State-resolved Cross Sections Research Articles

New Potential Energy Surface for the H + Cl2 Reaction and Quantum Dynamics Studies.

The reaction of H + Cl2 → HCl + Cl plays a crucial role in various fields. However, no previous study has investigated this reaction using accurate quantum mechanical methods. In this paper, we construct a global potential energy surface (PES) using the neural network method with more than 20,000 ab initio energies obtained by the MRCI-F12+Q method with the aug-cc-pV5Z basis and extrapolated to the complete basis set limit. The spin-orbit coupling of the Cl atom has been considered in the PES. With this new PES, product state-resolved quantum dynamics calculations for the H + Cl2 (v0 = 0, j0 = 0-2) → HCl + Cl reaction was carried out. Numerical results show that the initial rotational excitation of the Cl2 has negligible effects on the reactivity. Product state-resolved integral cross sections (ICS) and rate constants reveal that the HCl is most favorably produced in its v' = 2 vibrational state. The calculated product vibrational state-resolved and total reaction rate constants suggest that the new global PES is accurate enough, as compared with the available experimental measurements.

Product State-Resolved Reactive Scattering Studies of the H + Cl2 (v0 = 1-3, j0 = 0) → HCl + Cl Reaction by the Time-Dependent Wave Packet Method.

The typical hydrogen atom plus halogen molecule reaction H + Cl2 → HCl + Cl has implications across many fields. In this paper, product state-resolved quantum dynamics calculations for the vibrationally excited reaction H + Cl2 (v0 = 1-3, j0 = 0) → HCl + Cl were conducted using the time-dependent wave packet method on a newly developed accurate potential energy surface. Numerical results indicate that the initial vibrational excitation of Cl2 does enhance the reactivity for this early barrier reaction, although less than the enhancement of the translational energy. The calculated product vibrational state-resolved integral cross sections and rate constants reveal that the product vibrational state distribution and the initial vibrational state of Cl2 are highly correlated. The thermal rate constant in the temperature range from 100 to 1000 K was given and is found to be in reasonable agreement with the experimental measurements.

State-to-State Quantum Dynamics Study of Intramolecular Isotope Effects on Be(1S) + HD (v0 = 2, j0 = 0) → BeH/BeD + H/D Reaction.

The dynamic mechanisms and intramolecular isotope effects of the Be(1S) + HD (v0 = 2, j0 = 0) → BeH/BeD + H/D reaction are studied at the state-to-state level using the time-dependent wave packet method on a high-quality potential energy surface. This reaction can proceed along the indirect pathway that features a barrier and a deep well or the smooth direct pathway. The reaction probabilities, total and state-resolved integral cross sections, and differential cross sections are analyzed in detail. The calculated dynamics results show that both of the products are mainly formed by the dissociation of a collinear HBeD intermediate when the collision energy is slightly larger than the threshold. As the collision energy increases, the BeH + D channel is dominated by the direct abstraction process, whereas the BeD + H channel mainly follows the complex-forming mechanism.

Quantum State-Resolved Nonadiabatic Dynamics of the H+NaF → Na+HF Reaction

The H + NaF reaction is investigated at the quantum state-resolved level using the time-dependent wave-packet method based on a set of accurate diabatic potential energy surfaces. Oscillatory structures in the total reaction probability indicate the presence of the short-lived intermediate complex, attributed to a shallow potential well and exothermicity. Ro-vibrational state-resolved integral cross sections reveal the inverted population distributions of the product. The HF product favors an angular distribution in the forward hemisphere of 30°–60° within the collision energy range from the threshold to 0.50 eV, which is related to the nonlinear approach of the H atom to the NaF molecule. Quantum generalized deflection functions show that the low-J partial waves contribute primarily to the backward scattering, while the high-J partial waves govern the forward scattering. The correlation between the partial wave J and the scattering angle ϑ proves that the reaction follows a predominant direct reaction mechanism.

