Backward Electron Emission Research Articles

SECONDARY ELECTRON EMISSION INDUCED BY α-PARTICLES FROM Mg-MgO LAYERS

The paper presents the results of experimental study of forward and backward electron emission induced by α-particles from the deposited film of magnesium. It was shown that during the deposition of magnesium in residual gas atmosphere the deposited film contained a large amount of MgO component, which makes it possible to consider the resulting structure as Mg-MgO. The presence of magnesium oxide on the surface of the target and the collector leads to the fact that the previously obtained dependence of the ratio of forward and backward electron yields on specific energy loss of the ion for various metals is not applicable in the case of deposited magnesium. The differences are explained by the specificity of the emission from magnesium in the presence of a significant amount of MgO. The results obtained can be used to detect MgO on the surface of a magnesium substrate. It was shown the differences in the experimental data for the bulk magnesium collector and the collector with deposited magnesium layer

Secondary Electron Emission from Carbon Foils under O2+ Ion Impact

Secondary electron emission yields in forward and backward direction from carbon foils (thickness of 74 nm) induced by O2+ ions of energies form 1.9 keV/u to 11.3 keV/u have been measured. We find that the forward and the backward electron emission yields increase with the projectile kinetic energy. Further studies showed that the forward and the backward electron emission yields are approximately proportional to the electronic stopping power at the exit and entrance surfaces, respectively.

Secondary electron emission from carbon Foils by Ne2+ near Bohr velocity

We have measured secondary electron emission yields in forward and backward directions from the carbon foils with different thicknesses, which are penetrated by Ne2+ ion of an energy from 2 to 25 keV/u. In the experiment, the contributions of projectile potential energy deposition, electronic energy loss and recoil atom to the forward and backward electron emission yields have been studied systematically by changing the projectile ions energy. Results show that the ions potential energy deposition only have the impact on the backward electron emission yield, while the forward and backward electron emission yields increase approximately linearly with the increase of the electron energy loss at the exit and entrance surfaces of the thin film respectively. When the electron energy loss is low, the impact of recoil atoms on the electron emission cannot be neglected.

Monte Carlo study of ion-induced backward and forward secondary electron emission from thin Al foil

A direct Monte Carlo program has been developed to calculate the backward (γb) and forward (γf) electron emission yields from 20nm thick Al foil for impact of C+, Al+, Ar+, Cu+ and Kr+ ions having energies in the range of 0.1–10keV/amu. The program incorporates the excitation of target electrons by projectile ions, recoiling target atoms and fast primary electrons. The program can be used to calculate the electron yields, distribution of electron excitation points in the target and other physical parameters of the emitted electrons. The calculated backward electron emission yield and the Meckbach factor R=γf/γb are compared with the available experimental data, and a good agreement is found. In addition, the effect of projectile energy and mass on the longitudinal and lateral distribution of the excitation points of the electrons emitted from front and back of Al target has been investigated.

Backward and forward electron emission induced by helium projectiles incident on thin carbon foils: Influence of charge changing processes

The backward and forward electron emission yields γB and γF have been calculated by Monte Carlo simulations for helium (He++, He+ or He0) ions incident on thin amorphous carbon foils with energies around the electronic stopping power maximum (0.2–2MeV). Besides the direct excitation of target electrons by the incident projectile, we have taken into account the different charge changing processes (He++↔He+↔He0) undergone by the helium ion in the target. We discuss in particular the connection between the electron emission yield γ and the electronic stopping power (dE/dx)e. We compare our results with previously published experimental results.

Incident angle dependence of secondary electron emission from carbon induced by swift H 2 +

The incident angle dependence of secondary electron emission induced by a swift H2+ ion impinging on carbon is studied using the Monte Carlo method combined with the semiempirical theory. The relationships both between the electron emission yield and the project angle and between the statistics and the projectile angle are investigated. The results show that the backward electron emission yield deviates from the inverse cosine law, due to the effect of the valence electrons of H2+. The ratio of the forward electron emission yield to the backward electron emission yield at the inclining incidence is different from that at the normal incidence. The statistical distribution of electron emission is independent of the incident angle. The value of b, the deviation parameter from the Poisson distribution, increases with projectile energy.

Influence of the target thickness on the backward and forward electron emission characteristics induced by protons incident on thin carbon foils

The forward (γF) and backward (γB) electron emission yields have been measured for protons incident on thin carbon foils for incident energies between 2 and 7MeV as a function of the target thickness. Comparisons with theoretical results obtained by Monte Carlo simulations are presented. In particular, the Meckbach factor Rγ=γF/γB is discussed.

Electron emission induced by H+ and H0 incident on thin carbon foils: influence of charge changing processes

The forward and backward electron emission yields γF and γB have been calculated by Monte Carlo simulation for protons (H+) and hydrogen atoms (H0) (with energies between 25keV and 5MeV) incident on thin amorphous carbon foils. Direct electron excitations by the incident projectiles as well as electron excitations resulting from charge exchange processes undergone by H+ or H0 have been taken into account. For the latter, Auger and Shell processes have been considered. Subsequent electron transport has been considered in order to calculate the forward and backward electron emission yields γF and γB.

