Optical Energy Loss Function Research Articles

Electronic Stopping Power for 0.05–10 MeV Protons in a Group of Organic Materials

Electronic stopping powers for 0.05–10 MeV protons in a group of organic materials are systematically calculated. The calculations are based on Ashley's dielectric model, and an evaluation approach of optical energy loss function is incorporated into Ashley's model because no experimental optical data are available for most of the organic materials under consideration. The Barkas-effect correction and Bloch correction are included. The proton stopping powers for the considered organic materials except for mylar in the energy range from 0.05 to 10MeV are presented for the first time. The results may be useful for studies of various radiation effects in these materials and for space research.

Electronic stopping power for proton in amino acids and protein in 0.05–10 MeV range

The stopping powers for 0.05–10MeV protons in a group of 15 amino acids and a protein have been systematically calculated. The calculations are based on Ashley’s dielectric model. An approach of evaluating the optical energy loss function is incorporated into the Ashley’s model because no experimental optical data are available for these bioorganic compounds. The Barkas-effect correction and Bloch correction are included and an empirical minimum impact parameter a is used for the Barkas-effect correction. The proton stopping powers for the 15 amino acids and the protein in the energy range from 0.05 to 10MeV are presented here for the first time and might be useful for studies of various radiation effects in these materials.

Proton stopping power in a group of bioorganic compounds over the energy range of 0.05–10 MeV

The systematic calculations of the stopping powers (SPs) for 0.05–10 MeV protons in a group of 10 important bioorganic compounds, i.e. DNA, five bases, three fatty acids and cellulose, have been performed. The calculations are based on the dielectric model of Ashley and on an evaluation of optical energy loss function (OELF) for the bioorganic compounds considered, most of which have no available optical data. Higher-order corrections, Barkas-effect correction and Bloch correction, are included and a concise form of SP calculation corresponding to Barkas-effect is presented by incorporating the evaluated OELF. Especially, the dependences of SP calculations on the two choices of the minimum impact parameter a are analyzed in detail, and hence an appropriate parameter a is determined for organic media. For the tested organic compounds the proton SPs calculated by use of the determined a are in excellent agreement with the tabulated values based on experimental data. The presented data are the first results of the proton SPs for the 10 bioorganic compounds in the energy range from 0.05 to 10 MeV, and might be useful for studies of various radiation effects in these materials.

Electron stopping power and inelastic mean free path in amino acids and protein over the energy range of 20–20,000 eV

Systematic calculations of stopping power (SPs) and inelastic mean free path (IMFP) values for 20-20,000 eV electrons in a group of 15 amino acids and a simple protein have been performed. The calculations are based on the dielectric response model and take into account the exchange effect between the incident electron and target electrons. The optical energy-loss functions for the 15 investigated amino acids and the protein are evaluated by using an empirical approach, because of the lack of experimental optical data. For all the considered materials, the calculated mean ionization potentials are in good agreement with those given by Bragg's rule, and the evaluated SP values at 20 keV converge well to the Bethe-Bloch predictions. The data shown represent the first results of SP and IMFP, for these 15 amino acids and the protein in the energy range below 20 keV, and might be useful for studies of various radiation effects in these materials. In addition, the average energy deposited by inelastic scattering of the electrons on this group of 15 amino acids, on the protein, on Formvar and on DNA, respectively, has been estimated for energies below 20 keV. The dependences of the average energy deposition on the electron energy are given. These results are important for any detailed studies of radiation-induced inactivation of proteins and the DNA.

Electron inelastic interactions in bioorganic compounds in the energy range of 20–10000 eV

Systematic calculations of stopping powers (SPs) and mean free paths (MFPs) for 10 bioorganic compounds have been performed for electrons with energies lower than 10 keV, based on dielectric response theory and Penn’s statistical approximation. The exchange effect is also taken into account in the calculations. An empirical approach to obtain an optical energy loss function is presented for those organic compounds without available optical data. Using this method, the calculated values of the optical energy loss function are in good agreement with experimental data. Comparisons of SP and MFP values derived in this study with other published values are presented. Using the described model, the calculated mean ionization potentials agree well with the predictions from Bragg’s rule and the calculated SPs have also been compared with the Bethe–Bloch results at an energy of 10 keV.