Ultraviolet H2luminescence in molecular clouds induced by cosmic rays

Context. Galactic cosmic rays (CRs) play a crucial role in ionisation, dissociation, and excitation processes within dense cloud regions where UV radiation is absorbed by dust grains and gas species. CRs regulate the abundance of ions and radicals, leading to the formation of more and more complex molecular species, and determine the charge distribution on dust grains. A quantitative analysis of these effects is essential for understanding the dynamical and chemical evolution of star-forming regions.Aims. The CR-induced photon flux has a significant impact on the evolution of the dense molecular medium in its gas and dust components. This study evaluates the flux of UV photons generated by CRs to calculate the photon-induced dissociation and ionisation rates of a vast number of atomic and molecular species, as well as the integrated UV photon flux.Methods. To achieve these goals, we took advantage of recent developments in the determination of the spectra of secondary electrons, in the calculation of state-resolved excitation cross sections of H2by electron impact, and of photodissociation and photoionisation cross sections.Results. We calculated the H2level population of each rovibrational level of theX, B, C, B′,D, B″,D′, andastates. We then computed the UV photon spectrum of H2in its line and continuum components between 72 and 700 nm, with unprecedented accuracy, as a function of the CR spectrum incident on a molecular cloud, the H2column density, the isomeric H2composition, and the dust properties. The resulting photodissociation and photoionisation rates are, on average, lower than previous determinations by a factor of about 2, with deviations of up to a factor of 5 for the photodissociation of species such as AlH, C2H2, C2H3, C3H3, LiH, N2, NaCl, NaH, O2+, S2, SiH, l-C4, and l-C5H. A special focus is given to the photoionisation rates of H2, HF, and N2, as well as to the photodissociation of H2, which we find to be orders of magnitude higher than previous estimates. We give parameterisations for both the photorates and the integrated UV photon flux as a function of the CR ionisation rate, which implicitly depends on the H2column density, as well as the dust properties.

A global 2A″ state potential energy surface for the Al (2P) + O2 (Σg−3) → AlO (2Σ+) + O (3P) reaction based on the doubly hybrid functional XYG3

In this paper, a global and full-dimensional potential energy surface at the 2A″ ground state for the Al + O2 → AlO + O reaction was constructed, for the first time, based on extensive electronic structure calculations using the doubly hybrid functional XYG3 and potential energy surface fittings by neural networks. Details of the reaction paths have been analyzed. It was found that both two intermediates, the cyclic-AlO2 and the linear-OAlO, were able to dissociate to the AlO + O products, and the isomerization process between these two intermediates was controlled by conical intersections between two 2A″ states. Ro-vibrational state resolved integral cross sections have also been calculated at collision energies from 1.0 to 10.0 kcal/mol. The results support the harpooning mechanism in this metal-oxidant-involved reaction.

A Theoretical Study of Temperature-dependent Photodissociation Cross Sections and Rates for O2

The photodissociation of O2 is thought to play a vital role in blocking UV radiation in the Earth’s atmosphere and likely has great importance in characterizing exoplanetary atmospheres. This work considers four photodissociation processes of O2 associated with its four electronic states, whose potential energy curves and transition dipole moments are calculated at the icMRCI+Q/aug-cc-pwCV5Z-DK level of theory. The quantum-mechanical approach is used to compute the state-resolved cross sections for two triplet transitions from the ground X state to the excited B and E states, and for two singlet transitions from the a 1Δg and b states to the 1 1Πu state, with a consideration of photon wavelengths from 500 Å to the relevant threshold. Assuming the populations of the initial states satisfy a Boltzmann distribution, the temperature-dependent photodissociation cross sections are estimated at gas dynamic temperatures of 0–10,000 K, in which the discrete progressions of the B and E transitions are also considered. The photodissociation rates of O2 in the interstellar, solar, and blackbody radiation fields are also calculated using the temperature-dependent cross sections. The resulting photodissociation cross sections and rates are important for the atmospheric chemistry of Earth and may be also useful for the atmospheric exploration of exoplanets.

