Determination of Optical Properties of a Field Emission Gun Coupled with a Linear Accelerator for High Voltage Microscopy

The electron optical properties of a field emission gun plus a twenty-stage linear accelerator system have been determined in the case where the entrance pupil of the system is defined by one of the electrode apertures. The first part of the calculation concerns the electrical and geometrical parameters of triode and tetrode field emission guns which can give a fixed location of the source and a small spherical aberration coefficient. The second part is related to the possibility of placing a tetrode field emission gun at the top of the linear accelerator of the scanning high voltage electron microscope which is being constructed in the Toulouse Laboratory. Interesting conditions can be obtained when the electrical parameters are fixed as follows: either Vi/Ve= 0 or Vi/Ve= 20 (Ve is the extracting voltage, Vi is the potential of the second anode of the gun) whatever the accelerating voltage may be

Characteristic Energy Losses with High Energy Electrons up to 2.5 MeV

Some aspects of the influence of the energy of the incident electrons in electron energy loss spectroscopy (EELS) are considered. It is shown that this method of analysis used in high voltage electron microscopy, permits one to observe, with a better edge jump ratio than at lower accelerating voltages, the characteristic edges. One important question is to eliminate artefacts in the counting and to record only electrons from the true spectrum. Some recent examples are given. One of them concerns extended energy loss fine structures (EXELFS). It seems high voltage electron microscopy (HVEM) could be very useful in this domain

Monte Carlo Calculations on the Spatial and Angular Distributions of High Energy Electron Beams in Amorphous and Polycrystalline Films

We study the plural scattering of electrons in amorphous and polycrystalline films. The incident electron energy ranges from 0.1 to 3 MeV. The cross sections are obtained by measuring the transmission coefficient for targets of gold, silver, aluminium and carbon. The partial elastic cross section is calculated from Lenz\u27s theory using a Wentzel-Yukawa model for the atomic potential of the scattering atom. In the case of inelastic interactions, we take into account either scattering by a free atom (Morse\u27s approximation) or scattering by plasmon creation (relativistic theory of Ashley Ritchie). From these results, we solve the problem of electron transport in thin films with the aid of a Monte Carlo method. We use a single scattering model: each electron trajectory is followed through a succession of distinct scattering events in the target. For each scattering event, the direction and, if necessary, the electron energy are modified. We obtain the angular distributions of the transmitted electrons and the broadening of the electron beams in the specimen. This study sheds light on the amplitude contrast of electron microscope images

Monte Carlo Calculations on Electron Backscattering in Amorphous or Polycrystalline Targets

We propose an application of the Monte Carlo method in the field of backscattering. The results obtained for incident electron energies ranging from 0.3 to 3 MeV and for targets of Al, Cu, Ag and Au are compared with experimental values from several sources. An electron travelling through matter undergoes successive collisions between which it is assumed to travel in a straight line. In our case, we consider the elementary process of interaction electron-nucleus; we have used analytical models for the scattering cross-sections. In order to follow the electron through the specimen, we divide the real trajectory into elements of length much smaller than the mean free path. Pseudo-random number process permits us to determine whether or not an interaction occurs, also the type of interaction. For the energy losses, we introduced a relation derived from Landau\u27s theory. We then followed the electron until it is emerged from the material or halted. The backscattering coefficients obtained for thin and thick targets as a function of the incident electron energy are in good agreement with the experimental data. We have introduced the depth distribution function of the backscattered electrons, which allows us to test the predictions of various theoretical models proposed by other authors

Volkov-Pankratov states in topological heterojunctions

We show that a smooth interface between two insulators of opposite topological Z2 indices possesses multiple surface states, both massless and massive. While the massless surface state is non-degenerate, chiral and insensitive to the interface potential, the massive surface states only appear for a sufficiently smooth heterojunction. The surface states are particle-hole symmetric and a voltage drop reveals their intrinsic relativistic nature, similarly to Landau bands of Dirac electrons in a magnetic field. We discuss the relevance of the massive Dirac surface states in recent ARPES and transport experiments

Microscopie électronique à haute tension

The author recalls briefly the principle of transmission electron microscopy. He tries to define some domains which could be studied by mean of the high voltage technique. He thinks that two questions have priority for study : contrasts and point defects behaviour. One question is to know if it is possible to observe point defects clusters created by high energy electrons if the energy is high enough.L'auteur, après avoir rappelé brièvement le principe d'un microscope électronique par transmission, essaie de dégager quelques domaines d'application que la microscopie électronique à haute tension permettrait d'étudier efficacement. Il semble que deux questions sont à résoudre en priorité : l'étude des contrastes et celle des défauts ponctuels qui doivent être créés si la tension est assez élevée

GEOMETRICAL AND CHEMICAL KNOWLEDGE AT HIGH RESOLUTION

Parmi les différents intérêts actuels qu'elle présente, la microscopie électronique est de plus en plus utilisée dans les domaines de la résolution atomique, et de l'analyse chimique à l'aide des excitations de niveaux atomiques profonds. Cet article donne un survol des résultats qui ont été récemment obtenus autour de ces sujets. S'il est parfaitement possible d'observer dans les cristaux les colonnes atomiques et des atomes individuels plutôt lourds dans des molécules organiques, l'analyse chimique par excitation de niveaux atomiques profonds est spatialement moins précise (un ordre de grandeur environ).Amongst different tendencies in electron microscopy, the study of materials at atomic resolution and the use of energy losses (inner shell excitations) to do chemical analysis are quite promising. This article gives a rapid overview on the subject and some recent results in these domains. If it is possible to observe the column of atoms in crystals but also the rather heavy single atoms in stained organic molecules, the chemical analysis is less accurate (one order of magnitude about)

Défauts créés par bombardement ionique de couches minces d'or

The effects of ion bombardment on thin evaporated crystals of gold have been investigated by means of the electron microscope. Thèse effects are a function of the temperature of the sample. At room temperature the main effect is the production of a great number of dislocation loops. Elongated loops are observed. The behaviour of these defects during annealing shows that these elongated loops are dipoles which appear to be perfect. At 200°C the ion bombarment produces new defects which appear to be argon bubbles. It is possible to observe the condensation of bubbles on to the dislocation loops.Des monocristaux d'or préparés par évaporation ont été soumis à des bombardements ioniques, puis observés au microscope électronique. Les effets dus à ces bombardements dépendent de la température de l'échantillon. A la température ambiante, on observe un grand nombre de boucles de dislocations, parmi lesquelles on observe des boucles de dislocations allongées. Le comportement de ces derniers défauts durant un recuit montre que ce sont des dipôles de dislocations qui semblent être parfaits. A 200 °C le bombardement ionique entraîne la formation de bulles de gaz. Il est possible d'obtenir une condensation de telles bulles sur des boucles de dislocations