State-specific Cross Sections Research Articles

State-Specific Dissociation and Inelastic Rate Constants for Collisions of H2 with H and He

Recent reports have called for planetary probe missions to the outer planets of the solar system. To support these missions, kinetic data for processes involving molecular hydrogen are required in order to conduct the necessary computational and experimental studies. The data available in the literature are currently incomplete owing to the broad range of temperatures required for these computational studies. In this work, the rate constants for the state-specific dissociation and inelastic scattering collisions between molecular hydrogen and H and He were calculated using the quasi-classical trajectory method in a range of translational energy between 0.1 and 15 eV. Comparisons with the state-specific cross sections reported in the literature are made for both systems. The full set of ro-vibrational state-specific cross sections is then used to compute the corresponding dissociation and inelastic reaction rate constants between 1000 and 30,000 K. Thermal and state-specific data are made available.

Total and state-specific electron-ion recombination rates and photoionization of Ca XV for high temperature plasma

The inverse processes of photoionization and electron-ion recombination of (Ca XV + hν↔ Ca XVI + e), are studied using the unified method of Nahar and Pradhan for a large number of bound states, 582 in total with n≤10 and l≤9, for their characteristic features particularly in the high energy region that can impact plasmas at high temperature. The unified method which implements close-coupling (CC) approximation and the R-matrix method includes both the radiative recombination (RR) and dielectronic recombination (DR) for the total recombination and yields self-consistent sets of results for both the inverse processes. The present study, carried out in large scale computations, employs a large CC wavefunction expansion of 29 LS states belonging to n=2,3 complexes of the core ion Ca XVI, and appeared to be the first detailed study for the electron-ion recombination of the ion. The present results include (i) state-specific photoionization cross sections (σPI(nLS)) leaving the core ion in the ground state, (ii) state-specific recombination rate coefficients (αRC(nLS,T)) that include both the RR and DR, (iii) total recombination rate coefficients (αRC) with temperature that include contributions of infinite number of recombined states, and (iv) a spectrum of total electron-ion recombination cross sections and rates with respect to photoelectron energy. The study finds that the core ion excitations from the ground to high n=3 states have introduced Rydberg resonances which are stronger and Seaton resonances which are more enhanced in the high energy region of σPI compared to those introduced by excitations to the n=2 states. Such features have contributed to the shape of the temperature dependent state-specific αRC(nLS,T), and have resulted in three DR bumps/ shoulders in the high temperature region of the total αRC(T) where the third DR bump at temperature of ∼ 4 MK has raised αRC(T) at high T above all previous rates. The results are expected to be accurate within 10%–15% and provide more complete and precise spectral modelings for the high temperature plasmas where the highly charged Ca XV is more abundant.

Energy transfer between vibrationally excited carbon monoxide based on a highly accurate six-dimensional potential energy surface

Energy transfer between vibrational modes can be quite facile, and it has been proposed as the dominant mechanism for energy pooling in extreme environments such as nonthermal plasmas and laser cavities. To understand such processes, we perform quasi-classical trajectory studies of CO(v) + CO(v) collisions on a new full-dimensional potential energy surface fit to high-level ab initio data using a neural network method and examine the key vibrational energy transfer channels. In addition to the highly efficient CO(v + 1) + CO(v - 1) channel, there exists a significant, sometimes dominant, CO(v + 2) + CO(v - 2) channel for large v states at low collision energies. The latter is shown to stem from the substantially increased interaction between highly vibrationally excited CO, which has a much larger dipole moment than at its equilibrium bond length. Finally, the vibrational state-specific cross sections and their energy dependence on the thermal range are predicted from a limited dataset using Gaussian process regression. The relevance of these results to plasma chemistry and laser engineering and the recently observed flipping of highly vibrationally excited CO adsorbates on a cold NaCl surface is discussed.

Kinetics Of The H + CH2 → CH + H2 Reaction At Low Temperature.