Photodissociation cross sections and rates of NaO

ABSTRACT Photodissociation of NaO may be important for the sodium chemistry in various astrophysical regions. This work produces the photodissociation cross sections and rates of NaO over the temperature range from 0 to 15 000 K. First, the state-resolved cross sections for transitions from the ground and first excited states of NaO are investigated using ab initio potential energy curves and transition dipole moments. The temperature-dependent cross sections were then obtained by assuming a Boltzmann distribution to describe the population of the initial state. Detailed comparisons with experimental cross sections at 200 and 300 K reveal that the X 2Π → 1 2∆ and X 2Π → 2 2Σ− transitions may be the main photodissociation pathways for NaO in the wavelengths of about 2400–2580 Å, while the X 2Π → B 2Σ− transition may play a dominant role in the wavelengths of about 3534–4230 Å. Finally, photodissociation rates in the interstellar, solar, and blackbody radiation fields were determined. In the interstellar and solar radiation fields, the X 2Π → B 2Σ− transition dominates at low temperatures and the A 2Σ+ → 2 2Σ+ transition dominates at high temperatures. The total photodissociation rates in ultraviolet-rich and visible-rich radiation fields are almost insensitive to the temperature. The photodissociation cross sections and rates of NaO should be useful for investigating the chemical evolution of the sodium element in planetary exospheres, atmospheres of cool stars, and envelopes of evolved stars.

Polyatomic radiative association by quasiclassical trajectory calculations: Formation of HCN and HNC molecules in H + CN collisions.

We have developed the polyatomic extension of the established [M. Gustafsson, J. Chem. Phys. 138, 074308 (2013)] classical theory of radiative association in the absence of electronic transitions. The cross section and the emission spectrum of the process is calculated by a quasiclassical trajectory method combined with the classical Larmor formula which can provide the radiated power in collisions. We have also proposed a Monte Carlo scheme for efficient computation of ro-vibrationally quantum state resolved cross sections for radiative association. Besides the method development, the global potential energy and dipole surfaces for H + CN collisions have been calculated and fitted to test our polyatomic semiclassical method.

Electron scattering cross sections for the ground and excited states of tin

A comprehensive set of cross sections for electron scattering from the ground and first four excited states of tin has been calculated using the Relativistic Convergent Close-Coupling method. Elastic scattering, momentum transfer, total scattering, and total-inelastic scattering cross sections have been produced for the 5p23P0,1,2, 1D2 and 1S0 states of atomic tin over a projectile energy range of 0.1 eV to 1000 eV. Over the same projectile energy range, state-resolved cross sections for excitations to the 5p2, 5p6s, 5p5d and 5p6p manifolds from the ground and first four excited states of tin are presented. Total single-ionisation cross sections have been calculated which account for the direct ionisation of electrons in the valence 5p and closed 5s shells, as well as indirect contributions from excitation auto-ionisation. These ionisation cross sections are presented for projectile energies up to 1000 eV. Maxwellian rate coefficients have been calculated for all studied transitions over electron temperatures ranging from 0.5 eV to 200 eV and fitted with simple formulas. The fit coefficients are tabulated for use in modelling applications.

Complete collision data set for electrons scattering on molecular hydrogen and its isotopologues: III. Vibrational excitation via electronic excitation and radiative decay

We present state-resolved cross sections for vibrational excitation via electronic excitation followed by radiative decay (ERD), for electrons scattering on all bound vibrational levels of the ground electronic state (X1Σg+) of molecular hydrogen and its isotopologues (H2, HD, HT, D2, DT and T2). We consider excitation of the singlet n=2–3 states (where n refers to the united-atoms-limit principle quantum number) and account for all possible decay pathways back to the bound vibrational levels of the ground electronic state (X1Σg+) to produce an estimate for the ERD cross sections. A selection of the results are presented in graphical form and a full data set is provided as both numerical values and analytic fit functions in supplementary data files. The uncertainty in the cross sections is estimated to be 11%, except for scattering on the highest bound vibrational level of HD, HT, D2, and DT, where the uncertainty is 21%. The data can be downloaded from the MCCC database at mccc-db.org.

