Mean Energy Loss Research Articles

Charge state dependent energy loss of high velocity oxygen ions in the charge state non-equilibrium region

Mean energy losses of high velocity bare H-like, He-like and Li-like O ions in thin carbon foils were measured in the charge state non-equilibrium region and a clear charge state dependence was observed. The screening effect of bound projectile electrons on energy losses are deduced and compared with theoretical predictions.

Irreversible energy transfer in ionization of physisorbed adsorbates: II. Inelastic energy spectra of temporarily ionized adatoms

We have studied inelastic processes following core ionization of a physisorbed adsorbate by an external probe (electron impact or photon absorption), which typically occur in the intermediate, temporary ionized adatom states of electron (ESD) or photon (PSD) stimulated desorption and XPS of adsorbates. In the ionization process the adsorbate makes a diabatic or Franck-Condon transition from the neutral state potential V (0) (r) to the ionic potential V (+) (r) in which the charge of the moving ion is cou the charge density fluctuations of the substrate. This suddenly switched on dynamical interaction gives rise to inelastic energy transfer from the probe and the ionized adsorbate to the substrate degrees of freedom (heat bath). In order to qualitatively assess such dissipative processes we have calculated the characteristic energy loss spectrum of the adatom, the spectrum of energy absorbed by the heat bath and the characteristic energy spectrum of the whole system. From these spectral functions and their characteristics (e.g. the Debye-Waller factor, the shape of the inelastic side-band, mean energy loss, etc.) we can obtain information on the dynamics of each constituent of the system in the presence of dissipative coupling between the ionized adatom and the substrate heat bath. This enables us to discuss the role which inelastic effects may play in recently studied cases of stimulated desorption of neutral Ar atoms from metal surfaces.

Energy loss of 100 keV protons in single collisions with argon atoms

We have measured energy loss spectra of 100 keV protons interacting with argon atoms in small angle scattering under single collision conditions. The high overall energy resolution and the geometry of our setup allow a reliable determination of the mean energy loss and straggling. We have measured these quantities in dependence on the angle of scattering and find good agreement with recent calculations by Kabachnik et al.

Calculation of energy spectra of electrons transmitted through thin aluminium foils

The use of an analytical formula for the energy loss spectrum of plasmon, single-electron and inner-shell excitation of aluminium in the Landau theory of multiple scattering results in a better agreement with experimental values of the mean energy loss and the half width of the energy distribution of 10-100 keV electrons transmitted through aluminium foils than the original Landau theory. The same loss spectrum can be used in Monte Carlo calculations instead of a Bethe continuous slowing down approximation for a better consideration of plasmon losses and straggling and for calculating multiple scattering effects in electron energy loss spectroscopy (EELS).

Charge state dependent energy loss of high velocity carbon ions in the charge state non-equilibrium region

Energy losses of high velocity bare H-like and He-like carbon ions passing through thin carbon foils were measured in the charge state non-equilibrium region. The mean energy losses for each charge state increased proportionally to the foil thickness and a clear charge state dependence was observed.

Statistical theory of charged-particle stopping and straggling in the presence of charge exchange

General expressions have been derived for the mean energy loss and straggling of a heavy charged particle penetrating through a thin slice of matter. The main input parameter is a differential energy loss rate that allows for transitions within an arbitrary number of projectile states. Both spontaneous and collision-induced events are potentially induced, but different events are assumed statistically independent. Asymptotically, the mean energy loss is shown to approach a linear dependence on time or path length. The slope is given by the equilibrium energy loss rate, and the intercept at zero path length depends on the incident projectile state. Energy-loss straggling can be split into a collisional part and a contribution from variations in projectile state. The latter turns out to be related to the intercept in the mean energy loss for the pertinent projectile states. Previous results applying to a two-state system have been generalized to an arbitrary number of states. A full evaluation has been presented for a three-state system. A procedure is outlined that allows to empirically determine the magnitude of charge-exchange straggling.