A quasiclassical trajectory study of the kinetics of the title astrochemical reaction in a range of temperatures varying from 5 to 1000 K (corresponding to both the outer and the inner regions of the protostar and the circumstellar envelopes) was carried out and a clear dependence of the rate coefficient on the temperature is given, in contrast with the constant value adopted in kinetics astrochemical databases. Levering the massive nature of the performed calculations and of the detailed dynamical investigation of the reactive process, a rationalization of the temperature dependence of the released translational energy and of the rovibrational population of the CH and H2 diatomic products is also provided. Furthermore, the effect of the initial rovibrational energy of CH2 on the state-specific rate coefficients and cross sections is analyzed in order to single out the role played by the different regions of the potential energy surface on the dynamical outcomes and on the modeling the temperature dependence of the reactive efficiency of the investigated process. This led to a parametrization of the computed rate in terms of the following double Arrhenius expression (in cm3 s-1), k(T) = 2.50 × 10-10 exp(- 1.67/T) + 5.98 × 10-11 exp(- 280.5/T), alternative to the piecewise formulation into the three subintervals of temperature in which the overall 5-1000 K interval can be divided.

A quasiclassical trajectory calculation to compute the reaction cross section and thermal rate constant for the cesium exchange reaction 133CsI + 135Cs → 133Cs + I135Cs

To compute the reaction cross section and the thermal rate constant of the cesium exchange reaction 133CsI + 135Cs → 133Cs + I135Cs in the temperature range 500–1500 K, we performed a quasiclassical trajectory calculation using a potential energy surface developed from an ab initio calculation at the MP2/def2-QZVPPD level. The state-specific cross sections and their dependences on the initial vibrational and rotational quantum numbers of 133CsI (v and j, respectively) were analyzed. The v-j thermally averaged cross section and its dependence on the collision energy was also analyzed. The thermally averaged rate constant is above 3.6 × 10−10 cm3 molecule−1 s−1 and slightly increases with temperature within the investigated temperature range. The positive temperature dependence might indicate that the title reaction is controlled by long-range dispersion forces. The subsequent dissociation step might also contribute to the positive temperature dependence.

Imaging pair-correlated reaction cross sections in F + CH3D(νb = 0, 1) → CH2D(ν4 = 1) + HF(ν).

The title reactions were studied in a crossed-beam experiment at collisional energies (Ec) from 0.5 to 4.7 kcal mol-1. The νb (ν4) vibrational mode denotes the bending (umbrella) motion of the CH3D reactant (CH2D product). Using a time-sliced, velocity-map imaging technique, we extracted the state-specific, pair-correlated integral and differential cross sections. As with other isotopically analogous ground-state reactions, an inverted vibrational population of the HF coproduct was observed. Both the step-like excitation function near the threshold and the oscillatory forward-backward peakings in the Ec-evolution of the two dominant pair-correlated angular distributions at lower Ec suggest a resonance-mediated, time-delay mechanism. As Ec increases, the angular distribution of the HF(ν = 2) product evolves into a smooth and broad swath in the backward hemisphere, indicative of a direct rebound mechanism. One quantum excitation of the bending modes of CH3D(νb = 1) promotes the reaction rate by two- to three-fold up to Ec = 2.1 kcal mol-1. Broadly speaking, all major findings are qualitatively in line with previous results in the reactions of the F atom with other isotopologues. However, the rainbow feature recently observed in the CH2D(00) + HF(ν = 3) product channel is entirely absent. A possible rationale is put forward, which reinforces the previous reactive rainbow conjecture and calls for future theoretical investigations.