Electron emission induced by fast heavy ions in a thin silicon crystal

We report observations of backward and forward electron emission by a thin silicon crystal target traversed by $29\phantom{\rule{0.3em}{0ex}}\text{MeV}∕\mathrm{u}$ ${\mathrm{Pb}}^{56+}$ incident ions. For each incident ion we have performed measurements of backward and forward electron emission, of the energy loss and of the charge state of the transmitted ion. The crystal target was traversed by incident ions either in random incidence or in axial alignment conditions. In both cases these correlated measurements bring original information on electron emission. In random conditions, using an incident ion species with a charge quite far from equilibrium, we observe correlations between backward and forward electron emission, that we understand when analyzing the associated charge exchange and energy-loss data. In channeling conditions, we added electron emission measurements to simultaneous energy-loss and charge state measurements (that are known to characterize quite precisely the type of trajectory of a projectile transmitted through a thin crystal). This allowed us to observe the reduced electron emission due to hyperchanneled ions, that interact mainly with target valence electrons, and also the enhanced electron emission due to projectiles entering the crystal very close to atomic strings.

Molecular frame photoelectron angular distributions in dissociative photoionization ofH2 in theregion of the Q1 and Q2 doubly excited states

Dissociative photoionization ofH2 induced by VUV linearly polarized synchrotron radiationP has been studiedusing the (VH+,Ve,P) vector correlation method. The ion–electron kinetic energy correlationdiagrams obtained for the three photon excitation energieshν = 20, 28.5and 32.5 eV enable us to identify and select the dominant dissociative photoionization processes. TheIχ(θe,ϕe) molecular frame photoelectron angular distributions for any orientationχ of the molecular axis with respect to the polarization are reported for direct photoionization ofH2 into theH2+(2Σg+) ionic groundstate at hν = 20 eVand for the dominant DPI processes involving autoionization of theH2(Q1 1Σu+(1)) andH2(Q2 1Πu(1)) doubly excitedstates into the H2+(2Σg+) and H2+(2Σu+) continua at hν = 28.5 and 32.5 eV. They show the dominant excitation of ap σu partial wave inautoionization of the Q1(1Σu+(1)) state into the H2+(1s σg) ionic stateand that of a d πg partial wavein autoionization of the Q2(1Πu(1)) state into the H2+(2p σu) continuum. A molecular frame forward–backward electron emission anisotropy is observedwhen ionization takes place at large internuclear distance.

Influence of charge changing processes on the forward to backward electron emission yield ratio for light ions impinging on thin metallic foils

Stopping power d E/d x and electron emission yield γ are known to be strongly influenced by electron capture and loss processes. In this work we consider proton impact on thin Al foils for impact velocities below two atomic units. We investigate the effect of the charge changing processes on the forward/backward electron emission yields, γ F and γ B as well as on the specific yields, Λ F and Λ B ( Λ= γ/(d E/d x)). The dependence of the emission yields and of the specific yields as a function of the target thickness is discussed in terms of the charge changing processes.

Saturation effects in highly charged ion interaction with thin carbon foils

We present a set of experimental values for relative cross sections of Auger electron production in carbon foils for projectile charges QP ranging from 6 to 39 and fixed projectile velocity 19.2 atomic units. We deduce the 1s single and double ionisation cross sections and their relative variation with respect to the projectile charge. The cross section saturates for the largest projectile charges. However, the comparison of the experimental data with continuum distorted wave-eikonal initial state (CDW-EIS) and classical trajectory Monte Carlo (CTMC) calculations shows that this saturation effect is not well reproduced by the calculations. We suggest that the observed saturation effect in 1s-shell ionisation could be due to multiple electron effects not included in these models based on independent electron approximation. We have then simulated the backward electron emission yield γB taking into account this strong saturation effect for the 1s shell. We obtained a better agreement for both the emission yield γB and the stopping power. The electron emission simulation gives some insight on the relative contribution of valence and 1s-shell to saturation effects.

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

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 +, H 0, H − fractions as well as of electrons excited by the charge changing processes for incident H + (1< v<2 a.u.) on polycrystalline aluminium are also discussed.

Evaluation of the contribution fraction of close collision to the backward electron emission induced by He+ ion

Electron emission for He+ incident on solid surfaces of C, Cu and Al was simulated with the MonteCarlo method.The backward electron emission yields are calculated.The contribution fraction of electrons emitted by close collision to the total backward emission yield is evaluated with this code, and contribution fraction is 0.5,0.55 and 0.42 for C, Cu and Al, respectively. The effect of highenergy (E＞100eV) δ electrons on the backward electron yield is also considered in detail, and only those δ electrons with an energy of a few hundred eV plays an important role in the backward electron emission. For C, δ electrons will affect the behaviour of electron emission yield near the maximum electronic stopping power. Results of yield obtained are compared with experimental data of other authors, and a good agreement is found.

