Dynamics of the reaction H2+(He,H)HeH+. Influence of various forms of reactant energy on the total and differential cross section

Results of quasiclassical trajectory calculations of reactive processes between He atoms and H₂⁺ (υ, J) molecular ions in the collision energy interval 0.5–5.0 eV (c.m.) for a large number of selected υ, J combinations are analyzed with respect to the influence of the initial translational, vibrational, and rotational energy on the total and differential reaction cross sections. Vibrational energy is more effective in promoting the reaction than translational energy. Small rotational excitation has a negligible effect, whereas high rotational excitation has a similar influence on the reaction cross sections as the vibrational excitation of the same magnitude.     

Anharmonicity in rotational and vibrational excitation of H2 by Li+ collisions

Integral cross sections for pure rotational and vibrational-rotational excitation of H₂(X¹Σ⁺_g) by Li⁺(¹S) impact are computed by close-coupling methods at 0.2, 0.6, and 1.2 eV in the c.m. system using vibrational functions that are numerical solutions of the one-dimensional radial Schrödinger equation for harmonic, Morse, and adiabatically corrected Kolos-Wolniewicz (KW) potential functions. Comparison of results employing KW and Morse functions shows excellent agreement for all transitions studied. Findings using harmonic oscillator functions, however, differ noticeably from KW and Morse values for vibrational (0 → 1) and very large rotational (Δj = 10) transitions, but are satisfactory for lower order (0 → 2, 4, 6, 8) rotational transitions.     

Exploring the reaction dynamics of O(3P)+ (X2 )→OH+(X3Σ−)+ H(2S) reaction with time‐dependent wave packet method

The O(³P)+ reaction has been investigated by employing time‐dependent quantum wave packet with split operator method on potential energy surface of the doublet ground‐state H₂O⁺(1²A″). The reaction probabilities and integral cross sections are calculated using centrifugal sudden approximation, which basically agree with the quasi‐classical results of Paniagua et al. [Phys. Chem. Chem. Phys. 2014, 16, 23594]. Moreover, the effect of vibrational and rotational excitation of reactant is investigated. The results show that the vibrational and rotational excitation effects on the integral cross section are not obvious. The little differences between Coriolis coupling results and centrifugal sudden approximation ones show that the cheaper centrifugal sudden calculations here reported are effective for this reaction.     

Close-coupling elastic and inelastic integral and differential cross sections for Li+ + H2 collisions

The quantum mechanical close-coupling formalism is applied to the study of elastic and rotationally inelastic Li⁺ + H₂ collisions making use of the Kutzelnigg-Staemmler-Hoheisel potential energy surface. Integral and differential cross sections for j = 0 → 0 and j = 0 → 2 are obtained in the collision energy range 0.2 to 0.9 eV and for j = 1 → 1 and j = 1 → 3 at 0.6 eV. A rainbow structure is observed in both the elastic and inelastic angular distributions and a quenching of the fast oscillations is found in the cross sections for j = 1 initially compared to the case j = 0 initially. At 0.6 eV. the calculated quantum mechanical angular distributions are compared to those from a classical trajectory calculation using the same surface and to the experimental ones. The dynamics of rotational excitation in the Li⁺ + H₂ system is contrasted to rotational excitation in systems for which the atom-diatom interaction is predominantly repulsive.     

The reaction Ne+H 2 + (v=0, 1, 2, 3, 4)→NeH++H: 3D potential energy surface and quasiclassical trajectory calculations

A three-dimensional potential energy surface for the endoergic reaction Ne+H ₂ ⁺ →NeH⁺+H in the² A′ ground state of the system NeH ₂ ⁺ has been calculated by quantum chemical ab initio methods (CEPA approximation). The calculated points on this surface were fitted to an analytic ansatz in terms of an extended LEPS functional form augmented by a correction function. The latter was expanded in polynomials in inverse powers of the internuclear distances. This analytic form was used for quasiclassical trajectory calculations of reactive cross sections. In agreement with experimental investigations a strong vibrational enhancement is observed, i.e. the reaction is markedly favored if the necessary reaction energy is supplied as vibrational energy of H ₂ ⁺ rather than as relative translational energy. Other properties of the reaction dynamics such as the backward to forward scattering ratio, the lifetime of the collision complex NeH ₂ ⁺ , and final rotational and vibrational state distributions are also discussed on the basis of the quasiclassical trajectory calculations.     