Energy loss dependence of X-rays emitted by titanium ions channeled in a thin gold single crystal

X-rays generated by fully stripped titanium ions channeled along the 〈100〉 axis of a thin gold single crystal have been measured in coincidence with charge-changed ions as a function of ion energy loss. X-rays from the process of radiative electron capture are used to derive mean target electron densities for three values of mean energy loss. These densities are compared with densities calculated from a free-electron gas model for stopping. The results are used in a model calculation for Lyman series X-ray emission yields which includes effects of strong competition from secondary electron impact excitation and ionization of excited-state hydrogenic ions formed by electron capture in the crystal.

Energy-diffusion calculation of the electron-swarm distribution function.

An approximate treatment of the Boltzmann collision integral for electrons in a gas, valid for small fractional average energy loss, is studied. It is essentially a Fokker-Planck expansion in energy space, including mean energy loss (dynamical friction) and energy straggling (coefficient of diffusion). When applied to electron swarms in weakly ionized gases, treating angle variables in the two-term Legendre series, there results a useful, physically meaningful, differential equation for the time evolution of the energy spectrum in a time-dependent electric field. Elastic scattering, and inelastic and superelastic energy-transfer collisions are included. The time-independent solution in a constant field is a simple approximate expression for the steady-state energy spectrum of swarm electrons. The physical meaning of its functional form is made clear by showing its relation to ordinary diffusion-convection theory. Previous spectra by Pidduck [Proc. R. Soc. London Ser. A 88, 296 (1913); Proc. London Math. Soc. 15, 89 (1916); Q. J. Math. 7, 199 (1936)]; Druyvesteyn [Physica 10, 61 (1930)]; Davydov [Phys. Z. Sowjetunion 8, 59 (1935)]; Morse, Allis, and Lamar [Phys. Rev. 48, 412 (1935)]; Chapman and Cowling [The Mathematical Theory of Non-Uniform Gases, 2nd ed. (Cambridge University Press, Cambridge, 1952), p. 350]; and Wannier [Am. J. Phys. 39, 281 (1971)] are special cases.The reasons for the inadequacy of the continuous-slowing-down approximation (CSDA) become apparent. The new spectrum is exact in the limit of small quantum transition energies. It is further shown that the CSDA violates detailed balance. Consequences of detailed balance on the loss function and related functions are investigated, and the Boltzmann H theorem is studied. During non-steady-state behavior, collisions may increase or decrease the swarm entropy, but the effect of the electric field is always to increase entropy. The spectrum is used with experimental cross sections to compute transport coefficients in ${\mathrm{O}}_{2}$ and ${\mathrm{N}}_{2}$, in both of which fractional average energy loss is acceptably small over most energy ranges. Agreement with compiled swarm data is excellent over more than four orders of magnitude in E/N for most coefficients, except at certain energies in ${\mathrm{N}}_{2}$ that strain the approximation's validity. In the absence of an electric field, the inclusion of energy straggling provides a treatment of spectral relaxation valid for arbitrary energies that is an improvement over common mean-stopping-power formulas.

Photodissociation of sulfur dioxide cluster anions

Photodissociation of negatively charged sulfur dioxide clusters (SO2)n−, 2≦n≦11, was investigated in the wavelength range between 458 nm and 600 nm using a tandem mass spectrometer. The spectral position of the absorption band remains unchanged, however it exhibits narrowing with increasing cluster size. The smooth evolution of the spectra shows that the clusters are composed of a dimer anion core surrounded by neutral molecules. The analysis of the fragmentation products reveals that the absorption of a photon is followed by statistical evaporation of neutrals with a mean energy loss of 0.28±0.05 eV per evaporated monomer in the large cluster limit.

Charge-changing contribution to energy loss of 32 MeV 3He + in the charge state non-equilibrium region

Energy losses of 32 MeV 3He ions by carbon foils were measured in the charge state non-equilibrium region. The mean energy losses exhibited an abrupt change as a functiion of the foil thickness around 40–50 μg/cm 2. This behavior is quantitatively explained by considering the contribution from charge-changing collisions.

A superconductive junction detector as an energy proportional counter

A model to describe the linear regime in a Superconducting Tunnel Junction detector has been developed. In the limit of low temperatures, the only effective processes which involve quasi-particles and phonons are the phonon scattering mechanisms and the phonon escape from the area of the detector. The response of the STJ is linear if τ esc −1 >; τ ph −1 for low energy phonons and τ esc −1 < τ ph −1 for high energy phonons. The estimate of the mean energy loss ϵ by radiation per excess quasi-particle in the STJ is in agreement with previous determinations. A comparison with some experimental results taken from literature has been produced.