Dissociation dynamics of the water dication following one-photon double ionization. II. Experiment

Author(s): Reedy, D; Williams, JB; Gaire, B; Gatton, A; Weller, M; Menssen, A; Bauer, T; Henrichs, K; Burzynski, P; Berry, B; Streeter, ZL; Sartor, J; Ben-Itzhak, I; Jahnke, T; Dorner, R; Weber, T; Landers, AL | Abstract: We demonstrate the use of cold target recoil ion momentum spectroscopy to perform state-selective measurements of the dissociative channels following single-photon double-ionization of H2O. The two dominant dissociation channels observed lead to two-body (OH++H++2e-) and three-body (2H++O+2e-) ionic fragmentation channels. In the two-body case we observe the presence of an autoionization process with a double-differential cross section that is similar to the single-photon double-ionization of helium well above threshold. In the three-body case, momentum and energy correlation maps in conjunction with new classical trajectory calculations in the companion theory paper by Z. L. Streeter et al. [Phys. Rev. A 98, 053429 (2018)10.1103/PhysRevA.98.053429] lead to the determination of the eight populated dication states and their associated fragmentation geometry. For the latter case, state-specific relative cross sections, median kinetic energy releases, and median angles between asymptotic proton momenta are presented. This benchmark-level experiment demonstrates that, in principle, state-selective fixed-frame triple-differential cross sections can be measured for some dication states of the water molecule.

Electron-Impact Ionization of Atomic Nitrogen

New electron-impact ionization (EII) data are presented for neutral atomic nitrogen. The atom is treated as a 67-state system, incorporating Rydberg values up to . State-specific cross sections for the first three states are from published -spline results. Binary-encounter Bethe calculations have been performed for the remaining 64 states. These data are designed for modeling the hypersonic chemistry that occurs when a space vehicle enters Earth’s atmosphere from beyond orbit. The cross sections have been convolved into state-specific thermal rate coefficients and fit with the commonly used Arrhenius–Kooij formula for ease of use in shock-heated air plasma models. Additionally, rate coefficients are provided for a reduced 10-state system for use in coarse-grain models. For a given collisional–radiative model, these fine- and coarse-grain data can be used to generate state-specific rate coefficients for three-body electron–ion recombination, the time-reverse process of EII.

Inverse Laplace transformation for evaluation of state-specific cross sections for dissociation reaction and vibrational energy transitions

In the present work, we study the cross sections of VV (vibration-vibration) and VT (vibration-translation) energy exchanges in nitrogen and oxygen, as well as the vibrational state-specific cross sections of the dissociation reaction in N2 molecules. For VV- and VT-transitions the original approximations of the rate coefficients have been modified to make it possible to apply the inverse Laplace transformation in the analytical form. A satisfactory approximation of the state-resolved dissociation rate coefficient allowing for the application of the inverse Laplace transformation is also proposed. For all the considered reactions, analytical expressions for the cross sections are obtained. The results are analyzed in the wide range of energies and vibrational levels. It is shown that the cross sections of VV transitions increase almost linearly with the energy of the colliding particles. VT-exchanges and the dissociation reaction manifest threshold behavior and their cross sections are nonmonotonic. The dissociation threshold shifts significantly towards the low-energy region for high vibrational states. Using the hard sphere model for the dissociation cross section results in significant inaccuracy. The results of our work can be applied in nonequilibrium fluid dynamics, while simulating rarefied gas flows using the direct Monte-Carlo methods.

Development of a two-dimensional binning model for N2–N relaxation in hypersonic shock conditions

A high fidelity internal energy relaxation model for N2–N suitable for use in direct simulation Monte Carlo (DSMC) modeling of chemically reacting flows is proposed. A novel two-dimensional binning approach with variable bin energy resolutions in the rotational and vibrational modes is developed for treating the internal mode of N2. Both bin-to-bin and state-specific relaxation cross sections are obtained using the molecular dynamics/quasi-classical trajectory (MD/QCT) method with two potential energy surfaces as well as the state-specific database of Jaffe et al. The MD/QCT simulations of inelastic energy exchange between N2 and N show that there is a strong forward-preferential scattering behavior at high collision velocities. The 99 bin model is used in homogeneous DSMC relaxation simulations and is found to be able to recover the state-specific master equation results of Panesi et al. when the Jaffe state-specific cross sections are used. Rotational relaxation energy profiles and relaxation times obtained using the ReaxFF and Jaffe potential energy surfaces (PESs) are in general agreement but there are larger differences between the vibrational relaxation times. These differences become smaller as the translational temperature increases because the difference in the PES energy barrier becomes less important.