Time-dependent close coupling for vibrational excitation in three dimensions: Application to H+ - H2

Impact parameter calculations for the non-reactive H⁺ + H₂ (n_i = 0) → H⁺ + H₂ (n_f) collision are reported for energies 10 eV ? E_cm ? 200 eV describing the rotational motion of the molecule in the sudden limit. The time-dependent Schrödinger equation for the vibrational motion has been solved by close coupling techniques expanding the vibrational wavefunction into both harmonic and numerically exact H₂ bound states. The convergence in vibrational basis sets, where up to six vibrational levels are considered, becomes worse with decreasing energy and increasing inelasticity. Furthermore, the harmonic wavefunctions are not suitable over a large range of energies to calculate proper cross sections. The various integral and differential cross sections have been compared with the classical results of Giese and Gentry.     

Predissociation of the c3Πu state of H2, populated after charge exchange of H2+ with several targets at keV energies

A detailed study of the predissocitation of the c³Π_u state of H₂ has been made with a new, very sensitive, experimental technique. A resolution better than 1% is obtained in the measurement of the released kinetic energy of HH pairs after charge exchange of H₂⁺ with Ar, H₂, Mg, Na and Cs by detecting both fragments with a time- and position-sensitive microchannel-plate detector. Eighteen vibrational levels of the c³Π_u state can be clearly distinguished in the range of 7.2–10.2 eV. Detailed information is extracted from the spectra with the aid of a convolution procedure. The vibrational energy levels of the c³Π_u state as calculated by Ko?os and Rychlewski are found to be correct within the experimental accuracy of 5 meV. Predissociative lifetimes are measured, yielding 6.2±0.5 ns for the lowest rovibrational level (υ = 0, N = 1), which are in good agreement with theoretical calculations of Comtet and de Bruijn. The cross section for charge exchange is observed to increase gradually with the vibrational level and seems to follow the geometrical cross section of the molecule. Rotational excitation during the charge exchange is also found to increase considerably with the vibrational quantum number. The final rotational temperature further depends strongly on the target gas used and increases with the resonance energy defect ΔI in the charge exchange collision.     

Two-photon dissociation of vibrationally excited H2+. Complex quasi-vibrational energy and inhomogeneous differential equation approaches

We present two practical theoretical methods — the complex quasi-vibrational energy and the inhomogeneous differential equation approaches — for numerical computation of multiphoton dissociation cross sections. The methods are applied to the study of the two-photon dissociation of H₂⁺ (1sσg). The cross sections are small for low-lying vibrational states but increase very rapidly with increasing vibrational quantum number, suggesting that experimentally accessible powerful lasers can be used to probe the highly excited vibrational states of the ground electronic state of a homonuclear diatomic molecule.     

Excited states of gaseous ions. V. The predissociation of the h2o+ ion

The B?² state of H₂O⁺ is predissociated twice. First, by the ã⁴B₁ state, giving OH⁺ + H fragments via spinorbit coupling interaction. Secondly, by a²A state, giving H ⁺ OH fragments via spin-orbit coupling and Coriolis interactions. A vibrational analysis of the photoelectron band of the B? state of H₂O⁺ and D₂O⁺ is carried out. This provides the vibrational frequencies of the H₂O⁺, D₂O⁺ and HDO⁺ ions, as well as a vibrational assignment of the peaks. The H₂O⁺ ion in its B?²B₂ state is found to have a OH bond length of 1.12 A and a valence angie of 78°.In order to describe the unimolecular fragmentation process, a distinction is introduced between the totally symmetric, optically active vibrational modes, and the antisymmetric ones which are coupled to the continuum. The former are supplied with photon or electron impact energy, but only the latter are chemically efficient. The dynamics of the dissociation process depends therefore on the couplings among normal modes. This is studied in the framework of two models. In Model 1, it is assumed that, as a result of the anharmonicity of the potential energy surface, only even overtones of the antisymmetric vibration are excited by Fermi resonance. In Model II, excitation of the odd overtones is provided by vibronic coupling. Model II is in better agreement with experiment than Model I. Calculated and experimental results have been compared on the following points: isotopic shift on the appearance potential of OH⁺ and OD⁺ ions, shapes of the photoionization curves, fragmentation pattern with 21 eV photons, presence of a unimolecular metastable transition, production of O⁺ ions. All the vibrational levels situated above the dissociation asymptote are totally predissociated. Autoionization is shown in this case to contribute only to the formation of molecular H₂O⁺ ions, and not to that of the OH⁺ fragments. For 21 eV electrons, the contribution due to direct ionization is calculated to represent about 25% of the total cross section, the rest being due to autoionization.     

