Collision Energy Range Research Articles

Announcement of the winner of the Longuet-Higgins Early Career Researcher Prize 2021

ABSTRACT The reactions between ground state and forming and were investigated in the range of collision energies between and 10 K using a merged-beam approach. The reaction rates measured experimentally are compared to those obtained for the reaction between and forming under similar experimental conditions. Below 1 K, a clear enhancement of the reaction rate coefficient compared to the Langevin rate measured at higher collision energies was observed in both reaction systems. This enhancement is interpreted as originating from the interaction between the charge of and the quadrupole of para and ortho molecules in the J = 1 rotational level. The enhancement of the reaction with was found to be significantly less than that of the reaction with , reflecting the relative population of the J = 1 rotational level of (75%) and (33%) in natural samples at low temperatures. Simulations of the experimental results based on the theoretical predictions of the reaction cross sections by Dashevskaya et al. [J. Chem. Phys. 145, 244315 (2016)] reveal agreement within the experimental uncertainties. The branching ratio η of the reaction involving and and forming and near E coll = 0 was determined to be 0.341(15). Time-of-flight measurements of the velocity distributions of the reaction products are compatible with an isotropic product emission with an average total kinetic energy of 0.45(5) eV for both channels, representing about 30% of the total energy released by the reaction.

Cooper-Frye sampling with short-range repulsion

This work incorporates the effect of short-range repulsion between particles into the Cooper-Frye hadron sampling procedure. This is achieved by means of a rejection sampling step, which prohibits any pair of particles from overlapping in the coordinate space, effectively modeling the effect of hard-core repulsion. The new procedure---called the fist sampler---is based on the package thermal-fist. It is used here to study the effect of excluded volume on cumulants of the (net-)proton number distribution in central collisions of heavy ions in a broad collision energy range in conjunction with exact global conservation of baryon number, electric charge, and strangeness. The results are compared with earlier calculations based on analytical approximations, quantifying the accuracy of the latter at different collision energies. An additional advantage of the new method over the analytic approaches is that it offers the flexibility provided by event generators, making it straightforwardly extendable to other observables.

Decorrelation of participant and spectator angular momenta in heavy-ion collisions

High-energy heavy-ion collisions contain enormous angular momentum, $|\stackrel{P\vec}{J}|$, which is $O({10}^{3}--{10}^{6}\ensuremath{\hbar})$ in the range of collision energy, $\sqrt{{s}_{\mathrm{NN}}}$, spanned experimentally by the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). A fraction of $\stackrel{P\vec}{J}$ is transferred to the overlapping collision region, which is indispensable for measuring observables such as vorticity-driven hadron spin alignment with $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{J}$. Experiments estimate the orientation of $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{J}$ of the participant nucleons within the collision overlap region, ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{J}}_{\mathrm{part}}$, by using that of the forward- and backward-going spectating nucleons ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{J}}_{\mathrm{spec}}$. Using two models, we study the decorrelation between ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{J}}_{\mathrm{part}}$ and ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{J}}_{\mathrm{spec}}$, driven both by angular-momentum conservation and event-by-event fluctuations, as well as by the decorrelation between the orientation of the elliptic overlap region and the ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{J}}_{\mathrm{part}}$. $\sqrt{{s}_{\mathrm{NN}}}$-dependent decorrelation is observed in both of these cases and is large enough to be an important corrective factor used when experimentally observing phenomena driven by $\stackrel{P\vec}{J}$.

Imaging the Complex-Forming Reaction Dynamics in Al + CO2 → AlO + CO.

For indirect reactions involving more than one intermediate complex from reactant valley to product valley, the reaction dynamics becomes very complicated for researchers due to competition between pathways. In order to resolve the large discrepancy between theoretical and experimental studies on the linear or bent structures of complexes involved in the title endothermic reaction, we performed a crossed-beam experiment at a large collision energy (Ec) range with mapping of the velocity distributions of Al reactants and AlO products. We found that the reaction proceeds through different complex-forming mechanisms with the increase of Ec: at low Ec near the reaction threshold, only low impact-parameter collisions contribute through a collinear Al-OCO short-lived complex, and at high Ec, the bent-structure complexes, formed by either isomerization or noncollinear collisions, come into play.

Near-Threshold and Resonance Effects in Rotationally Inelastic Scattering of D2O with Normal-H2.

We present a combined experimental and theoretical study on the rotationally inelastic scattering of heavy water, D2O, with normal-H2. Crossed-molecular beam measurements are performed in the collision energy range between 10 and 100 cm-1, corresponding to the near-threshold regime in which scattering resonances are most pronounced. State-to-state excitation cross-sections are obtained by probing three low-lying rotational levels of D2O using the REMPI technique. These measurements are complemented by quantum close-coupling scattering calculations based on a high-accuracy D2O-H2 interaction potential. The agreement between experiment and theory is within the experimental error bars at 95% confidence intervals, leading to a relative difference of less than 7%: the near-threshold rise and the overall shape of the cross-sections, including small undulations due to resonances, are nicely reproduced by the calculations. Isotopic effects (D2O versus H2O) are also discussed by comparing the shape and magnitude of the respective cross-sections.

