contributions of J = 0-4 partial waves. Another partial wave resonance is also found on the total integral cross section at 1.2 meV, which is closely connected to the opening of the J = 7 partial wave. The opening of the J = 7 partial wave generates a notable forward scattering peak, and the Feshbach resonance can promote both the forward and backward scatterings. Moreover, the total and product vibrational state-resolved rate coefficients for the temperature range of 1-100 K are also reported.

Dynamics of C<sup>+</sup> + H<sub>2</sub> reaction based on a new potential energy surface

The multi-reference interaction method is explicitly dependent on the electron-electron distance, and ACVQZ basis set is used in the <i>ab initio</i> calculation. The potential energy surface (PES) is fitted by using the permutation invariant polynomial neural network method based on 18222 <i>ab initio</i> points. In addition, the topographical features of the PES are compared with available theoretical and experimental data. The results indicate that the present PES is more accurate and can be applied to any type of dynamic study. In order to validate the PES, the dynamic study of the C<sup>+</sup> + H<sub>2</sub> → H + CH<sup>+</sup> reaction is carried out by using the quasi-classical trajectory method in a collision energy range of 0.4–1.0 eV. The integral cross sections and differential cross sections are calculated and compared with previous theoretical studies. For the integral cross section, the present results are, in general, in good agreement with previous theoretical studies, both of which increase with collision energy increasing. The forward and backward symmetric differential cross sections indicate that the “complex-forming” mechanism plays a dominant role in the reaction.

Anionic states of C6Cl6 probed in electron transfer experiments.

This is the first comprehensive investigation on the anionic species formed during collisions of fast neutral potassium (K) atoms with neutral hexachlorobenzene (C6Cl6) molecules in the laboratory frame range from 10 up to 100 eV. In such ion-pair formation experiments we also report a novel K+ energy loss spectrum obtained in the forward scattering giving evidence of the most accessible electronic states. The vertical electron affinity of (-3.76 ± 0.20) eV has been obtained and assigned to a purely repulsive transition from the C6Cl6 ground state to a state of the temporary negative ion yielding Cl- formation. These experimental findings are also supported by state-of-the art theoretical calculations on the electronic structure of C6Cl6 in the presence of a potassium atom and are used for analysing the lowest unoccupied molecular orbitals participating in the collision process. From the time-of-flight mass spectra recorded in the wide collision energy range, more than 80% of the total anion yield is due to the undissociated parent anion C6Cl6-, C6Cl5- and Cl- formation. Other fragment anions such as C6Cl4-, C3Cl2-, C2Cl- and Cl2- that undergo complex internal reactions with the temporary negative ion formed after electron transfer account for less than 20% of the total yield. The joint experimental and theoretical methodologies employed in these electron transfer studies provide the most comprehensive and unique assignments of the hexachlorobenzene anionic species and the role of C6Cl6 electronic states in collision induced dissociation to date.

Quantum dynamics studies of the <inline-formula><tex-math id="M2">\begin{document}$\rm D+SiD^+ \to D_2+Si^ +$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20221155_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20221155_M2.png"/></alternatives></inline-formula> reaction

The quantum dynamics calculations are carried out for the title reaction D +SiD<sup>+</sup>→D<sub>2</sub>+Si<sup>+</sup> to obtain the initial (<inline-formula><tex-math id="M8">\begin{document}$ \nu = 0{\text{ }},j = 0 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20221155_M8.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20221155_M8.png"/></alternatives></inline-formula>)reaction probability, integral cross section (ICS) and rate constant on the potential energy surface (PES) of Gao, Meng and Song. A total of 110 partial waves are calculated by using the Chebyshev wave packet method with full Coriolis coupling (CC) and centrifugal sudden (CS) approximation in a collision energy range from 1.0 × 10<sup>–3 </sup>to 1.0 eV. The calculated probability decreases with the collision energy increasing except for <i>J≤</i>40. The calculation results indicate that the CS approximation will overestimate or underestimate the reaction probability . The ICS decreases with the collision energy increasing and shows an oscillatory structure due to the<inline-formula><tex-math id="Z-20221114130407">\begin{document}$\rm{SiH_2^+} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20221155_Z-20221114130407.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20221155_Z-20221114130407.png"/></alternatives></inline-formula>well on the reaction path. The results show that the neglect of the Coriolis coupling leads to the overestimation of the cross section and the rate constant. Besides, the discrepancy between the integral cross sections from the CC and CS calculations decreases clearly with collision energy increasing. Comparison with the corresponding results of H+CH<sup>+</sup> reaction indicates that isotope substitution reaction makes the cross section and the rate constant underestimated. The resulting integral reaction cross section displays less oscillatory structure, especially in the exact quantum calculation with the full Coriolis coupling effect taken into consideration. The kinetic isotope effect <inline-formula><tex-math id="Z-20221117061024-1">\begin{document}$(\kappa_{\rm H+SiH^+}(T)/\kappa_{\rm D+SiD^+}(T))$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20221155_Z-20221117061024-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20221155_Z-20221117061024-1.png"/></alternatives></inline-formula>is found to decrease with temperature increasing. It can be seen that the reduced mass of reactant can exert a certain effect on dynamic behavior.