Inverse Laplace transformation for evaluation of state-specific cross sections for dissociation reaction and vibrational energy transitions

Inverse Laplace transformation for evaluation of state-specific cross sections for dissociation reaction and vibrational energy transitions

Photoionization and electron-ion recombination of Ti I

Study of the inverse processes of photoionization and electron-ion recombination of (Ti I + hν⇋ Ti II + e) using the unified method is reported. The method, based on close coupling (CC) approximation and R-matrix method, subsumes both the radiative recombination (RR) and dielectronic recombination (DR) in a unified manner and provides state-specific and total electron-ion recombination rate coefficients which are self-consistent with the state-specific photoionization cross sections. The present results include state-specific electron-ion recombination rates (αRC(i))and partial photoionization cross sections (σPI(i)) leaving the ion in the ground state of 813 bound states with n ≤ 10 and l ≤ 9 of Ti I. Various features of state-specific and total electron-ion recombination with temperature, and the corresponding photoionization cross sections with energies are discussed with illustrations. Due to closely lying excited states near the ground state of the core, photoionization cross sections show presence of narrow Rydberg resonances in low energy region near the ionization threshold. Many excited states also show broad and enhanced Seaton resonances due to PEC (photo-excitation-of-core) which contribute to the high temperature recombination. The total recombination rate coefficient is found to show a low hump around temperature 280 K and a high dielectronic recombination peak at temperature 25,000 K. Total spectrum of recombination cross sections and rates with photoelectron energy are also presented for experimental observation. Calculations were carried out using a CC wave function expansion of 36 states of the core ion Ti II. The large set of data for recombination rates and partial photoionization cross sections with resonances should provide a complete and accurate modelings of plasmas.

Rovibrational Energy Transitions and Coupled Chemical Reaction Modeling of H+H2and He+H2in DSMC

A method of describing the rovibrational energy transitions and coupled chemical reactions in the direct simulation Monte Carlo (DSMC) calculations is constructed for H( 2 S)+H₂(X 1 Σg) and He( 1 S)+H₂(X 1 Σg). First, the state-specific total cross sections for each rovibrational states are proposed to describe the state-resolved elastic collisions. The state-resolved method is constructed to describe the rotational-vibrational-translational (RVT) energy transitions and coupled chemical reactions by these state-specific total cross sections and the rovibrational state-to-state transition cross sections of bound-bound and bound-free transitions. The RVT energy transitions and coupled chemical reactions are calculated by the state-resolved method in various heat bath conditions without relying on a macroscopic properties and phenomenological models of the DSMC. In nonequilibrium heat bath calculations, the state-resolved method are validated with those of the master equation calculations and the existing shock-tube experimental data. In bound-free transitions, the parameters of the existing chemical reaction models of the DSMC are proposed through the calibrations in the thermochemical nonequilibrium conditions. When the bound-free transition component of the state-resolved method is replaced by the existing chemical reaction models, the same agreement can be obtained except total collision energy model.

Rotational Mode Specificity in the F(-) + CH3Y [Y = F and Cl] SN2 Reactions.

More than 12 million quasiclassical trajectories are computed for the F(-) + CH3Y(v = 0, JK) [Y = F and Cl] SN2 reactions using full-dimensional ab initio analytical potential energy surfaces. The initial (J, K = 0) and (J, K = J) [J = 0, 2, 4, 6, 8] rotational state specific cross sections are obtained at different collision energies (Ecoll) in the 1-20 kcal mol(-1) range, and the scattering angle and initial attack angle distributions as well as the mechanism-specific opacity functions are reported at Ecoll = 10 kcal mol(-1). The tumbling rotation (K = 0) inhibits the F(-) + CH3F reaction by a factor of 3 for J = 8 at Ecoll = 10 kcal mol(-1). This tumbling rotational effect becomes smaller at low and high Ecoll, and the tumbling motion affects the cross sections of F(-) + CH3Cl by only a few percent. The spinning rotation (K = J) hinders both reactions by factors in the 1.3-1.7 range for J = 8 at low Ecoll, whereas slight promotion is found as the Ecoll increases. The tumbling rotation may counteract the attractive ion-dipole forces, and the spinning motion hinders the complex formation, thereby decreasing the reactivity.