Modeling of the ion in cold helium plasma: dynamics of collisions and cross-sections

A detailed dynamical study is presented for /He collisions running in the electronic ground state of the collision complex. Hybrid, quantum–classical dynamical calculations have been performed considering a broad range of collision energies (E coll = 0.01–100 eV) and various initial rotational–vibrational excitations of the ion. Both non-reactive and reactive ( collision-induced dissociation) cross-sections have been calculated with the momentum-transfer approximation applied to the former ones. A thorough comparison with pseudo-experimental data obtained from mobility measurements reported in the literature via an inverse-method approach has been performed and the effect of the rotational alignment of the ion on calculated cross-sections has been assessed and analyzed.

Dynamics of the proton transfer reaction O + H2 +(v = 0, j = 0) → OH+ + H on the ground 12A″ potential energy surface

The dynamics of the exothermic complex-forming reaction O + H2 +(v = 0, j = 0) → OH+ + H has been studied by the quasi-classical trajectory method on a global ab initio potential energy surface for the ground state 12A″ fitted by Miguel Paniagua et al. [Phys. Chem. Chem. Phys. 2014, 16, 23594]. The reaction probabilities and integral cross-section (ICS) were calculated and compared with available theoretical results in the collision energy range of 0–1.0 eV. Differential cross-section (DCS) exhibits forwardbackward symmetry, indicating the domination of indirect mechanism. The complex-forming reaction mechanism is investigated in detail. The results show that the contribution of indirect trajectories to ICS and the lifetime of the intermediate complex are strongly dependent on the collision energy. Another reaction mechanism, i.e. direct reaction, also exists. However, it is suppressed in the whole collision energy range; as a result, its contribution to the reaction is negligible.

Controlled Formation of Protonated and Radical Cation Precursor Ions by Atmospheric Pressure Photoionization with μLC-MS Enables Electron Ionization and MS/MS Library Search.

Atmospheric pressure photoionization (APPI) was developed as an alternative to electrospray ionization (ESI) for the generation of protonated molecules using liquid chromatography and optimized using dopants such as toluene, which predominantly forms protonated molecules, and chlorobenzene, which favors the formation of radical cations, although the latter has not been fully exploited. Based on 40 diverse low-molecular-weight compounds and micro liquid chromatography (μLC) coupled with APPI tandem mass spectrometry (APPI-MS/MS), the potential of radical cations was investigated. Chromatographic and ionization conditions were decoupled by post-column addition of methanol, allowing separate study and optimization. Due to the mass flow sensitive behavior of APPI, sensitivity is not affected by post-column dilution, and for 8 of 35 analytes, the radical cation response with μLC-APPI is better than for protonated molecules using μLC-ESI. Collision-induced fragmentation (CID) of radical cations produced within a collision energy range from 10-115 eV have, in the median, 65% of the fragments found in electron ionization (EI) spectra. This similarity allowed identification of 86% of the analytes using data-dependent acquisition (DDA) of radical cations and NIST EI library searches. We propose a workflow that uses multimodal DDA of protonated precursor molecules using ESI or APPI with toluene as a dopant, and radical cations produced by chlorobenzene-assisted μLC-APPI with post-column addition of methanol. This increases the confidence of molecular identification by allowing orthogonal library searches using MS/MS libraries for protonated precursor CID spectra and EI libraries for radical cation CID spectra.

Full-dimensional automated potential energy surface development and dynamics for the OH + C2H6 reaction.

We develop a full-dimensional analytical potential energy surface (PES) for the OH + C2H6 reaction using the Robosurfer program system, which automatically (1) selects geometries from quasi-classical trajectories, (2) performs ab initio computations using a coupled-cluster singles, doubles, and perturbative triples-F12/triple-zeta-quality composite method, (3) fits the energies utilizing the permutationally invariant monomial symmetrization approach, and (4) iteratively improves the PES via steps (1)-(3). Quasi-classical trajectory simulations on the new PES reveal that hydrogen abstraction leading to H2O + C2H5 dominates in the collision energy range of 10-50 kcal/mol. The abstraction cross sections increase and the dominant mechanism shifts from rebound (small impact parameters and backward scattering) to stripping (larger impact parameters and forward scattering) with increasing collision energy as opacity functions and scattering angle distributions indicate. The abstraction reaction clearly favors side-on OH attack over O-side and the least-preferred H-side approach, whereas C2H6 behaves like a spherical object with only slight C-C-perpendicular side-on preference. The collision energy efficiently flows into the relative translation of the products, whereas product internal energy distributions show only little collision energy dependence. H2O/C2H5 vibrational distributions slightly/significantly violate zero-point energy and are nearly independent of collision energy, whereas the rotational distributions clearly blue-shift as the collision energy increases.