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.

Theoretical investigation of orbital alignment of x-ray-ionized atoms in exotic electronic configurations

We theoretically study orbital alignment in x-ray-ionized atoms and ions, based on improved electronic-structure calculations starting from the Hartree-Fock-Slater model. We employ first-order many-body perturbation theory to improve the Hartree-Fock-Slater calculations and show that the use of first-order-corrected energies yields significantly better transition energies than originally obtained. The improved electronic-structure calculations enable us also to compute individual state-to-state cross sections and transition rates and, thus, to investigate orbital alignment induced by linearly polarized x rays. To explore the orbital alignment of transiently formed ions after photoionization, we discuss alignment parameters and ratios of individual state-resolved photoionization cross sections for initially neutral argon and two exotic electronic configurations that may be formed during x-ray multiphoton ionization dynamics induced by x-ray free-electron lasers. We also present how the orbital alignment is affected by Auger-Meitner decay and demonstrate how it evolves during a sequence of one photoionization and one Auger-Meitner decay. Our present work establishes a step toward investigation of orbital alignment in atomic ionization driven by high-intensity x rays.

Effective one-electron approach to proton collisions with molecular hydrogen

The two-centre wave-packet convergent close-coupling approach to ion–atom collisions is extended to study proton collisions with molecular hydrogen including electron-capture channels. We use a model potential to represent the molecular target as an effective one-electron spherically symmetric system. This greatly simplifies the target structure, allowing us to use already existing code developed for ion collisions with single-electron targets. Calculated total cross sections for electron capture, single ionisation, and excitation processes generally agree well with experimental data and other theoretical calculations where available. However, the total electron capture cross section is found to overestimate the experimental data at low energies, while the total ionisation cross section is slightly underestimated. Additionally, we present state-resolved cross sections for capture into the 1s, 2ell , and 3ell states of the projectile where deviation between various previous calculations is substantial. Our results lead to overall improvement over previous theoretical studies although discrepancies with experiment are observed for 3p and 3d capture. We conclude that treating molecular hydrogen as an effective one-electron system within the two-centre coupled-channel approach to one-electron targets can give reasonably accurate total cross sections at intermediate and high energies, without the need for a complex and computationally demanding two-electron target representation.

Destruction of AlF: a quantum study of its ground-state photodissociation

ABSTRACTPhotodissociation by ultraviolet photons is the key destruction pathway for aluminium monofluoride (AlF) in the envelope of the carbon star IRC +10216 from the stellar photosphere up to the outer layers. However, there is no available photodissociation data for AlF, which hampers the prediction of the abundances of Al-bearing molecules in astrochemical models. Here, we present an ab initio study of AlF photodissociation. Potential energy curves of seven singlet states for AlF were computed by the internally contracted multireference single and double configuration-interaction method and aug-cc-pCV5Z-DK basis set, along with the transition dipole moments from excited singlet states to the ground state. State-resolved cross sections for the direct photodissociation from 36 349 ground rovibrational levels ( υ″≤120, J″≤360) to six singlet excited states were calculated by the quantum mechanical method. We found that the 21Π←X 1Σ+, 31Π←X 1Σ+, and 41Π←X 1Σ+ transitions have extremely strong absorption for lower wavelengths, especially between the Lyman and Lyman α ones. Photodissociation cross sections in local thermal equilibrium were estimated for gas temperatures from 500 to 20 000 K. Finally, the cross sections were utilized to calculate the photodissociation rates in the interstellar and blackbody radiation fields. The obtained photodissociation cross sections and rates can be used to determine the abundance of AlF in astrochemical models.

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.

