The reactivity of slow electrons with molecules at different degrees of aggregation: gas phase, clusters and condensed phase

This paper presents nanoscopic aspects of electronic aging in dielectrics, as the result of the interaction of excess non-thermalized carriers with the media. Hot electrons are hypothesized as the major source of non-thermal carriers and shown to be a decisive factor in electrical breakdown. The interaction of hot electrons with atomic and molecular solids forming dielectric films is investigated using high-resolution monochromatic electron beam and cryogenic techniques. In these experiments, electrons of a well-defined energy impinge on a dielectric nanometer-thick film grown at cryogenic temperatures in an ultra-high vacuum system. It is possible to observe directly the species created at specific energies by hot (i.e. low-energy) electrons within the range 0 to 20 eV; including highly reactive radicals, positive and negative ions and excited atoms and molecules, as well as the new compounds that result from non-thermal reactions. Energy losses to electronic, vibrational and phonon excitation also can be measured as a function of electron energy. The results of these experiments are reviewed. They serve to explain the creation of local defects, which then can act as inter- and intramolecular (or atomic) traps for electrons and holes in inhomogeneous media. This nanoscale information also allows one to identify the effects of the polaron and that of cumulative material damage and increases in leakage currents, observed on the macroscopic scale. More particularly, it is shown from the results of low-energy electron bombardment of slices of PE HV cables, that simpler molecular solids such as n-alkane films provide insight on the effect of 'hot' electrons in commercial insulating material. Recommendations are given for further research necessary to represent the evolution of the aging process in practical macroscopic materials of interest at the engineering level.

Nanoscopic aspects of electronic aging in dielectrics

Electron attachment processes in isolated molecules in the gas phase, in clusters and in molecules condensed on surfaces are reviewed, and a special section is dedicated to electron attachment to biologically relevant molecules. For isolated molecules in the gas phase the emphasis is on the comparison between direct dissociation from strongly repulsive * states and the more indirect predissociation processes associated with the initial occupation of a* MO. These processes are discussed with regard to the temperature dependence of the dissociative electron attachment processes and with regard to its high bond selectivity. Specific examples of high bond and site selectivity at particular electron incident energies are discussed in terms of thermo-chemically controlled and state controlled selectivity of the dissociative electron attachment process. Different processes that are specific for electron attachment to molecules in clusters or condensed on surfaces are discussed for prototypical cases. These include suppression and enhancement of DEA channels, negative ion formation through “electron/exciton-complexes”, scavenging processes, and low energy electron induced synthesis in thin molecular films and in clusters. Finally we discuss gas phase electron attachment to the DNA and RNA building blocks, i.e., the nucleobases, the sugar unit and the phosphate group. The relevance of these gas phase experiments to actual DNA damage such as single strand breaks is discussed and first gas phase electron attachment experiments on larger DNA units are discussed.

From isolated molecules through clusters and condensates to the building blocks of life A short tribute to Prof. Eugen Illenberger's work in the field of negative ion chemistry

From isolated molecules through clusters and condensates to the building blocks of life

Low energy electron driven reactions in free and bound molecules: from unimolecular processes in the gas phase to complex reactions in a condensed environment

Alexander Kühn

Papers

Gas phase negative ion chemistry