H2 Collisions Research Articles

State-to-state rate coefficients for HCS+ in rotationally inelastic collisions with H2 at low temperatures

ABSTRACT HCS+ ions have been detected in several regions of the interstellar medium (ISM), but an accurate determination of the chemical-physical conditions in the molecular clouds where this molecule is observed requires detailed knowledge of the collisional rate coefficients with the most common colliders in those environments. In this work, we study the dynamics of rotationally inelastic collisions of HCS+ + H2 at low temperature, and report, for the first time, a set of rate coefficients for this system. We used a recently developed potential energy surface for the HCS+–H2 van der Waals complex and computed state-to-state rotational rate coefficients for the lower rotational states of HCS+ in collision with both para- and ortho-H2, analysing the influence of the computed rate coefficients on the determination of critical densities. Additionally, the computed rate coefficients are compared with those obtained by scaling the ones from HCS+ in collision with He (an approximation that is sometimes used when data is lacking), and large differences are found. Furthermore, the approximation of using the rates for the HCO+ + H2 collision as a rough approximation for those of the HCS+ + H2 system is also evaluated. Finally, the complete set of de-excitation rate coefficients for the lowest 30 rotational states of HCS+ by collision with H2 is reported from 5 to 100 K.

Double electron capture probability at He-=SUP=-2+-=/SUP=- ion Xe atom collisions with different impact parameters

Differential cross sections for scattering of helium atoms formed at collisions of He2+ ions with kinetic energies of 1.97, 3.00, and 7.17 keV/a.m.u with Xe atoms in processes with the formation of slow xenon ions with charges 2-4 have been measured. The projectiles deflection function is calculated. The probability of all these processes occurring at various values of the impact parameter of colliding particles is determined. The role of the electron shells of the Xe atom 5(s,p) and 4(s,p,d) for the capture of two electrons is determined depending on the speed of approach of the colliding particles, the impact parameter and the charge of the formed xenon ions. Keywords: Double electron capture, capture with ionization, scattering differential cross section, impact parameter.

An accurate many-body expansion potential energy surface for AlH2 (22A') and quantum dynamics in Al(3P) + H2 (v0 = 0-3, j0 = 0, 2, 4, 6) collisions.

An accurate potential energy surface is constructed for the excited state of AlH2 by fitting extensive ab initio points calculated at the multi-reference configuration interaction level based on aug-cc-pV(Q+d)Z and aug-cc-pV(5+d)Z basis sets. All the calculated energies are corrected via the many-body expansion method and extrapolated to the complete basis set limit. The various topographic features of the new potential energy surface are investigated to demonstrate the correct behavior of Al(3P) + H2(X1Σg+) and AlH(a3Π) + H(2S) dissociation limits. By employing the time-dependent wave packet approach, the integral scattering cross-sections obtained from the Coriolis coupling calculation and the centrifugal sudden approximation, respectively, are compared in detail and show that the former has a higher effect on the reaction. Moreover, the thermal rate constants for Al(3P) + H2 (v0 = 0-3, j0 = 0, 2, 4, 6) in the temperature range of 0-5000 K are calculated, thereby providing insights into the influence of ro-vibrational quantum numbers on the thermal rate constants.

B2.5-Eunomia simulations of Magnum-PSI detachment experiments: II. Collisional processes and their relevance

Detachment is achieved in Magnum-PSI by increasing the neutral background pressure in the target chamber using gas puffing. The plasma is studied using the B2.5 multi fluid plasma code B2.5 coupled with Eunomia, a Monte Carlo solver for neutral species. This study focuses on the effect of increasing neutral background pressure to the plasma volumetric loss of particle, momentum and energy. The plasma particle and energy loss almost linearly scale with the increase of neutral background pressure, while the momentum loss does not scale as strongly. Plasma recombination processes include molecular activated recombination (MAR), dissociative attachment, and atomic recombination. Atomic recombination, which includes radiative and three-body recombination, is the most relevant plasma process in reducing the particle flux and, consequently, the heat flux to the target. The low temperature where atomic recombination becomes dominant is achieved by plasma cooling via elastic H+−H2 collisions. The transport of vibrationally excited H2 molecules out of the plasma serves as an additional electron cooling channel with relatively small contribution. Additionally, the transport of highly vibrational H2 has a significant impact in reducing the effective MAR and dissociative attachment collision rates and should be considered properly. The relevancy of MAR and atomic recombination occupy separate electron temperature regimes, respectively, at eV and eV, with dissociative attachment being relevant in the intermediary. Plasma cooling via elastic H+−H2 collisions is effective at eV.

Ab Initio Study of Single- and Double-Electron Capture Processes in Collisions of He2+ Ions and Ne AtomsSupported by the National Natural Science Foundation of China (Grant Nos. 11774344 and 11474033), and the National Key Research and Development Program of China (Grant No. 2017YFA0402300).