Temperature-dependent direct photodissociation cross sections and rates of AlCl

ABSTRACT The photodissociation process of aluminium monochloride (AlCl) plays an important role in modelling the chemistry of the circumstellar envelope. In this work, direct photodissociation cross sections of AlCl have been computed for transitions from the ground X1Σ+ state to six low-lying excited electronic states by using ab initio potential energy curves and transition dipole moments, which are obtained by the internally contracted multireference configuration-interaction method with Davidson correction and the aug-cc-pV6Z basis set. State-resolved cross sections for transitions from 38 958 rovibrational levels (υ″ ≤ 100, J″ ≤ 400) of the ground X1Σ+ state have been obtained for photon wavelengths from 500 Å to the dissociation threshold. Photodissociation cross sections in local thermal equilibrium are evaluated for gas temperatures from 500 to 10 000 K. Using the computed cross sections, temperature-dependent photodissociation rates of AlCl in the interstellar and blackbody radiation fields are determined. The results can be applied to the investigation of the chemical evolution of Al in the envelope of carbon-rich and oxygen-rich stars.

Rovibrationally Resolved Photodissociation of AlH via Excited Electronic States

Abstract Photodissociation processes are of great importance for modeling interstellar chemistry since it is a key destruction pathway for small molecules. Here, we present a detailed ab initio study of AlH photodissociation. Potential energy curves and transition dipole moments for AlH are obtained by using the internally contracted multireference configuration interaction method and the Davidson correction (icMRCI+Q), as well as the aug-cc-pV6Z basis set. Except for the X1Σ+, A1Π, and C1Σ+ states, five higher excited 31Σ+, 21Π, 31Π, 41Σ+, and 41Π states are considered in order to investigate the photodissociation pathways in the vacuum ultraviolet region. State-resolved cross sections of transitions from all the rovibrational levels of the X1Σ+ state to seven singlet excited states are computed for photon wavelengths ranging from 500 Å to the threshold. Photodissociation cross sections in local thermal equilibrium (LTE) are obtained at temperatures from 500 to 10,000 K. Applications of the LTE cross sections to compute photodissociation rates in the standard interstellar radiation field and blackbody radiation field are briefly discussed.

State-to-state dynamics of reactions H + DH’(ν = 0,j = 0) → HH’(ν’,j’) + D/HD(ν’,j’) + H’ with time-dependent quantum wave packet method* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11504206 and 12004216), the Ph. D. Research Start-up Fund of Shandong Jiaotong University (Grant No. BS2020025), and the Shandong Natural Science Foundation, China (Grant Nos.

State-to-state time-dependent quantum dynamics calculations have been carried out to study H + DH’ → HH’ + D/HD + H’ reactions on BKMP2 surface. The total integral cross sections of both reactions are in good agreement with earlier theoretical and experimental results, moreover the rotational state-resolved reaction cross sections of H + DH’ → HH’ + D at collision energy E C = 0.5 eV are closer to the experimental values than the ones calculated by Chao et al. [J. Chem. Phys. 117 8341 (2002)], which proves the higher precision of the quantum calculation in this work. In addition, the state-to-state dynamics of H + DH’ → HD’ + H reaction channel have been discussed in detail, and the differences of the micro-mechanism of the two reaction channels have been revealed and analyzed clearly.

Isotopic effects of the N(2D) + H2 → NH + H reaction: a quantum time-dependent wave packet investigation

Based on the potential energy surface reported by Li and co-workers (J. Comput. Chem. 34 1686–1696 (2013)), the dynamics calculations of N(2D) + H2(v 0 = 0, j 0 = 0) reaction and its isotopic variants HD and D2 are studied using time-dependent wave packet method in the collision energy range of 0.01–1.0 eV. Dynamics properties such as reaction probability, differential cross section, and integral cross section are studied at state-to-state level of theory. Present values are compared with available theoretical and experimental results. The results indicate that the integral cross sections of N(2D) + D2 reaction are in general good agreement with the experimental data at collision energy below 0.15 eV. The rotational state-resolved integral cross sections of N(2D) + H2/HD/D2 reactions are compared with experimental values for the first time, with the obtained values being in good agreement with the experimental data.