M. Rösler

Experimental and theoretical study of target thickness dependent electron yields induced by electrons in carbon

Atomic collisions in solids lead to ionisation and electron ejection from the target atoms. The subsequent migration of electrons through the solid contributes significantly to energy deposition and damage creation. Several groups developed theoretical models on electron transport in solids and electron emission from solid surfaces. Before including ionisation by ions or atoms, such models have to deal correctly with electron induced ionisation. A basic test is the comparison to experimental data on electron emission. We therefore measured the electron yields induced from both surfaces of thin carbon foils (10-2000 nm) induced by electrons (0.2-5 keV) and compared them to simulation results obtained with two Monte Carlo codes developed independently and using different approximations for the electron interactions in the foil.

Monte Carlo simulation of kinetic electron emission induced by MeV He + and He ++ ions incident on polycrystalline aluminium

A theoretical study of the characteristics of kinetic electron emission induced by He+ and He++ ions (in the MeV-range) incident on polycrystalline aluminium targets is presented in this paper. The models of interactions for the incident projectiles as well as for the excited electrons are described. Especially, the electron excitation induced by incident He+ is calculated by taking into account the structure of the composite projectile. Charge exchange processes by the projectiles are also considered. The subsequent electron transport and escape is calculated by means of a Monte Carlo simulation code. The calculated electron emission yield is compared to experimental results. The differences between He+ and He++ are discussed, the role of the electron lost by He+ being especially emphasised.

Electron capture and loss processes for protons in aluminium: Comparison between conduction band electron–hole assisted and plasmon assisted Auger processes

Abstract Charge changing processes are known to have a strong influence on ion-induced electron emission characteristics. However, up to now, only a few theoretical models of electron emission incorporate electron capture and loss cross sections. For incident protons with velocities around 1 a.u., a thorough theoretical model of the various charge changing processes undergone by the proton is necessary for a correct description of the electron emission induced by the projectile. Indeed, all the electrons excited in the various processes have to be taken into account. In this work, we consider conduction band Auger capture and loss processes. In particular, we consider plasmon assisted processes, which are not negligible with respect to electron–hole pair assisted Auger processes for electron capture.

Experimental and theoretical study of the ratio between the electron emission yield and the electronic stopping power for protons incident on thin carbon foils

Abstract It is often assumed that the kinetic electron emission yield γ is proportional to the electronic stopping power dE/dx for ions incident on solid targets. Though these two phenomena are based on the same physical process in which the incident ions lose their energy in the target in electron excitations, there is no reason why the ratio Λ=γ/(dE/dx) should be constant. We present in this paper a comparison between experimental and theoretical results for protons incident on thin carbon foils in a wide energy range (200 keV–9.2 MeV). The ratios ΛB=γB/(dE/dx) and ΛF=γF/(dE/dx) for both the backward (B) and forward (F) emission yields are studied as a function of the incident proton energy as well as the target thickness.

Experimental and computational comparative study of electron emission of thin carbon foils traversed by MeV protons and hydrogen atoms in frozen charge states

Abstract The secondary electron emission has been measured for 2 MeV H+ and for H0 projectiles passing through very thin carbon foils without charge changing. We have studied the statistics of the emission from both entrance and emergence surfaces. Simultaneously, Monte Carlo calculations have been performed for similar experimental conditions (500 keV

Influence of the charge changing processes on proton induced electron emission from polycrystalline aluminium

Abstract Charge changing processes are known to have a strong influence on ion-induced electron emission characteristics. However, up to now, only a few theoretical models incorporate electron capture and loss cross-sections. For protons with velocities around 1 a.u., a correct theoretical model of the various charge changing processes undergone by the proton is necessary. In particular, all the electrons excited in the different processes have to be taken into account. It is precisely the aim of the present paper to give a description of all the possible charge changing processes and to incorporate these processes in a Monte Carlo simulation of proton induced electron emission from polycrystalline aluminium. The influence of charge changing processes on backward electron emission yield is evaluated. The contributions of H+, H0, H− fractions as well as of electrons excited by the charge changing processes for incident H+ (1

THEORETICAL STUDY OF ELECTRON EMISSION INDUCED BY NEUTRAL PARTICLES

Abstract Charge state effects are known to be important for particle induced electron emission. Indeed, the charge state of the incident beam has a strong effect on the emission yields. Incident neutral hydrogen atoms can induce secondary electrons after an electron loss process. At high impact energies (above 200 keV) target electrons are excited by both the proton and the electron which can be treated in a first approximation as independent “primary particles”. A more precise description should include different microscopic processes: (i) target electron excitation by the undissociated neutral H0; (ii) excitation of the projectile electron by the loss process accompanied by target electron excitation; (iii) excitation of target electrons by the proton and the electron. At high impact energies it is justified to neglect the capture processes. It is the aim of this paper to calculate the electron emission characteristics of aluminum including all these processes. The various microscopic processes are incorporated in a Monte Carlo simulation code.

Microscopic considerations concerning the relation between electron yield and stopping power: calculations for aluminium and carbon

Abstract It has been evidenced experimentally that, for proton impact on a thick solid target, the ratio Λ of the backward electron yield γB and the electronic stopping power ( d E d x) e is approximately constant over a large energy range (a few keV to several MeV). The macroscopic theoretical models (Sternglass 1957; Schou 1980; …) assume constancy of this ratio. However, from a microscopic point of view, there is no obvious reason for that. We present in this paper a microscopic calculation (microscopic cross sections and Monte Carlo simulation method for the transport) for proton induced electron emission from aluminium and amorphous carbon. The electron yields and the factors Λ from thick and thin (forward and backward emissions) targets are calculated and compared to experimental results.

Theoretical and experimental study of the correlation between forward and backward electron emissions induced by H0 or H+ projectiles incident on carbon foils

Abstract The emission statistics of secondary electrons emitted from solid targets under ion impact has been recently the object of much interest from a theoretical as well as experimental point of view since it provides very useful information about the electron excitation process in the solid. In the case of very thin targets, it is possible to measure simultaneously the number of electrons emitted from both sides of the target and the exit charge state of the projectile. The resulting two variable (number of electrons emitted in the backward and in the forward directions) statistical distribution shows in some cases a clear correlation between the numbers of electrons emitted from both sides of the target. This correlation between backward and forward emission has been studied from Monte Carlo simulations and from experiments for H0 and H+ projectiles incident on thin carbon foils in the MeV energy range. It is shown that the charge exchange processes of the incident projectiles in the target play an important role in the interpretation of the backward–forward correlation of the secondary electron emission.

Electron emission induced by hydrogen atoms and protons incident on aluminium: effect of the projectile charge state

Abstract It is well known that the charge state of incident ions has an influence on the stopping power as well as on the electron emission characteristics. We have studied here electron emission for protons and hydrogen atoms incident on polycrystalline Al targets for impact velocities up to 2v0 (v0 is the Bohr velocity). Besides the direct excitation of target electrons by the incident projectile, we have considered different charge changing processes that give rise to additional electron excitation, hence contributing to electron emission. These processes are also responsible for the charge state dynamics of the moving projectile. We compare our results with previous results and in particular with the predictions of the simple model proposed by Ghosh and Khare [Phys. Rev. 125 (1962) 1254].