The single- and double-electron capture (SEC, DEC) processes of He2+ ions colliding with Ne atoms are studied by utilizing the full quantum-mechanical molecular-orbital close-coupling method. Total and state-selective SEC and DEC cross sections are presented in the energy region of 2 eV/u to 20 keV/u. Results show that the dominant reaction channel is Ne+(2s2p 6 2 S) + He+(1s) in the considered energy region due to strong couplings with the initial state Ne(2s 22p 6 1 S) + He2+ around the internuclear distance of 4.6 a.u. In our calculations, the SEC cross sections decrease initially and then increase whereby, the minimum point is around 0.38 keV/u with the increase of collision energies. After considering the effects of the electron translation factor (ETF), the SEC cross sections are increased by 15%–25% nearby the energy region of keV/u and agree better with the available results. The DEC cross sections are smaller than those of SEC because of the larger energy gaps and no strong couplings with the initial state. Due to the Demkov-type couplings between DEC channel Ne2+(2s22p 4 1 S) + He(1s 2) and the dominating SEC channel Ne+(2s2p 6 2 S) + He+(1s), the DEC cross sections increase with increasing impact energies. Good consistency can also be found between the present DEC and the experimental measurements in the overlapping energy region.

Charge transfer and excitation processes in low energy collisions of He+ ions with Li atoms

Electron capture between solar wind ions and neutral species has contributed to the understanding of X-ray production from solar system bodies. The charge transfer and excitation processes in solar wind ions of He+(1s) colliding with Li(1s 22s) atoms are studied by utilizing the full quantum-mechanical molecular-orbital close-coupling (QMOCC) method with impact energies of 0.003–2 keV amu−1. Comparisons of cross sections from single- and multi-configurational calculations for a self-consistent field (SCF and MCSCF) process are carried out. Results show that the dominant reaction channels are He(1s2l 1,3L) + Li+(1s 2 1S). Good consistency is found among present total and state-selective charge transfer and excitation cross sections with other theoretical and experimental data in the same energy region. Due to the differences between coupling matrix elements in high-energy states, the charge transfer cross sections calculated from SCF and MCSCF split slightly as E > 0.4 keV amu−1. Weak Stueckelberg oscillations for charge transfer appear in the present work. In addition, the differences of cross sections for electron excitation to Li(1s 22p) in the singlet/triplet molecular states with He+(1s) are much smaller than those of charge transfer processes because of the similar energy gaps from Li(1s 22p) to the ground state in singlet/triplet states in the large R region.

Experimental Investigations of Vibration–Vibration Energy Transfer in HBr(X1Σ+v'' = 5, 6)–H2 Collisions

Experimental Investigations of Vibration–Vibration Energy Transfer in HBr(X1Σ+v'' = 5, 6)–H2 Collisions

Properties of Highly Rotationally Excited H2 in Photodissociation Regions

Abstract Molecular hydrogen (H2) is the dominant molecular species in the vast majority of interstellar environments and it plays a crucial role as a radiative coolant. In photodissociation regions (PDRs), it is one of the primary emitters in the near- to mid-infrared, which is due to lines originating from highly excited levels. The sparseness of H2 collisional data for rotational levels J ≥ 9, particularly for H2–H2 collisions, has limited nonlocal thermal equilibrium (NLTE) studies in ultraviolet-irradiated regions. Utilizing new calculations for para- and ortho-H2 high rotational collisional rate coefficients colliding with H2 (up to the maximum value for v = 0: J = 31), existing data for H2–H and H2–He collisions, and approximate scaling relations for missing collisional data, we investigate the excitation properties of H2 in a range of astrophysical environments, with the focus on PDRs (including benchmark PDR models and the Orion Bar). In these NLTE models, H2 emission is illustrated and shown as a diagnostic for physical conditions, such as temperature and density. Furthermore, we demonstrated the effect of updates in the collisional rates on the modeling results of H2 excitation. The resulting data sets of H2 collisional data should find wide application to other molecular environments.

Rainbow scattering in rotationally inelastic collisions of HCl and H2.

We examine rotational transitions of HCl in collisions with H2 by carrying out quantum mechanical close-coupling and quasi-classical trajectory (QCT) calculations on a recently developed globally accurate full-dimensional ab initio potential energy surface for the H3Cl system. Signatures of rainbow scattering in rotationally inelastic collisions are found in the state resolved integral and differential cross sections as functions of the impact parameter (initial orbital angular momentum) and final rotational quantum number. We show the coexistence of distinct dynamical regimes for the HCl rotational transition driven by the short-range repulsive and long-range attractive forces whose relative importance depends on the collision energy and final rotational state, suggesting that the classification of rainbow scattering into rotational and l-type rainbows is effective for H2 + HCl collisions. While the QCT method satisfactorily predicts the overall behavior of the rotationally inelastic cross sections, its capability to accurately describe signatures of rainbow scattering appears to be limited for the present system.

