Product Energy Distributions Research Articles

Quantum mechanical partitioning of kinetic energy in collision-induced dissociation

Kinetic energy distributions of atomic products of the collision-induced dissociation A+BC → A+B+C on a model triatomic reactive collinear system have been obtained for the first time by an accurate quantum mechanical method and compared with the results of quasi-classical trajectory calculations.

Reaction hamiltonian and the adiabatic approach to the dynamics of chemical reaction

A new approach for the evaluation of product energy distributions of chemical reactions is presented. The approach is characterized by two main features. The first is the introduction of a specific reaction hamiltonian (in second quantization formalism) which facilitates the description of a chemical reaction as a quantum transition from reactants to products. The second is the use of a specific adiabatic method to describe the dynamics of nuclear motion of both reactants and products. To illustrate the method the determination of the product energy distribution is discussed for OH + D → OD + H and CH + D → Cl + ID and carried out for the latter system.

Monte Carlo random walk calculations of unimolecular dissociation of methane

Microcanonical rate coefficients and product energy distributions are computed for the CH3+H and CH2+H2 dissociation channels of CH4 by a random walk procedure. The formulation is based on Slater theory, but uses Metropolis Monte Carlo procedures to average over the reactant phase space. We find that the convergence rates of the calculations for the CH4 system are less rapid than that obtained in previous studies on more simple systems involving the dissociation of argon clusters. The convergence rate is found to decrease as the complexity of the process increases. Thus, convergence of the rate calculations for the simple two-center elimination reaction to form CH3+H is found to be at least an order of magnitude faster than that for the three-center channel leading to CH2+H2. When convergence is obtained, the computed rates and product translational energy distributions are in good accord with previously obtained quasiclassical trajectory results. The computer time required to obtain converged results for the two-center reaction is substantially less than that needed for the corresponding trajectory calculations. It is likely that the calculations for the three-center channel will likewise be more efficient than trajectory calculations provided importance sampling is included in the Metropolis procedure.

Theoretical studies of tunneling processes in three-body exchange reactions of van der Waals rare gas dimers

Rare gas atom–diatom collisions of Ar+Ar2, Xe+Ar2, Kr+Xe2, Kr+Ne2, and Kr+NeAr have been investigated to determine the importance of tunneling processes in exchange and dissociation reactions involving van der Waals molecules. Reaction cross sections, angular distributions, and product-energy distributions have been computed using Monte Carlo quasiclassical trajectories. The effect of tunneling through the rotational barrier upon these quantities has been computed using WKB methods. The results show that metastable diatomic products with energies above the classical dissociation limit, but below the rotational barrier, play a significant role in the dynamics of both exchange and dissociation reactions. Lifetime distributions of such metastable dimers illustrate their importance in crossed molecular beam studies of rare gas systems. The WKB calculations indicate that a significant, and possibly measurable, number of the metastables dissociate by tunneling before they would reach the detector in a molecular beam experiment. Close agreement has been found between these calculations and statistical state-counting calculations of metastable product dimer lifetimes. Experiments are suggested that might permit the direct observation of tunneling in these systems.

How to measure the polarization of top quarks

The production of polarized top quarks will be reflected in the polarization of T ∗ mesons and can be observed experimentally through their dominant weak decays. It is shown that energy and transverse momentum distributions of top-meson decay products are effected and eventually enhanced or depleted by a factor between 0.5 and 1.5. Parity-violating effects in the lepton distribution from top decay could be observed.

Chemical reaction as a quantum transition

In this theory a chemical reaction is treated as a quantum transition from reactants to products. The approach leads to a Franck–Condon-like factor for the evaluation of product energy distributions. Second-quantization representation is used to enable a Hamiltonian for reaction to be defined. A specific adiabatic method is used to describe the dynamics of nuclear motion. The theory is applied to the reactions HO + D OD + H and ClI + D Cl + ID. Polyatomic photodissociation can also be treated by a similar formalism.

A quasiclassical trajectory analysis of the ionization-dissociation behavior of the van der Waals molecule ArH2

The ionic species ArH+2 may be formed in an ion–molecule collision or by ionization of the van der Waals molecule ArH2. The ion is unstable in either case, but its internal energy distribution and, hence, its dissociation behavior depends on the method of formation. A trajectory calculation is described which simulates the ionization-dissociation of ArH2, initially formed in a supersonic expansion. Cross sections for the possible dissociation channels and product energy distributions are obtained. The vibrational and rotational energy of H2 in the ArH2 precursor are determining factors in the dissociation behavior.

