Simple semiclassical optical model of harpooning elementary proceses K + Br2

The ionic rearrangement, chemi-ionization, and dissociation processes in collisions K + Br₂ which proceed via the harpooning mechanism, are calculated in a classical trajectory approach using a semiclassical optical model for treating the electronic transition. For the transition matrix element in dependence on the nuclear configuration an approximate analytic expression, based on a one-electron model is derived, the ionic potential energy surface is represented by a semiempirical model function with polarization. Total and differential cross sections are obtained in qualitative agreement with molecular beam data. The results depend critically on potential and interaction parameters, inclusion of classically forbidden transitions is important. By an analysis of the product internal-state and translational-energy distributions the mechanism of the different processes is discussed.     

Model calculations of harpooning elementary processes in the system K + Br2

The dynamics of non-adiabatic elementary processes in the system K + Br₂ (exchange reaction, electron transfer, dissociation) are investigated using the quasiclassical trajectory method. Based on a estimation of the transition probability from the covalent to the ionic potential surface, the treatment is restricted to a calculation of the motion (1D, 2D, and 3D) on the ionic potential which is represented by a model function. Differential and total cross sections are determined, briefly discussed and compared with molecular-beam data.     

Transformation of spin-orbit excited states of halogen atoms in the chemical reactions F + Br2 → BrF + Br,I*+ Br2 → IBr + Br,and Br + IBr → Br2 + I

The small laser pulse gain method, based on photochemical Br and I lasers, is used to probe ²P_3/2 and ²P_1/2 states of iodine and bromine atoms in the reactions F + Br₂ → BrF + Br (I), I(²P_1/2) + Br₂ → IBr + Br (II), and Br + IBr → Br₂ + I (III). The results obtained are capable of formulating a conservation rule for the spin-orbit excited state.     

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.     

Calculations of CI2 Br2 potential curves

The asymptotic method is used to calculate Cl₂ and Br₂ potential curves up to distances slightly exceeding those where strong chemical interaction takes place. All the curves originating from the ground atomic configuration fine structure levels are considered. The calculated XO⁺_g and BO⁺_u curves are compared with the available RKR curves, revealing the satisfactory accuracy of the asymptotic method.     

On-the-energy-shell optical potential for He-H2 vibrational excitation

The one-the-energy-shell optical potential is calculated for vibrational excitation in atom-diatom collisions. Effects of energies and potential energy parameters on the imaginary part of the on-shell optical potential is studied. It is shown that detailed collisional dynamics can be investigated from the optical potential studies.     

Total cross sections for K + Br2 at relative energies between 0 and 8000 eV

Trajectory Surface Hopping (TSH) calculations have been applied to the non-elastic scattering in the K + Br₂ collision system over a wide range of relative kinetic energies from 0 to 8000 eV. Absolute total cross sections have been computed for the formation of various collision products with an accuracy of 5% with respect to statistical errors. The following non-elastic processes have been studied: chemical reaction, inelastic neutral scattering, neutral dissociation and ion pair formation, yielding atomic as well as molecular negative bromine ions together with PC ions. The absolute values of the respective total cross sections, obtained from the TSH calculations, are in close agreement with the available experimental data, both for chemical reaction and for ion pair formation, over the whole energy range considered. The three particle character of the collision system is important in describing the experimental results quantitatively at relative kinetic energies below 100 eV.     

The dissociation energies of gaseous Br2 and I2

New rotational analyses have been made of the B³Π_0⁺u—X¹Σ⁺_g systems of ⁷⁹Br₂, ⁸¹Br₂ and ¹²⁷I₂. The density of vibrational states near the dissociation limit in the upper states follows the LeRoy—Bernstein predictions for n = 5. From short extrapolations, the ground state dissociation energies are found to be: D₀(⁷⁹Br⁸¹Br) = 15895.6 cm^?1, D₀(¹²⁷I₂) = 12440.1 cm^?1.     

Molecular beam kinetics: The excitation function of the reaction Xe(3P0,2) + Br2 → XeBr* + Br

Fast beams of metastable xenon atoms, Xe(³P_0,2), have been generated by combining the high speed rotor technique with electron bombardment excitation. In a crossed-beam apparatus, the excitation function σ(E_t) of the reaction Xe(³P_0,2) + Br₂ → XeBr* + Br has been measured. The results show a close similarity to the behaviour of the alkali metal atoms, with which the metastable rate gas atoms have been previously compared.     

Investigation on the mechanism and applications of the reaction Cl2+2HBr=2HCl+Br2

The mechanism of reaction Cl₂+2HBr=2HCl+Br₂ has been carefully investigated with density functional theory (DFT) at B3LYP/6-311G^** level. A series of three-centred and four-centred transition states have been obtained. The activation energy (138.96 and 147.24 kJ/mol, respectively) of two bimolecular elementary reactions Cl₂+HBr→HCl+BrCl and BrCl+HBr→HCl+Br₂ is smaller than the dissociation energy of Cl₂, HBr and BrCl, indicating that it is favorable for the title reaction occurring in the bimolecular form. The reaction has been applied to the chemical engineering process of recycling Br₂ from HBr. Gaseous Cl₂ directly reacts with HBr gas, which produces gaseous mixtures containing Br₂, and liquid Br₂ and HCl are obtained by cooling the mixtures and further separated by absorption with CCl₄. The recovery percentage of Br₂ is more than 96%, and the Cl₂ remaining in liquid Br₂ is less than 3.0%. The paper provides a good example of solving the difficult problem in chemical engineering with basic theory.     

