Effect Of Electronic Excitation Research Articles

Electronic excitation effects in CeO2 under irradiations with high-energy ions of typical fission products

In order to understand the formation mechanism of a crystallographic re-structuring in the periphery region of high-burnup nuclear fuel pellets, named as “rim structure”, information on the accumulation process of radiation damage and fission products (FPs), as well as high-density electronic excitation effects by FPs, are needed. In order to separate each of these processes and understand the high-density electronic excitation effects, 70–210MeV FP ion (Xe10–14+, I7+ and Zr9+) irradiation studies on CeO2, as a simulation of fluorite ceramics of UO2, have been done at a tandem accelerator of JAEA-Tokai and the microstructure changes were determined by transmission electron microscope (TEM). Measurements of the diameter of ion tracks, which are caused by high-density electronic excitation, have clarified that the effective area of electronic excitation by high-energy fission products is around 5–7nmϕ and the square of the track diameter tends to follow linear function of the electronic stopping power (Se). Prominent changes are hardly observed in the microstructure up to 400°C. After overlapping of ion tracks, the elliptical deformation of diffraction spots is observed, but the diffraction spots are maintained at higher fluence. These results indicate that the structure of CeO2 is still crystalline and not amorphous. Under ion tracks overlapping heavily (>1×1015ions/cm2), surface roughness, with characteristic size of the roughness around 1μm, is observed and similar surface roughness has also been observed in light-water reactor (LWR) fuels.

Isotopic effects in the electronic spectra of tryptophan

No influence of isotopic substitution in deuterium-substituted tryptophan on the florescence excitation spectrum has previously been found out. Here, the isotopic effects of electronic excitation of deuterium-substituted tryptophan were experimentally and theoretically analyzed for first time. It was shown a short-wave shift of the UV-absorption maximum at 220 nm corresponding to the 360 cal/mol and short-wave shift for fluorescence spectrum corresponding to the 210 cal/mol. To account for this effect, the quantum chemical calculations of the geometric and electron structure, frequencies of normal vibrations and transition energies have been performed. The isotopic effects originate from the zero-point energies of ground and excited states. It was found that isotopic shifts depend on the position of isotope in the molecule and kind of transition. So, it can be utilized in the analysis of proteins structure and complexation.

Electronic excitation effects and phase formation in semiconductor compound nanoparticles

Electronic excitation effects and phase formation in semiconductor compound nanoparticles

Including the Effects of Electronic Excitations and Electron-Phonon Coupling in Cascade Simulations

AbstractRadiation damage has traditionally been modeled using cascade simulations however such simulations generally neglect the effects of electron-ion interactions, which may be significant in high energy cascades. A model has been developed which includes the effects of electronic stopping and electron-phonon coupling in Molecular Dynamics simulations by means of an inhomogeneous Langevin thermostat. The energy lost by the atoms to electronic excitations is gained by the electronic system and the energy evolution of the electronic system is modeled by the heat diffusion equation. Energy is exchanged between the electronic system and the atoms in the Molecular Dynamics simulation by means of a Langevin thermostat, the temperature of which is the local electronic temperature. The model is applied to a 10 keV cascade simulation for Fe.

Electronic excitation effects on nanoparticle formation in insulators under heavy-ion implantation

Kinetic processes of nanoparticle formation by ion implantation was studied for the insulators of a-SiO2, LiNbO3, MgO·2.4(Al2O3) and PMMA, either by changing ion flux or by using a co-irradiation technique of ions and photons. Under Cu-implantation of 60keV Cu−, nanoparticles spontaneously formed without thermal annealing, indicating radiation-induced diffusion of implants. The high-flux implantation caused instable behaviors of nanoparticle morphology in a-SiO2, LiNbO3 and PMMA, i.e. enhanced atomic rearrangement or loss of nanoparticles. The spinel MgO·2.4(Al2O3) also showed nanoparticle precipitation at 60keV, but the precipitation tendency is less than the others. Combined irradiation of 3MeV Cu ions and photons of 2.3eV or 3.5eV indicates that the electronic excitation during ion implantation significantly enhances nanoparticle precipitation, greatly depending on photon energy and fluence. The selectivity for photons can be applied to control nanoparticle precipitation.