Reagent and product energy distributions for the reaction F + OH (ν) → HF(ν′) + O

The hydroxyl radical is generated in various vibrational states by three reactions: F + H2O → HF + OH (ν = 0), H + NO2 → NO + OH (ν ≤ 3), and H + O3 → O2 + OH (ν = 7–9). The OH so produced reacts with F atoms according to F + OH (ν) → HF (ν′) + O. The vibrational populations of OH and HF are obtained by the low pressure infrared chemiluminescene technique.

Change of angular and energy distributions in nuclear reactions with long-living intermediate states due to collisions with atoms during the reaction

The influence of collisions of the intermediate nuclear system with target atoms on angular and energy distributions of reaction products is investigated with respect to the exploration of reaction times.

Mechanisms of atom-exchange reactions in rare gas atom–diatom collisions: Kr+NeAr, Ar+ArKr, Kr+Ar2, Xe+Ar2

Atom-exchange and dissociation reaction cross sections, angular distributions, and product-energy distributions have been computed for the Kr+NeAr, Kr+Ar2, and Xe+Ar2 van der Waals reactions using Monte Carlo quasiclassical trajectories. Angular and product-energy distributions were also computed for the Ar+ArKr atom-exchange reactions. Detailed collision mechanisms are suggested to explain the cross section ratios for the atom-exchange branching for the two reactions. The trajectory results indicate that the branching ratio is the result of a competition between statistical factors, which tend to favor the more exothermic pathway, and mechanistic factors which tend to favor abstraction of the lighter atom of the reactant diatom through a stripping mechanism.

Effect of the initial orientation on the reaction attributes for Li + FH → Lif + H on an AB initio surface

Results of a three-dimensional quasiclassical trajectory investigation of the orientation dependence of the reaction cross section and the product energy distribution are reported for the title reaction on an ab initio potential-energy surface. The reaction is preferred at the F end rather than at the H end of the molecule. Also, the attack by the Li atom at the F end of the molecule leads to a large product rotational excitation while a broadside attack favors a higher vibrational excitation. This is attributed to the collissions occuring at larger impact parameters for the former than for the latter.

Distinctive features of heavy ton dissipative collisions

Examples of experimental angular, charge (mass) and energy distributions of reaction products from damped heavy-ion reactions are presented. Correlations between these observables, as well as with the associated light particles emitted in the deexcitation stage, are illustrated and shown to give important insights into the reaction mechanism of dissipative collisions. Comparisons of experimental results with dynamical transport calculations based on one-body nucleon exchange give strong evidence that stochastic exchange of independent nucleons account for the dominant part of the dissipative and fluctuative phenomena observed in damped reactions.

Dynamics of ethyl radical decomposition. II. Applicability of classical mechanics to large‐molecule unimolecular reaction dynamics

AbstractThe classical trajectory method is used to investigate the unimolecular dynamics of ethyl radical dissociation. It is found that chaotic trajectories need not be backward integrable to yield accurate lifetime, and product energy and angular momenta distributions. This allows the use of large numerical integration step sizes in trajectory calculations. The product energy and angular momenta distributions are independent of the ethyl radical lifetime, and are obtained after only 50 dissociation events. Differences between classical and quantal unimolecular dynamics are discussed, and a prognosis for future trajectory studies of large‐molecule unimolecular decompositions is given.

Detailed energy partitioning in the decomposition of chemically energized C 2H 3F

The HF product energy distribution in the reaction of F atoms with ·C 2H 3 radicals has been directly measured using a modification of the arrested relaxation infrared chemiluminescence technique in which the vinyl radicals are prepared and react to form the observed products in successive chemical reactions. The experimentally observed vibrational and rotational distributions are correctly reproduced by a calculation in which the reaction exoergicity is partitioned statistically among the products of the unimolecular decomposition of energized vinyl fluoride. Some properties of the transition state for this process are thereby elucidated. The statistical energy partitioning likely results from the relatively high (125 kcal mol −1) excitation of the vinyl fluoride intermediate.

