Richard Wolfgang

KINETIC THEORY OF HOT-ATOM REACTIONS. II. EVALUATION OF COLLISIONAL ENERGY LOSS. APPLICATION TO REACTIVE MIXTURES

The kinetic theory of reactions of atoms of high translational energy undergoing moderating collisions is further developed. The nature and validity of the assumptions underlying the treatment and the range of applicability of the final expressions are discussed in detail. It is shown that experimental data on the total probability of reaction in which the hot atom is chemically bound can be used to evaluate two fundamental quantities: (1) the reactivity integral, or reaction probability integrated over energy on a logarithmic scale (I) ; (2) a measure of average energy transfer in collisions of hot atoms with the reactive species (α).The present treatment involves no variable or arbitrary parameters. As a result sufficient data may be obtained to provide stringent internal consistency tests of the applicability of the model and its assumptions to any given system.Two types of systems are specifically treated: (1) a single reactant moderated by inert gas; (2) competitive reactions in systems of two reacta...

A Crossed‐Beam Apparatus for Investigation of Ion‐Molecule Reactions

A crossed‐beam apparatus for the study of ion‐molecule reactions is described. The device can provide information on the velocity and angular spectra of reaction products as well as on the angular displacement between reactant and product ions. It can thus yield data sufficient to provide a detailed characterization of the dynamics of the system studied. Furthermore, the mass selected ion beam is variable in energy from 100 eV down to the critical low energy region of about 1 eV or less. Although the apparatus thus seems to have broader capabilities than previously described beam devices, it is quite small and comparatively inexpensive.

Beam studies of energy and lifetime dependence of unimolecular decay: the extent of internal equilibration

Abstract The reaction C 2 H 4 + + C 2 H 4 → CH 3 + C 3 H 5 + , known to pass through a persistent C 4 H 8 + complex, has been used to test accepted models of unimolecular decay. Experimental product translational energy spectra are interpreted using an expression derived from RRKM theory. Data and calculations agree well, but only if the assumption of complete energy equilibration prior to decay is discarded.

The reaction of atomic carbon with oxygen: significance for the natural radio-carbon cycle

The nuclear reaction responsible for producing natural radio-carbon in the upper atmosphere, N/sup 14/(n,p)C/sup 14/, can be duplicated in an atomic reactor. When C/sup 14/ atoms so produced are reacted with N/sub 2/-O/sub 2/ mixtures under a variety of conditions and when corrections are made for complicating radiation induced effects CO/sup 14/ is found to account for 90 to 100% of the C/sup 14/ produced. This result was confirmed by work with C/sup 11/ in much weaker radiation fields. Previously it was tacitly assumed that C/sup 14/ O/sub 2/ must be the product of such a reaction. Since C/sup 14/ as CO cannot be utilized in the photosynthetic process, suggested mechanisms for converting carbon monoxide into a chemical form suitable for entrance into the biosphere are reviewed. The relatively long mean life of atmospheric CO leads to the prediction that the specific activity of natural CO may quite possibly be greater than that of atmospheric CO/sub 2/ and therefore of the biosphere with which CO/ sub 2/ is in equilibrium. (auth)

Further Study of the Reaction of Hot Hydrogen with Methane

The reaction of hot hydrogen atoms with methane (hot hydrogen in the form of recoil tritium produced by nuclear reaction) has been studied in further detail. Earlier work had been incomplete because in the presence of moderator, small amounts of HT, the abstraction product, were produced by what were recognized as spurious processes. An exhaustive investigation of possible limitations of the recoil method indicated that if helium is used as a moderator, not all the recoil tritium reaches chemical energies as neutral ground‐state atoms. This finding is in accordance with the adiabatic principle. The use of neon or argon as moderators eliminated this difficulty and permitted estimation of the true yield of HT and the total reactivity integral for the reaction. The consequent complete analysis of the system results in several new significant findings: (1) Values of the reactivity integral IHT measured using a helium moderator are probably too high and require downward revision (other products are apparently ...

Crossed‐Beam Study of the Reaction N++O2→NO++O

The reaction N++O2→NO++O has been studied as a function of collision energy using the crossed‐beam apparatus EVA. This process is of considerable importance in the upper atmosphere. The system is also of particular interest because it may involve strong interactions between several intersecting or close‐lying electronic surfaces of NO2+. This is not the case for previously well‐studied ion–molecule reactions (e.g., Ar++D2), the dynamics of which are closely represented by simple models dominated by long‐range electrostatic forces (e.g., “polarization stripping”). Beams of N+ in the ground (3P) state were crossed with thermal O2 beams and the angular and velocity distributions of the NO+ product measured. The reaction exhibits the asymmetric distribution characteristic of a direct process. Its translational exoergicity (Q) declines with energy, but above 5 eV relative energy, becomes constant. These data were then corrected for angular and energy spreads of the beams. The position of the plateau in Q appea...

Billiard‐Ball Collisions in the Reactions of Hot Hydrogen Atoms. Isotope Effect on the Hydrogen Displacement in Methane

The nature of collisions leading to reactions of hydrogen atoms of high kinetic energy (in the form of recoil tritium) is investigated. Two extreme models, both originally due to Libby, are examined. One assumes very weak coupling by valence bonds in the molecule attacked, so that the reaction can be represented by a quasi‐elastic atom‐atom collision (billiard‐ball model) and the other assumes strong valence bond coupling (epithermal model). Calculations on the billiard‐ball (hard sphere) model of the reaction T*+CH4→H+CH3T leads to an estimate of its relative cross section as a function of energy. An isotope effect for reaction with CH4 and with CD4 is calculated and measured experimentally. Consideration of both the calculations and the experimental results indicate that the billiard‐ball model is unimportant in the reactions of gas phase hot hydrogen (as recoil tritium), and that reactions occur nearly exclusively at less than 10–20 ev of kinetic energy, by mechanisms involving strong bond coupling. Th...

Reaction of hydrogen atomic ions with hydrogen molecules: Experiment, ab initio theory and a conceptual model

Abstract Ab initio calculations on trajectories and non-adiabatic transitions in reactions between H+ and D2, are compared with experimental data on cross sections, and on angular and velocity distributions. Good agreement indicates the essential validity of the theoretical model. The latter provides interesting conceptual insights into basic kinetics.

Reactions of atomic carbon with π-bonded inorganic molecules

Abstract Atomic carbon (11C) produced by nuclear transformations has been reacted with O2, CO, CO2, SO2, N2, N2O, NO and NO2. With O2, CO, CO2, and SO2, 11CO is the only significant product. This is true even in liquid oxygen where there is no three-body restriction on CO2 formation. These findings suggest that the dominant mode of attack is “end-on” rather than by the usual “sideways π-complex” mode. An examination of the molecular orbitals involved indicates that this should in general be the preferred mechanism of carbon atom attack on π-bonded inorganic molecules. With N2, N2O, and NO the carbon atom can form a strong bond at either end of the molecule. In accordance with this both 11CO and 11CN are observed as major products. With NO2, 11CO is the major product, again as required by the “end-on” attack mechanism. These reactions occur with both hot and thermalized carbon atoms. Thus both hot and thermal 11C can undergo the exoergic reactions with O2 to form 11CO and with N2O to give 11CO and 11CN. By contrast the endoergic process in which 11C combines with N2 to form 11CN occurs only with hot atoms.

Energy Dependence of Energy Partition in Products of Direct Reactions: Crossed‐Beam Studies and a New Model

Initial kinetic energy dependence of reaction energy distribution between direct ion-molecule reaction products internal and translational modes from crossed beam experiments