Effect of electron flux on electronic-excitation-induced phase separation in GaSb nanoparticles

We studied the effect of electron flux on phase separation induced by electronic excitation in GaSb nanoparticles, in order to see whether the phase separation is characterized as a cooperative phenomenon, in which a nonlinear relation may be found between density of excited states introduced and the efficiency of the phase separation or not. The phase separation to two phases consisting of an antimony core and a gallium shell proceeds abruptly after incubation time with increasing electron dose and does only at the flux above a threshold value. It is suggested that such nonlinear behaviors take place as a cooperative phenomenon among electronic-excitation effect, nano-size effect and temperature.

Silicide formation study at the interface of Zr/Si system using high-energy swift heavy ion

The effects of large electronic excitation resulting in silicide formation at the zirconium/silicon interface were studied using the swift heavy ions (SHI). In the present study Zr was deposited on Si ( ∼ 100 nm ) over Si [1 0 0] substrate at a pressure of 4 × 10 - 8 Torr . The final layer was Si to avoid oxidation of Zr. The Zr/Si system was irradiated by 350 MeV Au 26 + ions at liquid nitrogen temperature at the fluences of 0.5 × 10 14 , 1.85 × 10 14 and 4.62 × 10 14 ions / cm 2 . Rutherford backscattering spectroscopy (RBS) was done for the compositional analysis of the different specimens. X-ray diffraction experiments were carried out on the as-deposited and irradiated samples to study the phase formation. RBS results indicated mixing at the interface and XRD revealed the silicide phase.

Adsorption and diffusion of Ga and N adatoms on GaN surfaces: Comparing the effects of Ga coverage and electronic excitation

We present density-functional-theory calculations of the binding and diffusion of Ga and N adatoms on GaN (0001) and (000-1) surfaces under different conditions, including stoichiometric and Ga-rich surfaces, as well as in the presence of electron-hole (e-h) pairs induced by light- or electron-beam irradiation. We find that both Ga-rich conditions and electronic excitations cause a significant reduction of the adatom diffusion barriers, as required to improve the quality of the material. However, the two effects are nonadditive, as the influence of e-h pairs are found to be less important for the more metallic situations.

Evolution kinetics of sp 2 ordering in tetrahedral amorphous carbon films induced by electron irradiation

The kinetics of sp 3 to sp 2 structural conversion in tetrahedral amorphous carbon (ta-C) films induced by sample irradiation with an electron beam of transmission electron microscope was studied by systematically measuring the temporal evolution of the graphitic phase detected by electron energy loss spectroscopy (EELS). The results analyzed in the framework of the Johnson–Mehl–Avrami model indicates that the phase front velocity increases in proportion to the irradiation beam current density, providing an experimental support for the notion that the effect is caused by an electronic excitation effect. It was inferred from the experimentally assessed kinetic order index that the three-dimensional growth of graphitic phases is enhanced by electronic excitations. Possible microscopic mechanisms are discussed.

Isotopic effect of electron excitation in l-[ 3H]tryptophan

Isotopic effect of electron excitation in organic compounds was for the first time experimentally and theoretically analyzed. The nature of this phenomenon is due to zero-point energies (ZPE) for the ground and the excited states of the isotope substituted tryptophan. l-[ 3H]tryptophan ( l-[ 3H]Trp) with a tritium–hydrogen substitution ratio of 0.2–0.45 has been produced by high-temperature solid-state catalytic isotope exchange and was used in an experimental study. A comparison of the UV spectra of l-[ 3H]Trp and l-[ 1H]Trp showed a short-wavelength shift of the absorption maximum in the tritium-substituted compound that corresponds to an energy of 130 cal/mol for the electronic transition. To account for this effect, quantum chemical calculations of the geometric and electronic structures, frequencies of normal vibrations, and transition energies have been performed using of the restricted Hartree–Fock method (RHF), density-functional theory (RB3LYP), and configuration interaction (RCIS). The shifts of all allowed electronic transitions have been shown to shift towards the short-wavelength region and to be equal to 152 cal/mol for the first singlet–singlet transition (within the 280 nm region), to 242 cal/mol for the third transition (within the 220 nm region), and to 557 cal/mol for the singlet–triplet transition. All the shifts obtained by calculations are due to zero-point energies for the ground and the excited states of [ 3H]Trp and [ 1H]Trp.