The dynamics of the reaction H+3+D2→HD+2+H2, and the effects of reactant vibrational excitation on the product energy and angle distributions

The dependence of the absolute total cross section and the product ion kinetic energy distribution on the reactant kinetic energy have been measured for the reaction H+3+D2→HD+2+H2 over the energy range from 0.002 to 11 eV, by means of the merged molecular beam technique. At low collision energies, the reaction occurs with high probability per close collision, and at all collision energies the product ion is scattered preferentially in the direction of the D2 reactant in center-of-mass coordinates. Because the reactants in these experiments are vibrationally excited, comparison of the present results with the crossed-beam data of M. L. Vestal, C. R. Blakly, P. W. Ryan, and J. H. Futrell [J. Chem. Phys. 64, 2094 (1976)] reveals the influence of reactant vibration on the reaction dynamics.

Mechanism of the metastable reaction H 2S + → S + + H 2; product energy distributions and their dependence on temparature

The fragmentation of metastable H 2S + ions to produce S + and H 2 has been investigated by mass analyzed ion kinetic energy spectrometry (MIKES). Kinetic energy release distributions have been derived from the MIKES peaks. Resolved structure is evident in the release distributions. When the metastable reaction is assigned to the specific H 2S +( 2A 1) vibronic, levels (known from fluorescence lifetime measurements to lie in the metastable lifetime range) good correlation is found between the position of the structure in the distributions and predicted transition energies. The data indicate that substantial rotational excitation of the product H 2 is occurring. The average kinetic energy released in the metastable fragmentation was found to increase with ion source temperature. This increase in kinetic energy with temperature is ascribed to the release of the increase in rotational energy of H 2S + as relative kinetic energy of the metastable products. A detailed analysis of the mechanism for the metastable reaction is given.

Reaction of boron(1+)(1Sg) with molecular deuterium

The angular and energy distribution of ionic products from the reaction of B/sup +/(/sup 1/S/sub g/) with D/sub 2/ was studied over a projectile energy range of 10 to 50 eV in the laboratory system. The product states as determined from the limiting values of the translational exoergicity were BD/sup +/(X/sup 2/..sigma../sup +/) and BD/sup +/(A/sup 2/II). The experimental measurements and theoretical considerations suggest that both of these reactions are proceeding through a persistent complex.

Chemical energy release in one-dimensional detonation waves in gaseous explosives

The Zel'dovich-von Neumann-Doring (ZND) model of a one-dimensional, steady state detonation wave in a gaseous explosive is extended to include the thermal relaxation processes which precede and follow the exothermic chemical reconstitution reactions. In this nonequilibrium model the detonation wave consists of four main zones: a very thin leading shock front in which the unreacted explosive mixture is compressed and accelerated in the direction of shock propagation; a much thicker relaxation zone in which the rotational and vibrational modes of the unreacted explosive gases approach thermal equilibrium; a relatively thin zone in which the chemical energy is released by rapid chain propagation and branching reactions into highly vibrationally excited reaction product gases; and another very thick relaxation zone in which the product gases expand and vibrationally relax toward thermodynamic equilibrium at the Chapman-Jouguet (CJ) state. Quantitative calculations for the H 2Cl 2, O 3, and H 2O 2 systems based on experimental chemiluminescent and crossed molecular beam product energy distribution data show that the amount of chemical energy initially released as product vibrational energy is greater than the sum of CJ equilibrium vibrational energy plus the amount of chemical energy required to sustain the constant velocity shock front. The physical mechanism by which this vibrational energy sustains the shock front is postulated to be the chemical amplification of transverse pressure waves, which propagate through the vibrationally excited products and eventually overtake the leading shock front.

Total angular momentum conservation in statistical unimolecular theory should be represented in energy space

A representation of total angular momentum conservation in energy space is described. Several advantages of such a representation are discussed, primarily in connection with statistical unimolecular theory, experimental branching ratios and product energy distributions. Specific examples are given for the systems K + CsF and O + I 2.

Crossed-beam studies of the reaction of O − with allene

Product energy and angular distributions have been measured for the exoergic reaction O − + C 3H 4 (allene) → OH − + C 3H 3 over the relative energy range 4.6–10.8 eV (6.5–15.3 eV lab). The reaction mechanism is found to be direct and well-approximated by the spectator-stripping model. We see no evidence of carbanions being produced in this energy region.