Scattering of reactive and inert ion beams from surfaces: energy loss, straggling and skipping

We compare and contrast the scattering behaviour of beams of the ‘chemically active’ species H + and N + with that of the ‘chemically inert’ species He + and Ar +. The 3–3.5 keV beams were incident at from 1–8° to a Cu(111) surface. The scattered neutral particle energy distributions were recorded at scattering angles in the range 1–9° with a 0.2° angular resolution using a time-of-flight (TOF) technique. We analyse the mean energy loss and straggling as a function of incidence, scattering and azimuthal angles, and beam species. Both quantities behave in a complex manner as a function of these variables. No universal behaviour was found. We report regularly spaced peaks in the scattered Ar atom energy distributions and evidence for such structures in the scattered N atom distributions. We tentatively associate the fine structure in at least the Ar distributions with transient adsorption or skipping motion of a portion of the beam on the surface.

Energy loss by electrons in gaseous saturated hydrocarbons

The stopping powers, the inelastic mean free paths, the ranges, and the distributions of energy loss events for electrons in some gaseous alkanes have been calculated from experimentally based dipole oscillator strength distributions. The results show very little difference between the members of the homologous series of straight-chain hydrocarbons, C{sub n}H{sub 2n+2} where n = 2, 4, 6, and 8. For instance, the integrated path length of a 1-MeV electron in ethane of unit density is 0.38 cm and increases to 0.40 cm in octane of the same density. The stopping power of a 1-MeV electron in unit density hexane is 0.2 eV/nm, and the inelastic mean free path is 220 nm. The corresponding values for the other alkanes differ by less than 10%. The distribution of energy loss events along the track of a high-energy electron in these gaseous hydrocarbons is not significantly affected by the value of n, and it is insensitive to the incident electron energy from 10 keV to 1 MeV. The most probable energy loss along the track of a 1-MeV electron is 14 eV in ethane, 15 eV in butane and hexane, and 16 eV in octane. The mean energy loss is 25 eVmore » in butane, hexane, and octane and is 1 eV less in ethane.« less

Energy flux density estimation over sea ice based on satellite passive microwave measurements

This paper discusses in detail the energy flux density estimation at the ice/atmosphere interface over the entire North Water region, based on the ice-type distribution during the winter (1978/79 to 1985/86). The analysis is based on passive microwave measurements of the scanning multichannel microwave radiometer (SMMR) onboard the Nimbus 7 satellite. Based on the relation of increasing polarization ratio (18 or 37 GHz) with decreasing ice thickness, a threshold technique was applied to classify the different young ice types. The polarization ratio method seems feasible for climatological applications, such as energy flux density estimations, over large areas with homogeneous young ice. More than 20% young ice (&lt;0.3m) was found in the North Water area (100,000 km2) throughout most winters, with maximal values of 60% and more, and only a few percent of open water were classified. In general, the young ice cover in the North Water was decreasing towards the end of the winter. The mean energy loss for the entire North Water region was found to be 77 W m−2 for the months of November to March. Considering an energy supply by refreezing of 38 W m−2, the remaining 39 W m−2 must be withdrawn from the enthalpy in the sea water.

Energy flux density estimation over sea ice based on satellite passive microwave measurements

This paper discusses in detail the energy flux density estimation at the ice/atmosphere interface over the entire North Water region, based on the ice-type distribution during the winter (1978/79 to 1985/86). The analysis is based on passive microwave measurements of the scanning multichannel microwave radiometer (SMMR) onboard the Nimbus 7 satellite. Based on the relation of increasing polarization ratio (18 or 37 GHz) with decreasing ice thickness, a threshold technique was applied to classify the different young ice types. The polarization ratio method seems feasible for climatological applications, such as energy flux density estimations, over large areas with homogeneous young ice. More than 20% young ice (&amp;lt;0.3m) was found in the North Water area (100,000 km2) throughout most winters, with maximal values of 60% and more, and only a few percent of open water were classified. In general, the young ice cover in the North Water was decreasing towards the end of the winter. The mean energy loss for the entire North Water region was found to be 77 W m−2for the months of November to March. Considering an energy supply by refreezing of 38 W m−2, the remaining 39 W m−2must be withdrawn from the enthalpy in the sea water.