Swift heavy ion induced magnetic phase transition of Fe–Rh alloy

We report the effects of swift heavy ion irradiation on structure and magnetic properties of Fe–50at.%Rh alloys. The alloys are irradiated with 120–200MeV heavy ions (Ni, Kr or Xe) at room temperature. Before and after the irradiations, the magnetization and the lattice parameter are measured by using superconducting quantum interference device (SQUID) and X-ray diffractometer (XRD), respectively. The lattice parameter at room temperature increases by about 0.3% and antiferromagnetic–ferromagnetic transition temperature decreases below 5K by the irradiations. Effects of electronic excitation due to swift heavy ions on the change in magnetic properties and lattice structure are discussed.

Structural stability of C60 films under irradiation with swift heavy ions

In order to investigate the structural stability of fullerene (C60) under swift heavy ion irradiation, the irradiation experiments of thin C60 films were performed with 22MeV/amu Fe56 ions delivered by HIRFL at Lanzhou in China. The irradiated C60 films were analyzed by means of Raman scattering and Fourier transform infrared (FTIR) spectroscopes. The analysis results indicated that the damage cross-sections σ of the C60 molecule deduced from the data of the Raman spectra are between 1.1 and 4.5×10−14cm2 for the electronic energy loss from 3.5 to 8.7keV/nm and electronic energy transfer dominates the damage process of C60 films. The partial recovery of the damage in irradiated C60 films at certain electronic energy loss is attributed to an annealing effect of strong electronic excitation.

Ion-velocity dependence of high-density electronic excitation effects in oxide superconductors

Thin films of EuBa2Cu3Oy oxide superconductor have been irradiated with high energy heavy ions (80MeV I, 125MeV Br, 1.1GeV Mo and 3.5GeV Xe) having same electronic stopping power, Se, in order to investigate the ion-velocity dependence of the electronic excitation effects under the constant electronic energy deposition. Although Se is constant, a strong reduction in the irradiation effect on lattice parameter with increasing ion-velocity is observed in the low ion-velocity region around E∼1MeV/nucleon, while the ion-velocity dependence is hardly observed in the high ion-velocity region of E>10MeV/nucleon. If the observed velocity-dependence is assumed to be due to the change in the fraction of Se contributing to defect creation, the fraction in the low velocity region (E∼0.6MeV/nucleon) is estimated to be about two times larger than that in the high velocity region (E>10MeV/nucleon).

Measurement of luminescence from silica glasses: an optical x-ray absorption fine structures study at Si K-edge

Soft x-ray excited optical luminescence (XEOL) of silica glasses was studied with respect to the excitation x-ray energy (near the Si K-edge) and irradiation time. The luminescence spectra showed emission bands at 3.1 eV and 2.7 eV, assigned to B2β and B2α oxygen deficient centers in silica glasses, and the luminescence yields of these bands changed drastically before and above the Si K-edge. Si K-edge photoluminescence yield (PLY) spectrum of the band at 3.1 eV was similar to Si K-edge XANES spectrum recorded in the photocurrent mode for the same silica sample, except for the negative edge jump in the PLY spectrum.The time evolutions of the band intensity at 3.1 eV were also investigated under the excitation by soft x-ray below and above Si K-edge. The band intensity in both cases decreased slightly with the irradiation. These results indicate that a part of B2β centers in the silica glass probably changed to other types of oxygen deficiencies by the electron excitation effect. In the early stage of the irradiation, a rapid increase in the intensity of the 3.1 eV band was also observed, indicating that soft x-ray irradiation produced luminescence sites such as B2β centers.