Rate coefficients for H2:H2 inelastic collisions in the ground vibrational state from 10 to 1000 K

Aims. In this work, we present a pruned set of state-to-state rate coefficients (STS rates) for inelastic H2:H2 collisions in the thermal range from 10 to 1000 K. The set includes all relevant rates needed for diagnostics based on the simulation of quadrupole infrared spectra of H2. Methods. The reported set was obtained from a quantum scattering close-coupling calculation employing a recent version of the MOLSCAT code, a high-level potential energy surface, and rotational energies of the H2 molecules with spectroscopic accuracy. These improvements have led to a significant increase in the accuracy with respect to previous computational results. The accuracy of the present STS rates is tested against recently reported experimental rates. Most dominant rates agree with the experiment within a 2σ uncertainty (2 to 6%). Results. In addition to the tables given in the main text, three machine-readable tables are available at the CDS. These tables include all the relevant numerical results of the paper, namely, the excitation and de-excitation STS rates for H2:H2 inelastic collisions at selected temperatures between 10 and 1000 K, and their functional description for interpolation at any intermediate temperature.

Fine-structure resolved rovibrational transitions for SO + H2 collisions.

Cross sections and rate coefficients for sulfur monoxide (SO) + H2 collisions are calculated using a full six-dimensional (6D) potential energy surface (PES). The coupled states (CS) approximation is used to compute fine-structure resolved cross sections for rovibrational transitions between states with v = 0-2, where v is the vibrational quantum number of the SO molecule. The CS calculations for Δv = 1 are benchmarked against close-coupling (CC) results for spin-free interactions. For Δv = 0, the present fine-structure resolved CS results are benchmarked against existing CC results obtained with a rigid rotor approximation. In both cases, the agreement is found to be satisfactory, which suggests that the present results may provide reliable estimates for fine-structure resolved rovibrational transitions. These estimates are the first of their kind based on a full 6D PES. Rate coefficients are reported for temperatures between 10 K and 3000 K for both para- and ortho-H2 colliders. A comparison of the para-H2 rates with mass-scaled results for He shows substantial differences that may be important in astrophysical models.

Вероятность процесса захвата двух электронов быстрым ионом Не-=SUP=-2+-=/SUP=- у атома Хе при различных параметрах удара

Differential cross sections for scattering of helium atoms formed at collisions of He2+ ions with kinetic energies of 1.97, 3.00, and 7.17 keV/a.m.u with Xe atoms in processes with the formation of slow xenon ions with charges 2 - 4 have been measured. The projectiles deflection function is calculated. The probability of all these processes occurring at various values of the impact parameter of colliding particles is determined. The role of the electron shells of the Xe atom 5 (s, p) and 4 (s, p, d) for the capture of two electrons is determined depending on the speed of approach of the colliding particles, the impact parameter and the charge of the formed xenon ions.

Experimental and theoretical investigation of resonances in low-energy NO-H2 collisions.

The experimental characterization of scattering resonances in low energy collisions has proven to be a stringent test for quantum chemistry calculations. Previous measurements on the NO-H2 system at energies down to 10 cm-1 challenged the most sophisticated calculations of potential energy surfaces available. In this report, we continue these investigations by measuring the scattering behavior of the NO-H2 system in the previously unexplored 0.4 cm-1-10 cm-1 region for the parity changing de-excitation channel of NO. We study state-specific inelastic collisions with both para- and ortho-H2 in a crossed molecular beam experiment involving Stark deceleration and velocity map imaging. We are able to resolve resonance features in the measured integral and differential cross sections. Results are compared to predictions from two previously available potential energy surfaces, and we are able to clearly discriminate between the two potentials. We furthermore identify the partial wave contributions to these resonances and investigate the nature of the differences between collisions with para- and ortho-H2. Additionally, we tune the energy spreads in the experiment to our advantage to probe scattering behavior at energies beyond our mean experimental limit.

