Position-dependent Stopping Power Research Articles

Energy loss of100−keVhydrogen atoms during grazing scattering from Cu(111)

The energy loss of $100\text{\ensuremath{-}}\mathrm{keV}$ hydrogen atoms scattered under a grazing angle of incidence from a clean and flat Cu(111) surface is studied in coincidence with the number of emitted electrons. From the dependence of the energy loss on the angle of incidence we deduce the position dependent stopping power which shows the same functional dependence but a factor of about 2 higher values than recent calculations. From the number of emitted electrons related to a specific energy loss we reveal processes related to electron emission as dominant channel for energy dissipation of scattered projectiles.

Effect of surface band structure in the energy loss of ions at surfaces

We have calculated the energy loss of protons scattered off the Cu(1 1 1) surface. The position dependent stopping power is calculated in terms of a surface response function, which is obtained from the surface band structure of the target. The role played by the surface state is analyzed. Comparison of our model results with those obtained using the specular reflection model is performed.

Stopping power of a KCl(0 0 1) surface for low energy Ne atoms

Energy losses of specularly reflected ions from a KCl(001) surface at grazing incidence of 15–30 keV Ne+ ions are measured in order to investigate the effect of the large band gap on the surface stopping power for low energy projectiles. The position-dependent stopping power of the KCl(001) surface for Ne projectile is derived from the observed energy losses. Because the direct electron excitation by the projectile–electron collision, which is the dominant stopping mechanism for low energy projectiles at metal or semiconductor surfaces, is almost prohibited at insulator surfaces due to the large band gap, the surface stopping power of the KCl(001) is expected to be much smaller than those of metals or semiconductors. The obtained stopping power is, however, found to be comparable to that of the SnTe(001) surface, which is a typical narrow-gap semiconductor. Charge exchange processes probably make an important contribution to the stopping at the KCl(001) surface for the low energy Ne projectile.

Position-dependent stopping power of low velocity rare gas atoms at a SnTe(0 0 1) surface

Energy losses of 15–30 keV Ne+, Ar+ and Kr+ ions specularly reflected from a SnTe(001) surface are measured. Because more than 98% of the reflected ions are neutral, the observed energy loss can be considered as the result of atom–surface interactions. From the observed energy losses, position-dependent stopping powers of the SnTe(001) surface for neutral atoms are derived. The obtained stopping powers are compared with the theory of the stopping power for low velocity ions with help of the local density approximation. While the agreement between the experimental and theoretical results is excellent for Ar and Kr atoms, the experimental stopping is about twice larger than the theoretical one for Ne atoms. This discrepancy can be explained in terms of the effect of excited atoms on the stopping power.

Energy loss of scattered ions at glancing-angle incidence on the crystal surface

Experimental findings of glancing-angle scattering of energetic ions are reviewed with particular emphasis on the energy loss of ion during scattering. The position-dependent stopping power of the surface which is derived from the angular dependence of observed energy loss is explained in terms of single and distance electron excitations of surface electrons. For the ions with velocities less than the Fermi velocity of target valence electrons, it is shown that the stopping power of a surface is determined only by the elastic collisions of the ion with valence electrons. From the analysis of the energy losses of 12–30 keV He ions reflected from the (001) surface of SnTe, a method is proposed to derive the electron density distribution averaged over the plane parallel to the surface from this position-dependent stopping power.

Surface investigations using monolayer-resolvable high-resolution Rutherford backscattering spectroscopy

Energy spectra of scattered 0.5 MeV He ions from a clean (001) surface of SnTe are measured with a 90° sector magnetic spectrometer ( ΔE/ E ∼ 0.1%). The ions scattered from successive atomic layers can be resolved in the energy spectra. Inelastic energy losses and charge state distributions of 0.5 MeV He ions scattered from the topmost atomic layer of the SnTe(001) are measured. A position-dependent stopping power at the surface is proposed from the observed energy losses. The observed charge state distribution shows the importance of the charge-exchange processes with valence electrons in the tail of the electron distribution at the surface.

Inelastic energy loss of ions at grazing scattering by solid surfaces

The inelastic energy loss of an ion at grazing scattering by a solid surface has been calculated with the use of different models: the Firsov model and the Lindhard-Scharff one. The formula for the energy loss obtained within the Firsov model does not contain any fitting parameters and describes the experimental data only for small transverse energies. The formula for the energy loss deduced within the Lindhard-Scharff model includes two parameters describing the position dependent stopping power above a solid surface. The calculated energy losses are compared both with the experimental data and the results of computer simulation.

Energy Losses of 12–32 keV H+, He+and N+Ions at Glancing Angle Scattering from Clean Surfaces of Silicon Crystals

The energy spectra of scattered ions of 12–32 keV H + , He + and N + have been measured at glancing angle incidence on the clean (001) and (111) surfaces of Si. A similar formula of the position-dependent stopping power which has been derived for high energy light ions was used to analyze the present results of low energy ions.