Interaction of slow (100 ev/atom) carbon clusters with gold: Penetration properties and collision cascades

The interaction of carbon clusters of variable size with thin gold targets has been analyzed by molecular dynamics simulation. The initial energy was 100 eV per cluster atom. Only a minor difference was found between the mean energy loss of transmitted cluster atoms and individual bombarding carbon atoms. The energy spectrum of transmitted cluster atoms is much broader than for atomic bombardment. In particular, a substantial fraction of the cluster atoms is transmitted with energies higher than their initial energy. This effect has been traced to collisions between cluster atoms. Similar, yet less pronounced effects were also found for reflected cluster atoms. The angular distribution of transmitted cluster atoms shows a sharp dip in the forward direction while their average energy in the forward direction even exceeds the initial energy per atom. Recoiling target atoms may achieve kinetic energies up to more than twice the maximum recoil energy in a single elastic collision. This effect has been traced to mu ltiple hits by different cluster atoms.

Energy loss by slow electrons in polar gases.

The mean energy loss by slow electrons moving in a gas and rotationally exciting the molecules is proportional to the rotational stopping cross section (RSCS) defined by multiplying the cross section for a rotational transition \ensuremath{\Gamma}\ensuremath{\rightarrow}\ensuremath{\Gamma}' by the transition energy and by summing the product over \ensuremath{\Gamma}'. The RSCS for linear and symmetric-top polar molecules are studied in the adiabatic-rotation approximation at scattering angles \ensuremath{\theta} larger than a critical value ${\mathrm{\ensuremath{\theta}}}_{\mathit{c}}$ and in the first-Born approximation for \ensuremath{\theta}${\mathrm{\ensuremath{\theta}}}_{\mathit{c}}$. Formulas independent of ${\mathrm{\ensuremath{\theta}}}_{\mathit{c}}$ are derived for the RSCS for any \ensuremath{\Gamma} as the sum of the RSCS for the ground rotational state \ensuremath{\Gamma}=0 and a term that depends on \ensuremath{\Gamma} in a simple analytic form and is proportional to the dipole moment D squared. For a vanishing D, this term vanishes and the formulas reduce to a previously proven theorem for nonpolar molecules that the RSCS is independent of \ensuremath{\Gamma}. Useful expressions are also derived for the average of the RSCS over the Boltzmann distribution of \ensuremath{\Gamma}.

Penetration of slow gold clusters through silicon

The penetration of gold clusters with energies of 0.1 and 1 keV atom through thin silicon targets has been investigated by molecular dynamics simulation. The stopping power per gold atom is noticeably smaller than for bombardment with gold atoms. For 0.1 keV atom , we observed a decrease by a factor of two. The energy spectrum of penetrating gold atoms is broader for cluster bombardment and the skewness smaller, compared with the case of atomic bombardment. Similar effects are observed in angular scattering distributions. The decrease in mean energy loss is demonstrated to be caused by a clearing-the-way effect. The increased energy loss straggling is shown to be due to collisions between projectile atoms. The results of this work are asserted to be characteristic for the case of clusters made up by atoms that are heavier than the target atoms.

Calculation of Energy Loss Characteristics of Accelerated Ions Passing through Thin Targets

AbstractThe possibility is discussed of checking the formula for the impact parameter dependence of energy losses in an inelastic collision, comparing calculated and experimental mean energy losses and straggling of ions passing a thin foil. Some results of Monte‐Carlo calculations are presented.

Energy distribution of electrons transmitted through thin foils

The energy distributions of electrons transmitted through thin foils are investigated especially with regard to the possibilities of an analytical representation of these spectra. Most probable energy losses are quantified and compared with mean energy losses. A mathematical description of the energy spectra is presented and its applicability in the electron acceleration energy region between ≈15 and 40 keV is proved by comparison with experimentally found distributions. Due to the simple form of the respective formulae several physical phenomena such as energy deposition by electrons in solids, scattering phenomena, etc., can easily be investigated and quantified.