Physical Conditions of Gas Components in Debris Disks of 49 Ceti and HD 21997

Abstract Characterization of the gas component in debris disks is of fundamental importance for understanding their origin. To address this goal, we have conducted non-LTE (local thermodynamic equilibrium) analyses of the rotational spectral lines of CO including those of rare isotopologues (13CO and C18O) that have been observed toward the gaseous debris disks of 49 Ceti and HD 21997 with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Atacama Compact Array (ACA). The analyses have been carried out for a wide range of H2 density, and the observed line intensities are found to be reproduced as long as the H2 density is higher than 103 cm−3. The CO column density and the gas temperature are evaluated to be (1.8–5.9) × 1017 cm−2 and 8–11 K for 49 Ceti and (2.6–15) × 1017 cm−2 and 8–12 K for HD 21997, respectively, where the H2 collision is assumed for the rotational excitation of CO. The results do not change significantly even if electron collision is considered. Thus, CO molecules can be excited under environments containing no H2 or a small number of H2 molecules, even where collisions with CO, C, O, and C+ would make an important contribution to the CO excitation in addition to H2. Meanwhile, our result does not rule out the case of abundant H2 molecules. The low gas temperature observed in the debris disks is discussed in terms of inefficient heating by interstellar and stellar UV radiation.

Projectile angular distribution for electron capture in p + H2 collision considering the vibrational motion

We show the results calculated with improved treatment for the interaction between incident proton and nuclei in H2 over our previous work [2019 J. Phys. B: At. Mol. Opt. Phys. 52 075204] for electron capture process from H2 by proton with including the vibrational motion of molecules. Moreover, the differential cross sections with respect to the projectile scattering angle are calculated and the results are compared with experimental data. It is found that the inclusion of vibrational motion of molecules is essential to reproduce the experimental angular distribution for the fixed molecular orientation parallel to the transverse momentum transfer.

Relaxation processes in non-reactive collisions of H2 and N2 at high translational energies

ABSTRACT The present work reported a full-dimension quasi-classical trajectory study of the activation and deactivation processes in the collisions of and , when one of the molecules is vibrationally excited. In the present calculations, for the first time in the scientific literature for the studied system, a wide range of high translational energies is covered. To simulate the interatomic potential energy, acting during collisions, a single-valued potential energy surface for the electronic ground state of previously reported is used. Cross-sections and rates for activation and deactivation processes of the initial vibrationally excited molecules and cross-sections and rates for activation of the molecule that initially is in the ground vibrational state are calculated. Additionally, probabilities of rotational excitation of both molecules and an estimation of the cross-section of the possible reactions for high vibrationally excited molecules are reported. These results could improve the modelling studies of the shock waves, the low-temperature plasma, the gas flows produced around hyper sonic flying objects, and others.

Saturn’s Atmospheric Helium Abundance from Cassini Composite Infrared Spectrometer Data

Abstract We use thermal infrared data from the Composite Infrared Spectrometer, which was on board the Cassini orbiter, to retrieve the helium abundance in Saturn’s visible atmosphere. We find that the data is most consistent with a helium-to-hydrogen mole ratio in the range between 0.04 and 0.075, near the lower end of previous measurements, and implying a significant sequestration of helium in Saturn’s interior. The primary sources of uncertainty are in the spectroscopic parameters for H2–H2 collision induced absorption and the nonuniqueness of the spectral inverse problem.

Formation Cross Sections of He+ Ions in Specific Electronic States in Collisions of He2+ Ions with Hydrogen Atoms

Formation Cross Sections of He+ Ions in Specific Electronic States in Collisions of He2+ Ions with Hydrogen Atoms

Full-dimensional quantum dynamics of SO(X[formula omitted]) in collision with H2

A six-dimensional (6D) potential energy surface (PES) for the SO(X -HSO(X3Σ-)-H2 system is computed using high-level electronic structure theory and fit using a hybrid invariant polynomial method. Full-dimensional quantum close-coupling scattering calculations have been carried out using this potential for rotational and, for the first time, vibrational quenching transitions of SO induced by H2. State-to-state cross sections and rate coefficients of SO are reported for rotational transitions from rotational levels j1 = 0–10 in the ground vibrational state neglecting fine-structure. Some selected state-to-state rotational rate coefficients are compared with previous theoretical results obtained using a rigid-rotor approximation. For vibrational quenching, state-to-state and total cross sections and rate coefficients were calculated for the transitions in SO(v1=1,j1) + H2(v2=0,j2) → SO(v1′=0,j1′) + H2(v2′=0,j2′) collisions with j1 = 0–5. Cross sections for collision energies in the range 1 to 3000 cm−1 and rate coefficients in the temperature range of 5–600 K are obtained for both para-H2 (j2 = 0) and ortho-H2 (j2 = 1) collision partners. The application of the results to astrophysics is discussed.

Low energy fragments from O2+ + H2 collisions following single and double electron removal from the target

SynopsisFragmentation of H2 by 20-keV O2+ was studied. A time of flight setup was developed to measure the low-energy fragment ions originating from singly ionized target molecules. High-energy fragments stemming from double electron removal were measured by an electrostatic spectrometer. The contribution of the low-energy fragments is significant but smaller than that of the energetic ones. Though single electron removal is more probable it less likely leads to dissociation. A statistical type model shows a good agreement with the experimental results.