Electron and photon dissociation cross sections of the D2 singlet ungerade continua

The dissociation of H2 and its isotopologues through excitation to the singlet ungerade continua is one of the major channels for the production of slow hydrogen atoms at high temperature (T > 10 000 K) and electron energy >25 eV. State-specific photodissociation cross sections and oscillator strengths for molecular deuterium from the X 1Σ+g(vi, Ji) levels to the continuum levels of the B 1Σ+u, C 1Πu, B′ 1Σ+u, D 1Πu, B, D′ 1Πu and 5pσ 1Σ+u states have been calculated. The corresponding (vi, Ji) state-specific electron impact dissociation cross sections have been obtained for the first time over a wide energy range using calculated continuum oscillator strengths along with previously published excitation functions of the Lyman and Werner bands. Estimated cross sections to the higher (n ⩾ 5) npσ 1Σ+u and npπ 1Πu continua are also provided. Both photon and electron impact excitation cross sections show strong dependences on the initial (vi, Ji) quantum numbers. Thermally averaged electron impact cross sections of all singlet ungerade states increase monotonically with temperature. While excitation to the B′ 1Σ+u continuum is the dominant dissociation channel at room temperature, the C 1Πu and B 1Σ+u continua become more important at high temperature (>5000 K). The large increase of the C 1Πu and B 1Σ+u cross sections with (vi, Ji) is primarily responsible for making the continuum dissociation from a minor break-up channel at room temperature into a major one at high temperature. The electron dissociation cross section of D2 via the singlet ungerade continua is smaller than its H2 counterpart, although this difference decreases with temperature. This work, along with the previous calculations of H2 by Liu et al (2009, 2012), provides the complete electron impact dissociation cross sections of H2 and D2 through the singlet ungerade continua. Thermally averaged electron dissociation cross sections are provided at various temperatures for applications in plasma physics.

Electron and photon dissociation cross sections of the H2 singlet ungerade continua

Photodissociation cross sections and oscillator strengths for H2 from the X 1Σ+g (vi, Ji) levels to the continuum levels of the B 1Σ+u, C 1Πu, B, D′ 1Πu and 5pσ 1Σ+u states have been calculated. The (vi, Ji) state-specific electron impact dissociation cross sections to the continuum levels of these states have been obtained for the first time over a wide energy range using calculated continuum oscillator strengths along with previously published excitation functions of the Lyman and Werner bands. Estimated cross sections to the higher (n ⩾ 5) npσ 1Σ+u and npπ 1Πu continua are also provided. Both photon and electron impact excitation cross sections show strong dependences on the initial (vi, Ji) quantum numbers. Thermally averaged electron impact cross sections of all singlet ungerade states increase monotonically with temperature. While excitation to the B′ 1Σ+u continuum is the dominant dissociation channel at room temperature, the C 1Πu and B 1Σ+u continua become more important at high temperature (>5000 K). This work, along with the previous calculation of the B′ 1Σ+u and D 1Πu states by Liu et al (2009a J. Phys. B: At. Mol. Opt. Phys. 42 185203), provides the complete electron impact dissociation cross section of H2 through the singlet ungerade continua. Electron dissociation cross sections of the singlet ungerade continua are provided for the purpose of modelling atmospheric heating, analysis of occultation measurements and the hot atomic hydrogen plume observed at Saturn.

