Charge trapping in dielectrics.

When a dielectric is irradiated by electrons with energy E of several kiloelectron volts, a large number of processes take place: backscattering of incident electrons, excitation and ionization of the electrons in the dielectric with binding energies lower than E, creation of excitons, radiative and nonradiative decays of the excited and ionized states, slowing down of the primary and secondary electrons, and thermalization in the conduction band. The thermalized electrons can move freely in the unoccupied conduction states of the material. If electric connection exists between the dielectric and the apparatus, then the charges normally flow out. Thermalized electrons can also be trapped in excited levels localized in the band gap of the dielectric and nonradiative and radiative recombinations from these levels can be observed. The number of the trapped electrons varies with the structural characteristics of the dielectric. In a monocrystal, this number is weak because the number of the defect states in the band gap is small, making the localization of the charges restricted. In contrast, in a polycrystal or amorphous material, the number of the trapped electrons can be large and increases with the disorder. Information on the charge effects suffered by the sample during its irradiation can be deduced by studying the trapping of electrons in localized states and, consequently, by analyzing radiations emitted from these states in the visible and X-ray ranges. In the case of oxides, F+ centers (oxygen-ion vacancy having trapped one electron) and F centers (F+ center having trapped a second electron) are generally present. We will show that the F+ <--> F conversion can be used to study the dynamic of the trapping in the oxides. Application to various samples of crystallized and amorphous alumina will be presented.

Xuv resonant transition radiations from periodic stratified media.

General formulation of the xuv intensity emitted when relativistic electrons cross through any periodic stratified medium is presented in the framework of electromagnetism in continuous media. Application is made to the resonant transition radiation emitted in the geometries of both normal and oblique incidence. In the first case, agreement is found between our calculated results and published values. Intensity emitted by a periodic multilayer stack used in the Bragg conditions is determined and the practical interest of a such a radiation source is discussed.