Cross sections of electron inelastic interactions in DNA

The cross sections of electron inelastic interaction in DNA are calculated using the dielectric response theory and Penn statistical approximation, with the exchange correction included. An empirical approach to obtain optical energy loss function is given for the organic compounds without available optical data. Comparisons of the calculated data with available experimental and theoretical results have been done to show the reliability of the approach proposed in this work. Using this approach, the total inelastic cross sections for five bases: guanine, adenine, thymine, cytosine and uracil have been calculated in the energy range of E< or =10 keV and compared with those recently obtained with the Deutsch-Mark formalism and the Binary-Encounter-Bethe model, respectively. An equivalent unit of the DNA molecule is constructed according to the contents of A-T and G-C base pairs in DNA, and is divided into five constituents, i.e. sugar-phosphate and four bases. The total inelastic cross sections for the constructed unit of the DNA molecule and its constituents have also been calculated.

Electron stopping power and mean free path in organic compounds over the energy range of 20–10,000 eV

An empirical method to obtain optical energy loss functions is presented for a large number of organic compounds, for which optical data are not available, on the basis of structure feature analysis of the existed optical energy loss functions for certain organic compounds. The optical energy loss functions obtained by using this method are in good agreement with the experimental data. Based on the Penn's statistical model, a set of systematic expressions have been given for the calculation of the stopping powers and mean free paths of electrons penetrating into the organic compounds in the energy range of E⩽10 keV. Detailed comparison of the calculated data with other theoretical results is presented. The stopping powers and mean free paths for a group of important polymers, without available optical data, have been calculated. In the calculations, three different cases have been considered, i.e. exchange correction not being considered, Ashley exchange correction being involved, and Born–Ochkur exchange correction being included. The results indicate that for these compounds the calculated stopping powers agree well with those obtained by using Bethe–Bloch theory at high-energy limit E=10 keV, as expected for a stopping power theory that should be converged to Bethe–Bloch theory at high energies.

Ion-track structure and its effects in small size volumes of silicon

A model for ion-energy deposition in silicon, based on experimental data of the optical energy loss function, is presented. The model yields the characteristics of ion- and electron-interactions in silicon. Monte Carlo transport calculations based on this model give the energy distribution and its moments including a full and consistent description of the spatial ion track structure. The model was used for estimating the influence of the track structure on energy deposition in submicron devices and on the shape of the single event upsets (SEUs) cross-section curves.

Electro-optical properties of single-walled carbon nanotubes

Optical conductivity tensor and electron energy loss function for individual single-wall carbon nanotubes (SWCNT) of arbitrary chirality are found. The reported two types of results obtained in the TEM measurements on the isolated SWCNT of the same diameter are interpreted. The difference in plasmon excitation probabilities is shown to be related to the different numbers of allowed interband transitions in the chiral and achiral tubes, while the peak position shift is to be attributed to the interrelationship between the parities of the electronic states of the armchair tubes and the polarization of the perturbing field.

Optical conductivity of the Bechgaard salts: the sum rules revisited

The validity of the optical sum rules has been addressed eversince and was always matter of debate. Particularly controversial is the proof that the partial sum rules can be extended to both optical conductivity and energy loss function. We show in this paper that for both transverse (optical conductivity) and longitudinal (energy loss function) absorption processes the corresponding sum rule can be theoretically established and through appropriate conditions for the integration limits exactly verified. We also focus our attention on the one-dimensional case within the microscopic Hubbard model. An application of these concepts to the quasi one-dimensional systems, for which we have chosen the organic (TMTSF)2PF6 material, will also be presented.