A detailed comparison of centrifugal sudden and J-shift estimates of the reactive properties of the N + N2 reaction

An extended comparison of the reactive properties of the N + N(2) exchange reaction calculated on a non-collinear dominant potential energy surface using both a centrifugal sudden and a J-shift quantum method is reported. The choice of carrying out such an investigation for N + N(2) is motivated by the fact that the best available (and currently used for spacecraft re-entry simulations) computed set of kinetic data has been worked out using the low level J-shift approximation though based on exact quantum zero total angular momentum probabilities. The fact that our investigation is carried out for a heavy system and a potential energy surface free of wells in the strong interaction region minimizes the occurrence of tunnel, resonance and interference effects which would make the rationalization of the result difficult and the centrifugal sudden treatment less accurate. The study has provided evidence of two important limits of the J-shift approximation: the wrong determination of the maximum value of the total angular momentum quantum number J contributing to reactivity and the lack of deformation of the partial reactive probability dependence on energy at fixed J value. Accordingly, it has been found that the J-shift state-specific cross sections underestimate the corresponding CS values when the initial diatomic rotational energy is low while the situation reverses when the initial diatomic rotational energy is high.

Nuclear dynamics in resonant electron collisions with small polyatomic molecules

The excitation and dissociation of polyatomic molecules by low-energy electron impact can be dominated by resonant collision processes. The formal resonance theory that has formed the basis for much of our understanding of these processes has, for the most part, treated the nuclear dynamics in one dimension. This talk will focus on dramatic effects in low energy electron scattering by small molecules that are purely polyatomic in origin and that can only be studied with a multi-dimensional treatment of the dissociation dynamics. Resonant vibrational excitation of CO2 and dissociative electron attachment to formic acid are briefly described to illustrate the discussion. The talk will then concentrate on the recent progress that has been made in studying dissociative electron attachment to water, including a completely ab initio evaluation of the three complex-valued resonance potential surfaces involved as well as the dynamical studies, carried out in full dimensionality, that give the state-specific cross sections and branching ratios into various two- and three-body channels.

Cross sections and rate constants for the C(P3)+OH(XΠ2)→CO(XΣ+1)+H(S2) reaction using a quasiclassical trajectory method

First quasiclassical trajectory calculations have been carried out for the C(3P)+OH(X 2Pi)-->CO(X 1Sigma+)+H(2S) reaction using a recent ab initio potential energy surface for the ground electronic state, X 2A', of HCO/COH. Total and state-specific integral cross sections have been determined for a wide range of collision energies (0.001-1 eV). Then, thermal and state-specific rate constants have been calculated in the 1-500 K temperature range. The thermal rate constant varies from 1.78x10(-10) cm3 s-1 at 1 K down to 5.96x10(-11) cm3 s-1 at 500 K with a maximum value of 3.39x10(-10) cm3 s-1 obtained at 7 K. Cross sections and rate constants are found to be almost independent of the rovibrational state of OH.

Highly excited states: New experimental windows in photoexcitation

In a recent series of experiments, it has been demonstrated that highly excited states, formed through photoexcitation using vacuum-ultra-violet (VUV) synchrotron radiation, have a significant probability of decaying via the fluorescence decay route. This decay route has products of VUV fluorescence photons and, in some cases, long lived metastable atoms. In helium, studies using high resolving powers at third generation synchrotron light sources utilising this decay route to the metastable atoms have enabled the identification of three series of triplet doubly-excited states below the N = 2 ionisation threshold which are excited by single photon impact from the singlet ground state. The pulsed nature of synchrotron light and the detection of fluorescence photons has been utilised to enable fluorescence lifetime measurements and the determination of the (2p3d) 1P lifetime. Fluorescence timing measurements have also allowed the nearly-energy-degenerate ion states 3 2S, 3 2P and 3 2D to be separated through their different lifetimes to allow ion state specific cross sections as a function of energy to be extracted. These groups of measurements have stimulated new theoretical work which explicitly includes both the fluorescence and autoionisation decay routes to reveal for some states a branching ratio in favour of fluorescence.