Position dependent stopping power for axially channeled 10 to 160 MeV alpha-particles in silicon

The slowing down of highly energetic charged incident along major crystallographic directions of a crystal is mainly due to conduction electrons, valence electrons and the local electron density due to inner shells at the position of the moving charged particles. Under channeling conditions the number density of atoms of the target seen by the projectile is slightly different from the bulk density of atoms. So the contribution of the valence electrons to the stopping power due to collective plasma oscillations will be different for the channeled and the random impact. Slater-type orbitals are used to calculate the local electron density due to the inner shells, and the cylindrically symmetric charge density around the channel axis due to all the six strings is obtained in the continuum approximation for the 〈110〉 axial channel is a diamond structure crystal. Using the above formulation, the position dependent electronic stopping power is calculated for 10 to 160 MeV alpha-particles channeled along the 〈110〉 axis in silicon, and the results thus obtained are compared with the experimental and theoretical results available.

Dynamic response of surface electrons to energetic ions at glancing angle scattering from crystal surfaces

A new method is proposed to derive the position-dependent stopping power of crystal surfaces for energetic ions from the dependence of energy loss of specularly reflected ions on the angle of incidence on the surface. The method is applied to the specular reflection of MeV ions from the (100) surfaces of NaCl, KCl, KBr, SnTe, PbTe and PbSe, and stopping powers S p( x). where x is the distance of the ion from the surface, are obtained. The method is also applied to the surface channelling of MeV He ions along the 〈100〉, 〈110〉 and 〈210〉 axes of SnTe and the position-dependent stopping powers S a( r), where r is the distance from the atomic row, are obtained.

Energy loss of energetic ions in surface blocking

The blocking effect at surface atomic planes on ions scattered from surface atoms (surface blocking) is investigated for 1 MeV He + ions incident on an atomically clean (001) surface of a SnTe crystal under UHV conditions. The observed energy of the surface-blocked ions depends on the exit angle relative to the surface plane. A new method is proposed to determine the position-dependent stopping power near the surface using surface blocking.

Specular reflection of fast ions at a single crystal surface

The interaction of fast ions with a solid surface is studied by the specular reflection of MeV H + and He + ions from clean (001) surfaces of SnTe single crystals. Oscillatory structure is observed in the energy spectra of the reflected He ions. This indicates that a part of the incident ions penetrate inside the crystal and travel for a few wavelengths of oscillatory motion in the (001) planar channel before reappearing at the surface. The position dependent stopping power is obtained to be proportional to E − 1 2 exp(−0.15x/a TF ) , where x is the distance from the surface. A large difference is observed in the charge state distribution between the specular reflection and the transmission through a self-supporting foil. This is attributed to the fact that the specularly reflected ions interact exclusively with the valence electrons. It is found that the velocity of convoy electrons emitted at the glancing-angle incidence of He ions is faster than that of the incident ions. This may be due to the acceleration of convoy electrons by the dynamical image potential induced by ions near the surface.

Energy loss of MeV light ions specularly reflected from a SnTe(001) surface.

Energy losses of specularly reflected MeV protons and He ions from a clean (001) surface of SnTe single crystal are observed. The position-dependent stopping powers near the surface are derived from the observed results. The obtained stopping power is proportional to ${E}^{\mathrm{\ensuremath{-}}1/2}$ exp(-${\ensuremath{\beta}}_{3}$x/2${a}_{\mathrm{TF})}$, where E is the ion energy, ${a}_{\mathrm{TF}}$ is the Thomas-Fermi screening distance, ${\ensuremath{\beta}}_{3}$ is 0.3, and x is the distance from the surface. This result is similar to the observed position-dependent stopping power for planar channeled ions.

Position-dependent stopping power for channeled ions

A method is developed for predicting the stopping power of crystals toward swift channeled ions, based on a semiempirical treatment of the dielectric response of the medium. Random stopping power is also obtained. Theoretical results are compared with the experimental channeled stopping power of gold as a function of ion velocity and position within channels. It is shown that the theory is accurate when the velocity of the ion equals at least twice the root-mean-square velocity (estimated from binding energies) of most of the electrons near its path.

Channeling stopping power for high energy ions

Making use of the inelastic scattering function introduced by Ohtsuki, the position dependent stopping power is derived for energetic ions in a channeling condition. We can interprete the Lindhard theory, Esbensen-Golovchenko theory, and Burenkov-Komarov-Kumakhov theory for the channeling stopping power in terms of our method. Our results agree very well with experiment.

Inelastic scattering of ions at the surface

A systematic study of the interaction between surface collective modes and surface-channeled ions is made. A complex image potential is introduced and the position-dependent stopping power is obtained. It is shown that the surface collective modes play a significant role outside the solid. The energy loss spectra of the surface-channeled ions are obtained by computer simulation.

Isotope effects in Raman spectra of crystalline alkali-metal chlorates and bromates

A new method is proposed to derive the position-dependent stopping power of crystal surfaces for energetic ions from the dependence of energy loss of specularly reflected ions on the angle of incidence on the surface. The method is applied to the specular reflection of MeV ions from the (100) surfaces of NaCl, KCl, KBr, SnTe, PbTe and PbSe, and stopping powers Sp(x). where x is the distance of the ion from the surface, are obtained. The method is also applied to the surface channelling of MeV He ions along the 〈100〉, 〈110〉 and 〈210〉 axes of SnTe and the position-dependent stopping powers Sa(r), where r is the distance from the atomic row, are obtained.