C2 and CN formation by optical pumping of CO/Ar and Co/N2/Ar mixtures at room temperature

A continuous wave carbon monoxide laser is used to excite the vibrational mode of CO in CO/Ar and CO/N₂/Ar mixtures flowing through a gas absorption cell. High steady-state excitation of the CO vibrational mode (0.3 eV/molecule) is achieved, while a translational—rotational temperature near 300 K is maintained by the steady flow of cold gas into the cell. These non-equilibrium conditions result in extreme vibration—vibration pumping, population high-lying vibrational quantum levels (to V = 42) of CO. N₂ can also be pumped by vibrational energy transfer from CO. Under these conditions, C₂ and CN molecules are formed, and are observed to fluoresce on various electronic band transitions, notably C₂ Swan (A ³Π_g—X ³Π_u) and CN violet (B ²Σ⁺—X²Σ⁺).     

On the possibility of charge-transfer processes in low-energy proton- HF1Σ+ collisions

Potential energy surfaces for the (HFH)¹ system that asymptotically lead to H¹ + HF(¹Σ⁺) or to H(²S_g) + HF⁺(²Π)) fragments are compared over a wide range of relative geometries by using the results of ab initio calculations via the CEPA method and DIM model calculations. Recent molecular beam experiments on H⁺ + HF suggest negligible vibrational excitation, strong rotational excitation and do not detect any charge-transfer effect. Analysis of the relative shapes of the surfaces as functions of all coordinates involved shows structural reasons for the experimental behaviour and suggests qualitative dynamical arguments for excluding the possibility of temporary charge-transfer intermediates.     

Eclipsed and staggered conformations of (SIH3)2F+: An ab initio study

Ab initio calculations at 6–31G**, 6–31++G**, and MP2/6–31G** levels were performed on disilyl–fluoronium, (SiH₃)₂F⁺, with the SiH₃ group eclipsed or staggered. Optimized geometries, total energies, dipole moments, atomic charges, electronic density, and vibrational frequencies were computed. The results were compared with calculated structural parameters and vibrational frequencies of H₃SiF, H₂SiF⁺, H₂SiF^?, and H₄SiF⁺ ions. The basis-set effects were studied. Several thermochemistry parameters—ZPE, thermal energy, rotational constants, and entropies—were also calculated. © 1994 John Wiley & Sons, Inc.     

Rotational energy transfer in NO (A2Σ+, v = 0 and 1) studied by two-color double-resonance spectroscopy

Collision-induced rotational relaxation in the A²Σ⁺, v = 0 and 1 states of NO has been measured by using step-wise double-resonant ionization spectroscopy. Multiple quantum rotational energy transfer is occurring to at least |ΔJ| = 6 and the observed cross sections ranging from tens to hundreds of A² are larger than the gas-kinetic collisional cross section. The energy-transfer efficiency is slightly enhanced by the vibrational excitation. Energy-based scaling laws are successfully applied to reproduce the observed rotational distribution.     

A quasiclassical trajectory study of the collisional dissociation of H2 by H atoms

The collision induced dissociation of H₂ by H atoms was studied by quasiclassical trajectories using the Liu-Siegbahn-Truhlar-Horowitz potential energy surface. Dissociation cross sections were obtained for five highly internally excited initial states of H₂ for translational energies up to 100 kcal mol^?1. Rate constants for dissociation out of these states were calculated for temperatures of 1000 to 10,000 K. Initial internal energy strongly enhances the probability of collisional dissociation, vibrational energy being more effective than rotational. The results are compared to those from a similar study of the H₂?He system, and are discussed in relation tothe respective potential energy surfaces. The implications for the kineticsof thermal dissociation are also considered.     

