120 research outputs found
Kramers-Kronig relations beyond the optical approximation
We extend Kramers-Kronig relations beyond the optical approximation, that is
to dielectric functions that depend not only on the
frequency but on the wave number as well. This implies extending the notion of
causality commonly used in the theory of Kramers-Kronig relations to include
the fact that signals cannot propagate faster than light in vacuo. The results
we derive do not apply exclusively to electrodynamics but also to other
theories of isotropic linear response in which the response function depends on
both wave number and frequency.Comment: 1 figur
Surface excitations in the modelling of electron transport for electron- beam-induced deposition experiments
The aim of the present overview article is to raise awareness of an essential
aspect that is usually not accounted for in the modelling of electron transport
for focused-electron-beam-induced deposition (FEBID) of nanostructures: surface
excitations are on the one hand responsible for a sizeable fraction of the
intensity in reflection-electron-energy-loss spectra for primary electron
energies of up to a few keV and, on the other hand, they play a key role in the
emission of secondary electrons from solids, regardless of the primary energy.
In this overview work we present a general perspective of recent works on the
subject of surface excitations and on low-energy electron transport,
highlighting the most relevant aspects for the modelling of electron transport
in FEBID simulations.Comment: 17 pages, 5 figure
Simulation of electron transport in electron beam induced deposition of nanostructures
We present a numerical investigation of energy and charge distributions
during electron-beam-induced growth of W nanostructures on SiO2 substrates
using Monte Carlo simulation of electron transport. This study gives a
quantitative insight into the deposition of energy and charge in the substrate
and in already existing metallic nanostructures in the presence of the electron
beam. We analyze electron trajectories, inelastic mean free paths, and
distribution of backscattered electrons in different deposit compositions and
depths. We find that, while in the early stages of the nanostructure growth a
significant fraction of electron trajectories still interact with the
substrate, as the nanostructure becomes thicker the transport takes place
almost exclusively in the nanostructure. In particular, a larger deposit
density leads to enhanced electron backscattering. This work shows how
mesoscopic radiation-transport techniques can contribute to a model which
addresses the multi-scale nature of the electron-beam-induced deposition (EBID)
process. Furthermore, similar simulations can aid in understanding the role
played by backscattered electrons and emitted secondary electrons in the change
of structural properties of nanostructured materials during post-growth
electron-beam treatments.Comment: 22 pages, 14 figures, 1 tabl
Electromagnetic interaction models for Monte Carlo simulation of protons and alpha particles
Electromagnetic interactions of protons and alpha particles are modeled in a
form that is suitable for Monte Carlo simulation of the transport of charged
particles. The differential cross section (DCS) for elastic collisions with
neutral atoms is expressed as the product of the DCS for collisions with the
bare nucleus and a correction factor that accounts for the screening of the
nuclear charge by the atomic electrons. The screening factor is obtained as the
ratio of the DCS for scattering of the projectile by an atom with a point
nucleus and the parameterized Dirac-Hartree-Fock-Slater (DHFS) electron
density, calculated from the eikonal approximation, and the Rutherford DCS for
collisions with the bare point nucleus. Inelastic collisions, which cause
electronic excitations of the material, are described by means of the
plane-wave Born approximation, with an empirical simple model of the
generalized oscillator strength (GOS) that combines several extended
oscillators with resonance energies and strengths determined from the atomic
configurations and from the empirical mean excitation energy of the material.
The contributions from inner subshells are renormalized to agree with realistic
ionization cross sections calculated numerically from the DHFS self-consistent
model of atoms by means of the plane-wave Born approximation. The resulting DCS
allows analytical random sampling of individual hard inelastic interactions.Comment: 48 pages, 12 figure
SBETHE: Stopping powers of materials for swift charged particles from the corrected Bethe formula
The Fortran program sbethe calculates the stopping power of materials for swift charged particles with small charges (electrons, muons, protons, their antiparticles, and alphas). The electronic stopping power is computed from the corrected Bethe formula, with the shell correction derived from numerical calculations with the plane-wave Born approximation (PWBA) for atoms, which were based on an independent-electron model with the Dirac–Hartree–Fock–Slater self-consistent potential for the ground-state configuration of the target atom. The density effect correction is evaluated from an empirical optical oscillator strength (OOS) model based on atomic subshell contributions obtained from PWBA calculations. For projectiles heavier than the electron, the Barkas correction is evaluated from the OOS model, and the Lindhard–Sørensen correction is estimated from an accurate parameterization of its numerical values. The calculated electronic stopping power is completely determined by a single empirical parameter, the mean excitation energy or I value of the material. The radiative stopping power for electrons, and positrons, is evaluated by means of Seltzer and Berger's cross section tables for bremsstrahlung emission. The program yields reliable stopping powers and particle ranges for arbitrary materials and projectiles with kinetic energy larger than a certain cutoff value , which is specific of each projectile kind. The program is accompanied by an extensive database that contains tables of relevant energy-dependent atomic quantities for all the elements from hydrogen to einsteinium. sbethe may be used to generate basic information for dosimetry calculations and Monte Carlo simulations of radiation transport, and as a pedagogical tool
Collisions of nucleons with atoms: calculated cross sections and Monte Carlo simulation
After a summary description of the theory of elastic collisions of nucleons with atoms, we present the calculation of a generic database of differential and integrated cross sections for the simulation of multiple elastic collisions of protons and neutrons with kinetic energies larger than 100 keV. The relativistic plane-wave Born approximation, with binding and Coulomb-deflection corrections, has been used to calculate a database of proton-impact ionization of K-shell and L-, M-, and N-subshells of neutral atoms These databases cover the whole energy range of interest for all the elements in the periodic system, from hydrogen to einsteinium (Z = 1-99); they are provided as part of the penh distribution package. The Monte Carlo code system penh for the simulation of coupled electron-photon-proton transport is extended to account for the effect of the transport of neutrons (released in proton-induced nuclear reactions) in calculations of dose distributions from proton beams. A simplified description of neutron transport, in which neutron-induced nuclear reactions are described as a fractionally absorbing process, is shown to give simulated depth-dose distributions in good agreement with those generated by the Geant4 code. The proton-impact ionization database, combined with the description of atomic relaxation data and electron transport in penelope, allows the simulation of proton-induced x-ray emission spectra from targets with complex geometries
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