Effect of the reactant vibration on quantum dynamics of the reaction H + CH(v = 0,1,2, j = 0) → H2 + C based on a new CH2(X3A″) potential energy surface

Based on the new ab initio potential energy surface of CH2 (X3A″) system, the influence of the reactant vibration on the H + CH → H2 + C reaction has been studied by using quantum time-dependent wave packet method. The results show that for the collision energy lower than 0.47 eV, the vibrational excitation of reactants inhibits the abstraction reaction H + CH → H2 + C, but promotes the exchange reaction H + CH → HC + H. In addition, the vibrational excitation of reactants has also a certain influence on the Coriolis coupling effect. Finally, because of the wider collision energy range (0.010–1.0 eV) in CC and QCT calculations in this work, the calculated reaction rate constants of H + CH(v = 0, j = 0) → H2 + C reaction in the temperature range of 1500–2000 K are much closer to the experimental center value than the existing theoretical results, which shows that the calculation results in this work are more reasonable and accurate.

Measurement of n- and l-resolved State-selective Charge Exchange in Ar8+ Collision with He

The state-resolved capture cross sections for principal n and orbital angular momentum l play an important role in modeling soft X-ray emissions induced by charge exchange for many astrophysical environments. However, the empirical and semiclassical theories used to produce these data of n- and l-resolved state-selective capture have not been well tested. Using the cold target recoil ion momentum spectroscopy apparatus at Fudan University, we perform a series of measurements of Ar8+ ion charge exchange with He in the collision energy range from 1.4 to 20 keV u−1. We find that electrons are mainly captured in the n = 4 state of Ar7+ ions. This agrees with the prediction of the scaling law for n capture. Moreover, the relative cross sections are reported for 4s-, 4p-, 4d-, and 4f-resolved state-selective capture. The often used analytical l distributions in the astrophysical literature are evaluated by comparing to the measurements.

Three-Body Recombination of Cold 3He–3He–T− System

The atom-atom-anion three-body recombination (TBR) and collision induced dissociation (CID) processes of the 3He–3He–T− system at ultracold temperatures are investigated by solving the Schrödinger equation in the adiabatic hyperspherical representation. The variations of the TBR and CID rates with the collision energies in the ultracold temperatures are obtained. It is found that the JΠ = 1− symmetry dominates the TBR and CID processes in most of the considered collision energy range. The rate of TBR (CID) into (from) the l = 1 anion is larger than those for the l = 0 and l = 2 anions, with the l representing the rotational quantum number of 3HeT−. This can be understood via the nonadiabatic couplings among the different channels.

Two-photon annihilation of positrons with K -shell electrons of H-like ions

The two-photon annihilation of a positron with an electron bound in the $1s$ state of a H-like ion is calculated within the fully relativistic QED framework. The interaction with the nucleus is treated nonperturbatively, thus allowing the calculations to be carried out for the annihilation with strongly bound inner shells of heavy ions. Infrared divergences, appearing when one of the emitted photons approaches the low-frequency limit, are accurately eliminated from final expressions. The total cross section of the two-photon and one-photon annihilation processes are compared for a wide range of collision energies and nuclear charge numbers. It is demonstrated that the two-photon annihilation channel dominates over the one-photon channel for the low- and medium-$Z$ ions, whereas for the high-$Z$ ions the situation reverses.

Quarkonium production in p+p collisions at RHIC and LHC

Quarkonium production mechanism is still an unresolved question. This paper provides an overview of recent quarkonium production studies performed at RHIC and LHC in p + p collisions at a wide range of collision energies. After a brief explanation of representative theoretical models, experimental results of the quarkonium spectra and polarizations are presented with comparison to the model calculations. The spectra are overall rather well described by production model calculations, with a few exceptions, while no significant polarization is observed beyond large uncertainties for J/ψ and ϒ, and thus no constraint on the models is obtained at present. Recently, new observables have been measured such as charged particle multiplicity dependence of quarkonium production, which provides a way to understand the interplay between hard and soft processes, and associated production of quarkonium (J/ψ in jets and double production), which provides more information about production mechanism. A trend of non-linear increase of quarkonium production with increasing the particle multiplicity is observed both at RHIC and LHC, but it is reproduced by most models and further studies and more data are needed in order to differentiate them. Meanwhile, the first measurements of J/ψ in jets show that it is produced with larger activity within a jet than simulated by PYTHIA. A comment on the study of J/ψ-hadron femtoscopic correlations to measure their mutual interaction is also given. In summary, the production mechanism remains a puzzle, but more precise studies will be possible with upcoming RHIC and LHC runs.

Measurement of the Charge Exchange Cross Section for N7+, O7+ Ions in Collision with Atomic H

The absolute total cross sections for the charge exchange between highly charged ions 15N7+, O7+, and atomic H have been measured with the ion-atom merged-beams apparatus at Oak Ridge National Laboratory. The collision energy range is from 1224 down to 2 eV u−1, which covers outflowing hot components of astrophysical charge exchange plasmas like stellar-wind and supernova remnants. Good agreement with the previous measurements and theory is found for the collision energies above 100 eV u−1, while below 100 eV u−1 limited agreement is achieved with the available calculations. These cross-section data are useful for modeling X-ray emission resulting from the charge exchange at the interface of hot plasma interacting with ambient neutral gas.

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.