Triatom-triatom collisions: Fixed angle close-coupling study of vibrational excitation in the 12CO2+13CO2 system

A close-coupling approach to the calculation of quantal vibrational transition probabilities for the fixed angle scattering of a linear triatomic molecule with another linear triatomic molecule is described. The method is applied to the ¹²CO₂+¹³C0₂ collisional system. For a calculated inelastic transition probability to have an appreciable magnitude, it is found that the amount of energy transferred in a transition must be very small and just one quantum of energy must be exchanged between either the symmetric stretch or the asymmetric stretch vibrational modes of ¹²C0₂ and ¹³CO₂. For collisional energies away from threshold, the probabilities for transitions involving the symmetric stretch ¹²CO₂ and ¹³CO₂ modes are insensitive to long range multipole terms in the potential energy surface, while the probabilities for energy exchange between the asymmetric stretch modes are considerably diminished when the long range terms are removed from the potential energy surface. A brief discussion is presented on the possibilities of extending the technique to the calculation of vibrational excitation cross sections for three-dimensional triato—triatom collisions.     

Dissociative excitation of water by metastable argon

The reaction Ar(²P_2,0) + H₂O → Ar + H + OH(A²Σ⁺)was studied in crossed molecular beams by observing the luminescence from OH(A²Σ⁺). No significant dependence of the spectrum on collision energy was found over the 22–52 meV region. Spectral simulation was used to obtain the OH(A) vibrational distribution and rotational temperature, assuming a Boltzmann rotational distribution. Since predissociation is known to strongly affect the rovibrational distribution, the individual rotational state lifetimes were included in the simulation program and were used to obtain the average vibrational state lifetimes. Excellent agreement with experiment was obtained for vibrational population ratios N₀/N₁/N₂ of 1.00/ 0.40/0.013 and a rotational temperature of 4000 K. Correction for the different average vibrational lifetimes gave formation rate ratios P₀/P₁/P₂ of 1.00/0.49/0.25. The differences between these results and those from flowing afterglow studies on the same system are discussed. Three reaction mechanisms are considered, and the vibrational prior distributions are calculated from a simple density-of-states model. Only fair agreement with experiment is obtained. The best agreement for the mechanisms giving OH(A) in two 2-body dissociation steps is obtained by assuming 1.0 eV of internal energy remains in the second step. The OH(A) vibrational population distribution of the present work is similar to that found in the photolysis of H₂O at 122 nm, where there is 1.10 eV of excess internal energy.     

Fully converged sudden rotation total integral cross sections for 4He + H2 (ν = 0, j = 0) → 4He + H2 (ν′ = 1, j′)

Total integral cross sections for ⁴He + H₂ (ν = 0, j = 0) → ⁴He + H₂ (ν′ = 1, j′ = 0, 2) have been calculated in the total energy range 1.2 to 5.5 eV, according to a quantal sudden approximation for the H₂ rotational degrees of freedom and a close coupling expansion of the vibrational degree of freedom. Convergence of the above cross sections is investigated by employing four vibration basis sets in the close coupling calculations, i.e., ν = 0,1, ν = 0,1, 2, ν = 0, 1, 2, 3 and ν = 0, 1, 2, 3, 4. Between 4.2 and 5.5 eV calculations were done with three vibration basis sets; ν = 0.–4, ν = 0–5, and ν = 0–6. It is found that at least four vibrational basis functions are needed to converge (to within 5–10%) these cross sections in the above energy range. Comparison of breathing sphere calculations and summed sudden rotation results shows good agreement for the (weakly anisotropic) Mies-Krauss potential. However, as expected the former results underestimate the vibrational 0 → 1 total integral cross sections.     

Cross sections and rate constants for the vibrational relaxation of D2 (υ = 1,j = 0) in collisions with 4He

Fully converged quantum cross sections for ⁴He—D² (υ = 1,j = 0) vibrational relaxation were determined using the coupled-states method and a modified version of the Gordon—Secrest surface. First-order forbidden rotational transitions play a significant role, comparable to that observed previously for the He—H₂ system. At 60 K the υ = 1,j = 0 level of D₂ is predicted to relax ≈4 times slower than the corresponding level of H₂. This difference decreases with increasing temperature.     

A model potential for vibrational excitation of diatomic molecules

A model interaction potential for the vibrational excitation of H₂ by Li⁺ of the form C exp(−αx+βy) is studied using non-iterative integral techniques. The results show that vibrational excitation is sensitive to small quantitative change in qualitatively similar surfaces.