Optical and electron energy loss experiments in ionic CrBr3 crystals

The fundamental reflectance of CrBr3 crystals has been measured over the energy range 2-31 eV from 300 to 30 K using synchrotron radiation. The high-frequency dielectric tensor elements epsilon xxT(E) have been determined by Kramers-Kronig analysis. Energy losses of 50 keV electrons incident along the c axis of the crystal have also been measured up to 30 eV. The results have been described in terms of single-particle excitations (charge transfer, inter-band excitons and transitions) and collective valence electron plasma oscillations. The assignments of direct inter-band transitions both at the critical points Gamma and Z and along symmetry lines of the Brillouin zone ( Lambda line) have been made by considering the intersecting-sphere model calculation of the band structure of CrBr3. The fundamental energy gap, assigned to Gamma 3- to Gamma 1+ transitions, at the zone centre is observed around 8.00 eV. Plasma resonance has been observed at 19.8 eV in the electron energy loss spectrum in fine agreement with the maximum of the optical energy loss function. Finally, the fractional ionic character fiDT=0.79 has been evaluated in the framework of the Phillips dielectric theory.

Infrared reflectance properties of surface thin films

Analytic expressions for the reflectance of an isotropic thin film with flat, parallel sides on a semi-infinite substrate are developed for the tin film limit. Proper expressions are given for the limiting cases of ideal dielectric substrates (k{sub d} = O) and metallic substrates (k{sub b} >> 1). The interesting case of the intermediate substrate (k{sub b} {approx} 1) is treated for the first time. The reflectance spectra in the limiting cases are written as linear combinations of the transverse optical and longitudinal optical energy-loss functions of the thin film. Spectra of films on an intermediate substrate with constant extinction coefficient will have distorted line shapes as a result of contributions from the real part of the film's dielectric function. For a substrate with an absorption resonance, additional bands from the substrate will appear, one of which shifts in frequency with incident angle, even in a spectrum normalized to the reflectance of the bare substrate.

Optical and electron energy loss experiments in ionic CrCl3crystals

The low-temperature reflectance of CrCl3 crystals has been measured in the ultraviolet region up to 31 eV with synchrotron radiation, and the high-frequency dielectric tensor elements epsilon xxT(E) have been deduced by Kramers-Kronig analysis. Energy losses of 50 keV electrons incident along the c axis of the crystal have also been measured as far as 80 eV. The results of the optical and electron loss experiments are described in terms of single-particle (charge transfer and interband transitions, excitons and inner core excitations) and collective excitations (valence-electron plasma oscillations). The energy gap Eg approximately=9.7 eV is assigned to direct allowed transitions Gamma -3 to Gamma 1+ at the zone centre; higher energy strong interband transitions occur at the symmetry points Gamma and Z, and along symmetry lines of the Brillouin zone ( Lambda line). The plasma resonance has been observed at 22.6+or-0.2 eV in the electron loss spectrum in excellent agreement with the maximum of the optical energy loss function. The high-energy losses observed beyond 40 eV have been attributed to Cr3+ 3p-shell excitations. Finally, the fractional ionic character (fiDT approximately=0.80-0.85) of CrCl3 has been evaluated in the framework of the Phillp's dielectric theory.

Straggling and plasmon excitation in the energy loss spectra of electrons transmitted through carbon

A model is described for the calculation of differential inverse mean free paths (DIMFPs) for the inelastic interactions of electrons with solids. The energy loss function Im{-1/ ε( q, ω)} is represented as a sum of Drude-type functions. The adjustable parameters in the energy loss function are fixed by a fit to the energy loss function in the optical limit ( q → 0) obtained from data on the optical constants of glassy carbon. The extension of the optical energy loss function to arbitrary values of q in this model leads to an analytical expression for the DIMFP. Energy loss spectra for low energy electrons (less than 2000 eV) transmitted through carbon foils of various thicknesses are determined by solving the transport equation using a convolution method and the DIMFPs determined as described above. Comparisons of these calculated spectra with those measured by Jacobi show that the model provides a good description of both the broad straggling distribution and the fine structure due to plasmon excitation.

Plasma oscillations in Ag-Au alloys

The existence of plasma oscillations in Ag-Au alloys is discussed by means of the complex dielectric constant and the optical energy loss function deduced from optical measurements on thin films. The role of interband transitions in the plasmon damping is investigated. The results are compared with those of different authors.