John L. Magee

Capture Collisions between Ions and Polar Molecules

The manner in which a permanent dipole in a molecule affects capture collisions of the molecule with ions is investigated. Analytical capture cross sections are considered, and three cases are distinguished. None of these is strictly applicable to real collisions, but they are useful in establishment of the general magnitudes of cross sections. The Langevin cross section is somewhat of a lower limit. A strict upper limit to the cross section is derived; it depends on the magnitude of the dipole moment. A quantum‐mechanical treatment at small relative velocity uses the effective potential of interaction determined by the Stark effect; cross sections for this case depend explicitly on the rotational energy of the polar molecule. Molecules which have a first‐order Stark effect have a term e−1 in their cross sections, whereas the second‐order Stark effect only leads to the e−1/2 term of the Langevin theory (e is the relative translational energy).Numerical cross sections are obtained by using a computer study...

Theory of Radiation Chemistry. IV. Chemical Reactions in the General Track Composed of N Particles

The chemical reactions which occur in particle tracks are diffusion‐controlled. In this paper a set of partial differential equations is presented which describes the dissipation of a track composed of N particles. These equations involve functions which can be interpreted as correlation functions for the relative distribution of the particles. A superposition approximation is introduced which greatly simplifies the equations and a method for obtaining the correlation functions is presented.As an example of application of these equations a model for the action of ionizing radiation on water is discussed. It is shown that the radiation chemistry of water is adequately described for reasonable values of the parameters of the model.

THEORY OF RADIATION CHEMISTRY. III. RADICAL REACTION MECHANISM IN THE TRACKS OF IONIZING RADIATIONS

This paper develops a model for the dissipation of the tracks of high‐energy particles. It is assumed that all chemical effects are due to one kind of radical and that there are no effects of overlapping of neighboring tracks, i.e., that there is a sufficiently high concentration of scavenger to prevent such overlapping. The model of Samuel and Magee is used and extended to take into account the interaction of randomly distributed spurs along the track. Calculated values of the extent of scavenger reactions and radical reactions are presented in graphical form. General trends of experimental results to be expected according to the model are indicated. Quantitative correlation with experiments was not attempted because of the uncertainty in the values of various parameters used and because of the serious limitations of the one‐radical model of tracks.

The Energy of the Hydrogen Molecule

A simple two‐parameter variational function has been used to calculate the energy of normal H2. The first parameter is the usual scale factor; the second parameter is a distance of displacement of the atomic orbitals (see Fig. 1). The best potential energy is obtained as approximately 96 kcal per mole for internuclear separation 1.45 a0. The behavior of the new parameter as a function of internuclear separation is discussed.

The early events of radiation chemistry

Abstract A critical review of the early events of radiation chemistry is presented with special reference to aqueous solutions and liquid hydrocarbons. It is concluded that a judicious combination of both direct and indirect methods must be used to get a complete picture of the early events. Proper interpretation of scavenging experiments plays a key role in the indirect methods. Contact reactions at very short times (≈ 10 −13 s) must also be considered in concentrated solutions. The aim of the paper is not to develop any particular model in detail but to analyze the natural limitations of model construction. It appears that, for different reasons, there are “barriers” on the short time scale for both direct and indirect ways of investigating early events. The proper use of indirect methods combined with early real-time measurements allows investigation of events behind these barriers.

A theoretical survey of the radiation chemistry of water and aqueous solutions.

In this paper the various kinds of available evidence are examined in an attempt to determine the detailed mechanism of the action of high-energy radiation on water and aqueous solutions. The analysis employed is not exhaustive, but rather is concerned with effects which are presently believed to be most important. For example, it is now generally accepted that local heating, as such, is relatively unimportant and that most effects of high-energy radiations are due to electronic excitation, ionization, and dissociation of molecules. Specifically, in the case of water, it has for some years been customary to explain most radiation phenomena on the basis of action of H and OH radicals formed in irradiated water. In most respects such a radical mechanism seems to be able to explain experimental results, although it is by no means certain that other entities are unimportant. A detailed description of the radiolysis of water must be in agreement with experimental and theoretical requirements which can be put into two categories: a. Requirements from chemical experiments. b. Requirements from physical and theoretical considerations. At present it appears that the requirements of chemical experiment are better defined than those of theory, and for this reason they are discussed first. It is well known that a complete theoretical description of the tracks of ionizing radiations in water has not been given. The great unknown is the fate of the slow electron from ionization processes and, as we shall see, this uncertainty makes the geometrical arrangement of H and OH radicals impossible to predict from theory. An attempt has been made to draw conclusions, from the assembled requirements, regarding the nature of the radiolysis mechanism. This attempt is aided by a detailed calculation made by Samuel and Magee (1) on a radical diffusion model. Such a model was suggested by Lea (2, 3) and Allen (4), but no detailed study had previously been made. A critical examination of this model is made on the basis of experimental results.

The Clustering of Ions in Irradiated Gases

The general theory of the clustering of ions in irradiated gases is considered. A systematic comparison of theory with experiment is impossible due to lack of data, however a limited comparison is made, and a brief consideration of the effects of clustering on the radiation chemistry of gases is given. (J.R.D.)

Evidence for long-lived ion-molecule collision complexes from numerical studies

Abstract Collision lifetimes τ c have been calculated numerically for ion-molecule collision. Dipoles cause multiple reflections leading to τ c values much longer than specular reflection periods. Langevin collisions yield simple specular reflections. The interaction has been studied via a computer-plotter system.

Dissociation Processes in Electronically Excited Molecules. Linear Chain Model

Dissociation mechanisms in a chain of coupled molecules have been considered. The attempt has been made to look at general features of the influence of electronic coupling on such processes. Actual calculations have only been made for the special case of a chain of H2+ ions. The weak‐coupling and strong‐coupling cases have been discussed explicitly, and various criteria for applicability of the special cases have been presented in terms of the physical constants of the system.

Central‐Field Approximation for the Electronic Wave Functions of Simple Molecules

The use of a central‐field type of molecular orbital for the electronic wave functions of simple molecules has been investigated and application has been made to H2, H2O and CH4.Ionization potentials calculated with the use of these wave functions are generally in good agreement with experimental values. No empirical data have been used in these calculations except the known equilibrium dimensions of the molecules. Interactions of all electrons are included. Configuration interaction, however, is not used.Excitation potentials for H2O and CH4 are not in very good agreement with experimental values, which possibly means that configuration interaction cannot be neglected in such calculations.