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.     

Laser induced photodissociation spectroscopy: Br2

The continuous absorption spectrum of molecular bromine has been examined using laser induced photodissociation spectroscopy. In this technique, Br₂ molecules are photolyzed using a flashlamp-pumped dye laser; the atomic products of the dissociation are then monitored by time-resolved resonance absorption spectroscopy in the vacuum ultraviolet. The relative absorptivities for the transitions B³Π_o⁺u ← X¹Σ⁺_g and ¹Π_1u ← X¹Σ⁺_g have been obtained at 18350, 21010 and 22125 cm⁻¹.     

Total reactive cross section for K + Br2 in the energy range of 0–4 eV

3-D classical trajectory calculations were performed using diabatic as well as adiabatic potential energy surfaces. Non-adiabatic transitions were allowed and localized at the avoided crossing of the two adiabatic surfaces. The transition probability was calculated according to the Landau—Zener formalism. The total cross sections for the reaction K + Br₂ → KBr + Br were calculated and compared with experimental data. The total cross sections, calculated with the aid of adiabatic potential energy surfaces, were, contrary to those with diabatic surfaces, in very good agreement with the measured total reactive cross sections over the whole energy range of 0–4 eV.     

Reactive scattering of Sn from Cl2 and Br2 and a lower limit on D00 (SnBr)

Laboratory (lab) angular and time-of-flight (TOF) recoil velocity spectra of SnCl product formed by crossing a thermal Sn beam with a Cl₂ nozzle beam have been measured in a molecular beam apparatus equipped with an electron bombardment ionizer—quadrupole mass filter detector unit. Product center of mass (c.m.) recoil angle and energy distributions have been fit to the data by convoluting the c.m. → lab transformation over measured beam speed distributions and the instrumental TOF resolution. The inferred distributions for the Sn + Cl₂ reaction are strikingly similar to those reported for the Li + Cl₂ reaction. In addition, SnBr product was observed from crossed beams of Sn and Br2 which establishes a lower limit for D⁰₀ (SnBr) of 131 kJ/mole.     

X-ray study of Hg2Cl2Br2 and HgCl2HgBr2 reactions in solid state

The reactions (I) Hg₂Cl₂(s) + Br₂(g) and (II) HgCl₂(s) + HgBr₂(s) have been investigated by an X-ray method. Both the reactions yield two forms of the mixed halide HgClBr, designated as α-HgClBr and β-HgClBr. The cell parameters of the two are as follows:α-HgClBr: $\text{a = 6.196}\text{A}\text{?}$, $\text{b = 13.12}\text{A}\text{?}$, $\text{c = 4.37}\text{A}\text{?}$, z = 4, ? = 5.91 g/cm³. The powder pattern and cell parameters are similar to that of HgCl₂. Therefore it is probable that the chlorine atoms, in the linear halogenHghalogen molecules of HgCl₂ structure have been replaced by bromines, and since the radius of the bromine atom is larger than that of chlorine, the lattice is larger in this case.β-HgClBr: $\text{a = 6.78}\text{A}\text{?}$, $\text{b = 13.175}\text{A}\text{?}$, $\text{c = 4.17}\text{A}\text{?}$, z = 4, ? = 5.40. These parameters are the same as those reported in the literature for β-Hg(ClBr)₂, and its X-ray powder pattern is similar to HgCl₂. Therefore this phase also has linear halogenHghalogen molecules but the distribution of Cl and Br atoms is perhaps random.Heating the products (I) and (II) up to the melting point increases the amount of α phase and decreases the β phase, whereas crystallization increases the β phase. DTA study has supported the X-ray findings.     

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.     

Quantum calculations of vibrational harmonic energy transfer in collinear collisions of X2Y2 + A and X2Y2 + A2

Quantum calculations of translational-vibrational energy transfer in atom-four-atom and diatom-four-atom collinear collisions are reported. Energy transfer shows a strong dependence on the reduced mass of the collision system and on whether the incoming particle is an atom or a molecule. Atom-molecule collisions are more efficient for energy transfer purposes.     

A new look at rotational and vibrational excitation in the scattering of Li+ from N2 and CO at energies between 4 and 17 eV

New experimental time-of-flight distributions are reported for Li⁺-N₂ and Li²-CO at two center-of-mass energies of about 8 and 16 eV and large scattering angles θ_lab ? 120°. The Li⁺-N₂ spectra show two widely spaced maxima, whereas the Li⁺-CO spectra show two and sometimes three maxima. The results are consistent with the model of rotational rainbows, and have also been analyzed in terms of an impulsive model involving collisions with the individual atoms of the molecules with energy-dependent masses. Classical trajectories for a simple model potential reveal only small contributions from vibrational excitation.     

The emmission spectrum of Br2 in argon

We have observed and identified positive ions produced in a spark discharge in SF₆ at high pressure. The ion species were found to be very sensitive to impurities. In purified SF₆ the ions were sulfur-rich and complex before reacting to form chemically stable long-lived products.