Recombination Of Molecular Positive Ions With Electrons

Results of studies of the recombination of electrons with molecular positive ions are presented. Theoretical arguments indicate that the dissociative process -- in which a molecular ion captures an electron without change of energy, forming an unstable excited molecule which then dissociates -- provides a highly efficient recombination mechanism. The nature of the process responsible for the large recombination rates observed in Laboratory afterglow studies is investigated using combined microwave, optical spectrometric and interferometric, and mass spectrographic techniques. Dissociative recombination is shown to be the process operative in the laboratory studies, since it is found that (a) molecular positive ions must be present to observe a large recombination rate, (b) when diatomic ions are present, excited atoms are formed in the recombination process, and (c) these excited atoms have kinetic energy of dissociation substantially in excess of thermal energy. (Author).

Information obtained from laboratory experiments and theoretical calculations concerning electron ion and ion-ion recombination processes is re viewed. It is concluded that the two-boc esses of dissociative recombination and radia tive recombination between electrons and positive ions and of mutual neutralization of positive and negative ions are of importance for the iono sphere. Laboratory aferglow measurements on ions of ionospheric interest yield values for the dissociative recombination coefficients. Theoretical calculations yield radiative recom bination values for H(), He(), O() ions and electrons of 4-5 x 10 to the -12th power cc/sec at 250 K and a T(sub e) sup-0.5 dependence on electron temperature. Auxiliary processes affect ing ionospheric recombination, such as electron attachment to O2 molecules, charge transfer both for positive ions and negative ions, and ion conversion, are briefly considered. (Author).

This final report describes briefly (a) a microwave system for measuring the electron density in decaying plasmas over t density range of about five thousand, (b) an experimental tube for measuring the value of the mobility of positive ions by means of the transit-time method, and (c) the results obtained during the period of the contract. The studies were confined to the rare gas ions and excellent agreement was found between the value of the mobility of atomic and molecular ions as determined from ambipolar diffusion coefficient measurements and those found by means of the ion transit-time method. The influence of the presence of negative ions on the ambipolar diffusion coefficient of electrons was demonstrated. The recombination coefficients of molecular neon and molecular argon ions were found to be independent of pressure and the values were in excellent agreement with previous studies. (Author).

In a recent paper, Bauer and Wu proposed a self-consistent model for the application of perturbation theory to calculating the general order of magnitude of cross sections of electron-positive molecular ion dissociative recombination. In applying their model to the calculation on a hypothetical system of electron capture by molecular hydrogen ions, it was necessary to make several approximations, without which it would have been exceedingly difficult to evaluate certain matrix elements. It is shown that Monte Carlo methods can be used for evaluating these matrix elements without restoring to approximations other than the model itself. The Monte Carlo method of stratified sampling has been tested successfully on the capture of electrons by molecular hydrogen ions. The dissociative recombination coefficient is found to be of the order of 3 x 10 to the -8th power cu cm/sec at room temperature, which is in agreement with reported experimental results. (Author).

Some of the earliest civilizations regarded the universe as organized around four principles, the four "elements" earth. water, air, and fire. Fire, which was the source of light and as such possessed an immaterial quality related to the spiritual world. was clearly the most impressive of these elements, although its quanti tative importance could not have been properly discerned. M- ern science has changed the names, but macroscopic matter is still divided into four states. The solid, liquid, and gaseous states are ordinary states, but the fourth state of matter, the plasma state, has retained a somewhat extraordinary character. It is now recognized that most of the matter of the universe is in the ionized state. but on the earth. the plasma state is still the exception. Hence the importance and also the difficulty of investigations dealing with ionized matter, which have been greatly furthered by the development of thermonuclear fusion research. The study of matter in the ionized state comprises a large diversity of problems belonging to many different branches of phys ics. A number of them relate to the microscopic properties of plasmas and concern the structure and the collisional behavior of atomic constituents. Although they are clearly of basic importance, their relevance to thermonuclear research was at first overlooked, at a time when most of the effort was concentrated on designing fusion devices and understanding macroscopic phenomena, mostly of an electromagnetic nature. At present.

Proceedings of a NATO ARW held in Saint Jacut de la Mer, Brittany, France, May 3-8, 1992

Electron-Molecule Interactions and Their Applications, Volume 1 presents a comprehensive account of electron-molecule interactions in high- and ultra-high-pressure gases and liquids. Topics covered include elastic scattering of electrons by molecules; excitation, ionization, and dissociation of molecules by electron impact; electron-molecule resonances; and electron attachment and detachment processes. This volume is comprised of seven chapters and begins with a discussion on non-resonant elastic scattering and rotational excitation of molecules by electrons, followed by a review of non-resonant vibrational and electronic excitation. The reader is then introduced to resonance effects in electron scattering; electron-induced ionization and dissociation of molecules; and electron-molecule resonances. The ionization mechanisms and types of ions produced are highlighted, along with differential ionization cross sections. The final two chapters focus on electron attachment and detachment processes, paying particular attention to modes of electron capture by molecules such as via negative-ion resonant states. The collisional dynamics for a few selected atomic reactants are also described. Physicists will find this book extremely helpful.