Monte Carlo simulation of electron emission induced by swift highly charged ions: beyond the linear response approximation

Numerical simulations of ion induced electron emission from solids mostly use the first order Born approximation within the dielectric formalism to describe valence electron excitation. As a result, the yield of emitted electrons is found to scale with the square of the projectile charge QP in contrast to experimental findings obtained with carbon targets [1]. Since similar deviations from QP2 scaling were observed for the electronic stopping power, at least a part of this deviation must be related to primary ion-electron interaction, for which an alternative description needs to be developed. We thus present here a distorted wave approach for the modelling of primary interaction, which can be expected to give better results in view of its success in describing ion-atom collisions at large impact velocity. Keeping the same description of the electron transport through the target, we show that both the electron yield and the stopping power ratios (with respect to the same quantities for C6+), as a function of the projectile charge, are better reproduced by this alternative approach. We show that low energy electron excitation is responsible for the deviation from the QP2 scaling. We also analyse the effect of the transport on the primary electrons. This distorted wave approach successfully explains the shape of the ratio of energy differential spectra for two different QP obtained in earlier experiment for Al and C. Furthermore, we predict a different behaviour of the forward and backward electron emission with respect to QP in qualitative agreement with experimental results.

Monte-Carlo Simulation of Secondary Electron Emission from Carbon Induced by H2+

A Monte-Carlo code for calculating secondary electron emission from carbon induced by H2+ ion has been developed. The projectile H2+ splits into cluster H+ ion and e- at a certain depth in the solid. H2+, cluster H++H+ and e- make different contributions to the total backward electron yield, and the contribution fractions are calculated using the code. The calculated molecular effect [R(Yb)] in the backward electron emission is from 0.85 to 1.09 in the energy range from 400 keV to 3000 keV. The calculated ratios of forward to backward electron yield increase with energy and decease with target thickness. The molecular effect [R(Yf)] in the forward electron emission decreases with target thickness for a given energy. When the projectile energy is greater than 3700 keV, ratio R(Yf) in the forward electron emission will be greater than one [R(Yf)>1] for the target thickness of 1000 Å.

Theoretical study on the role of bound electron in backward secondary electron emission induced by He + incident on solid surfaces

A theoretical study on the role of bound electron in the process of backward secondary electron emission for He + incident on Al, Cu and C has been made. A Monte-Carlo computer code based on the theoretical model for the simulation of this phenomenon has been developed. The code can be used to calculate the yield, distribution statistics and other physical parameters of emitted electrons. The calculated results are compared with experimental data of other authors, and a good agreement is found. We also calculated the yield for He + incident on Al and Cu at the energy near the electronic stopping power maximum.

Electron energy spectra from intense laser double ionization of helium.

The double ionization of helium in the strong-field limit has been studied using an electron-ion coincidence technique. The observed double ionization electron energy spectra differ significantly from the single ionization distributions. This gives new support to the rescattering model of double ionization and explicitly reveals the role of backward electron emission following the e-2e ionizing collision.

Backward Secondary Electron Emission Yield of Thick Targets Induced by MeV Ions

The backward secondary electron emission yields of MeV ions (H+, He+, He++, Cl, Si, and Cu) impinging on thick carbon and gold targets are studied. The measured results for H+ (1 MeV ⩽ E ⩽ 5 MeV) on carbon are proportional to the electronic stopping power. Our experimental data and fitting formula of yields for H+ (1 MeV ⩽ E ⩽ 4.5 MeV) impacting Au are compared with the theoretical expectation. The influence of the collective field and the charge state of ions on the secondary electron emission yield is discussed.

Backward electron emission from carbon and gold foils under impact of MeV Au+n clusters

Gold cluster beams delivered by the 2.5 MV Van de Graaff accelerator of the Institut de Physique Nucléaire de Lyon, equipped with a liquid metal ion source (LMIS), were used for secondary electron emission measurements. We investigated backward emission from thin carbon and gold targets bombarded by Au+n projectiles (1⩽n⩽9) of energies between 150 and 500 keV/atom in a standard vacuum of 10−7 mbar. For Au+ ions incident on carbon and gold targets it is observed that the electron yield follows the velocity dependence of the energy deposited at the surface of the target in ionization processes induced both by the incident projectile and by the recoiling target atoms. For Au+n clusters of a given velocity the electron yield per incident gold atom is observed to decrease when the cluster size increases. Given the large contribution of the recoil atoms to the electron emission, we studied the dependence of the cluster effect on the target thickness, for thicknesses of the order of the size of the atomic displacement cascades. The comparison of the electron yields of thin carbon targets of various thicknesses at the impact of Au+ and Au+n projectiles of the same velocity has effectively evidenced a thickness effect for cluster induced electron emission.