Global Λ -hyperon polarization in Au+Au collisions at sNN=3 GeV

Global hyperon polarization, $\overline{P}_\mathrm{H}$, in Au+Au collisions over a large range of collision energy, $\sqrt{s_\mathrm{NN}}$, was recently measured and successfully reproduced by hydrodynamic and transport models with intense fluid vorticity of the quark-gluon plasma. While na\"{i}ve extrapolation of data trends suggests a large $\overline{P}_\mathrm{H}$ as the collision energy is reduced, the behavior of $\overline{P}_\mathrm{H}$ at small $\sqrt{s_\mathrm{NN}}<7.7$ GeV is unknown. Operating the STAR experiment in fixed-target mode, we measured the polarization of $\Lambda$ hyperons along the direction of global angular momentum in Au+Au collisions at $\sqrt{s_\mathrm{NN}}=3$ GeV. The observation of substantial polarization of $4.91\pm0.81(\rm stat.)\pm0.15(\rm syst.)$% in these collisions may require a reexamination of the viscosity of any fluid created in the collision, of the thermalization timescale of rotational modes, and of hadronic mechanisms to produce global polarization.

Detection of particles of dark matter from the spectrum of secondary particles in high-energy proton-proton collisions in a thermodynamic model

In a simple hydrodynamic model, the transverse momentum distributions are found for A hyperons formed in pp collisions at ultrarelativistic energies. The calculated spectra are compared with the experimental data obtained for various colliders in a wide range of proton collision energies, including the data from experiments at the Large Hadron Collider. An interpretation of the transverse momentum spectra of soft photons in pp collisions is proposed, taking into account the X17 boson with a mass of 17 MeV - a new particle, a possible candidate for the role of dark matter particles. A tube model is proposed on the basis of combining two-dimensional quantum chromodynamics and quantum electrodynamics. An interpretation is proposed for the detection of a 38 MeV boson in the spectra of photons emitted in the reactions of protons with carbon nuclei at an incident proton momentum of 5.5 GeV/c. The X38 boson with a mass of 38 MeV has a mass close to the boson mass obtained by us, equal to 35 MeV for an electromagnetic tube. This new particle was discovered in experiments carried out recently in Dubna for the reactionp p + C→2γ + X. To interpret the obtained experimental data on the spectra of emitted photons depending on their mass, it is proposed to use the formulas obtained for massive particles, setting the mass of a boson decaying into two photons equal to 38 MeV. It was proposed to consider bosons X17 and X38 as particles of dark matter.

Binary-Encounter Model for Direct Ionization of Molecules by Positron-Impact

We introduce two models for the computation of direct ionization cross sections by positron impact over a wide range of collision energies. The models are based on the binary-encounter-Bethe model and take into account an extension of the Wannier theory. The cross sections computed with these models show good agreement with experimental data. The extensions improve the agreement between theory and experiment for collision energies between the first ionization threshold and the peak of the cross section. The models are based on a small set of parameters, which can be computed with standard quantum chemistry program packages.

Examination of the universal behavior of the η -to- π0 ratio in heavy-ion collisions

We demonstrate that the $p_T$ dependence of the $\eta/\pi^0$ ratio is universal within a few percent for high energy $p$+$p$, $p$+A and $d$+A collisions, over a broad range of collision energies. The $\eta/\pi^0$ ratio increases with $p_T$ up to 4 to 5 GeV/$c$ where it saturates at a nearly constant value of 0.487$\pm$0.024. Above $p_T = 5$ GeV/$c$ the same constant value is also observed in A+A collisions independent of collision system, energy, and centrality. At lower $p_T$, where accurate $\eta/\pi^0$ data is absent for A+A collisions, we estimate possible deviations from the universal behavior, which could arise due to the rapid radial hydrodynamic expansion of the A+A collision system. For A+A collisions at RHIC we find that possible deviations are limited to the $p_T$ range from 0.4 to 3 GeV/$c$, and remain less than 20% for the most central collisions.

Revisiting the oscillator strengths and cross sections of atomic neon by fast electron scattering

The generalized oscillator strengths of the valence-shell excitations from the ground state to the 2p5(3s, 3s′, 3p, 3p′, 4s, 4s′) states of atomic neon have been determined by means of fast electron scattering, with an incident electron energy of 1500 eV and an energy resolution of about 80 meV. The adopted relative flow technique improved accuracy of the experimental measurements prominently. The validity condition of the first Born approximation is clarified by comparing the present measured data with previous experimental and theoretical results. We found that the first Born approximation is not applicable for the monopolar transition in the whole momentum transfer region even at the incident electron energy of 1500 eV. However, the high-order Born terms are found dominating only at the minimum for the dipolar transitions. The good agreement of the present extrapolated optical oscillator strengths with previous available data confirms the reliability of the measurement. With the cross-checked generalized oscillator strengths, the state-resolved integral cross sections in a wide collision energy range have been obtained by adopting the BE-scaling method. The analysis of the feasibility of the BE-scaling method demonstrates that the scaling law applies for the typical dipole-allowed transitions while it would overestimate the cross sections for the dipolar transitions with significant dipole-forbidden components. Present data compilation constitutes a benchmark for developing and testing the theoretical methods or computational codes.

Vibrational mode-specific dynamics of the F(2P3/2) + C2H6 → HF + C2H5 reaction.

We investigate the competing effect of vibrational and translational excitation and the validity of the Polanyi rules in the early- and negative-barrier F(2P3/2) + C2H6 → HF + C2H5 reaction by performing quasi-classical dynamics simulations on a recently developed full-dimensional multi-reference analytical potential energy surface. The effect of five normal-mode excitations of ethane on the reactivity, the mechanism, and the post-reaction energy flow is followed through a wide range of collision energies. Promoting effects of vibrational excitations and interaction time, related to the slightly submerged barrier, are found to be suppressed by the early-barrier-induced translational enhancement, in contrast to the slightly late-barrier Cl + C2H6 reaction. The excess vibrational energy mostly converts into ethyl internal excitation while collision energy is transformed into product separation. The substantial reaction energy excites the HF vibration, which tends to show mode-specificity and translational energy dependence as well. With increasing collision energy, direct stripping becomes dominant over the direct rebound and indirect mechanisms, being basically independent of reactant excitation.