Electronic excitation effects on radiation damage in insulators under ion irradiation

To extract electronic excitation effects from the synergistic damage processes, we have studied photon-irradiation effects on insulators under heavy ion irradiation. Copper ions (Cu 2+) of 3 MeV energy at an ion flux of 2 μA/cm 2 and 2.3 eV photons at 0.2 J/cm 2pulse were used to amorphous SiO 2 (KU-1) and spinel MgO·2.4(Al 2O 3), either sequentially or simultaneously to fluences up to 5 × 10 17 ions/cm 2. Atomic force microscopy and cross-sectional TEM were conducted to study the surface morphology and internal microstructure, respectively. The simultaneous photon irradiation at high photon densities significantly enhanced surface damage for the insulators, but alleviated bulk defects. The electronic excitation gave rise to significant reduction in dislocation loops in MgO·2.4(Al 2O 3), whereas single ion irradiation produced copious dislocation loops. The results demonstrate that intense electronic excitation, coexistent with heavy ions, excite transient sub-gap states and the absorbed energy results in enhancement of atomic migration, either damaging the surface or annealing the internal defects.

Electron emission and fragmentation of C 60 in collisions with energetic particles

We have studied ionization and fragmentation of C 60 induced in collisions with fast helium, silicon and gold ions. Fragment ions and electrons produced were measured simultaneously in coincidence with outgoing charge-selected projectile ions. By means of this triple coincidence method we obtained successfully detailed information about the correlated emission process between electrons and fragment ions. It was found that the number of emitted electrons n e depends strongly on the cluster size of fragment ions and n e increases with decreasing cluster size. Also, we found that, with increasing atomic number of the projectile ions, the C 60-multi-fragmentation becomes more prominent and the value of n e increases as well, indicating clearly the importance of electronic excitation effects. As a more important new result, it was found that the multi-fragmentation is induced rather strongly even in such collisions where only a few electrons are emitted. It is safely concluded that the accompanying internal excitation plays an essential role in the C 60-multi-fragmentation.

Protein-Drug Interaction in the Nanocomposites Prepared by UV and IR Pulsed Laser Deposition

ABSTRACTProtein (bovine serum albumin; BSA)-drug (indomethacin; IM) nanocomposites were prepared by pulsed laser deposition (PLD) at two wavelengths, infrared (1064nm) and ultraviolet (266nm), from uniformly dispersed mixture of BSA and IM as a target. Composite particulates under 50nm were obtained with the coexistence of larger agglomerates over 200nm. Primary structure of BSA is preserved after laser irradiation by both wavelengths. Effects of electronic and vibrational excitation by UV and IR laser respectively on the secondary structure of the nanocomposites were examined by Fourier transform infrared spectroscopy (FT-IR). Chemical shift towards lower wavenumber and a broadening of the amide I band due to PLD treatment were observed by FT-IR. From curve fitting of the amide I into five components, we found the decrease in the ratio of α-helical / β-sheet components with increasing the laser fluence. The secondary structure of BSA is more sensitive to the laser fluence than the wavelength.

Comparative study of depth and lateral distributions of electron excitation between scanning ion and scanning electron microscopes

In order to study the contrast difference between scanning ion microscopes (SIM) and scanning electron microscopes (SEM), the depth and lateral distributions of secondary electrons escaped from surfaces of 17 metals with atomic numbers, Z2, of 4-79 were calculated for bombardment with 30 keV Ga ions and for 10 keV electrons. For both projectiles, the excitation depth generally decreased with increasing Z2, while showing the same periodic change as the secondary-electron yield. However, an opposite trend in Z2 dependence between the Ga ion and electron bombardments was calculated with the lateral distribution of secondary electrons escaped from the surface. Except for low Z2 metals, the lateral distribution, which is much narrower for 30 keV Ga ions than for 10 keV electrons, indicates that the spatial resolution of the secondary-electron images is better for SIM than for SEM, if zero-sized probe beams are assumed. Furthermore, the present calculation reveals important effects of electron excitation by recoiled material atoms and reflected electrons on the lateral distribution, as well as the secondary-electron yield, for the Ga ion and electron bombardments, respectively.

Study of the effect of high electronic excitations in quasicrystals irradiated with heavy ions

We have studied the effect of irradiation with 900 MeV Pb and 780 MeV Xe ions on quasicrystals Al 62Cu 25.5Fe 12.5 and the related cubic α-phase Al 55Cu 27Fe 11Si 7. The fluences ranged from 10 10 to 5.8 × 10 13 ions/cm 2. Irradiations were performed at 80 K and at room temperature. The structural changes induced by the electronic excitations were studied by high-resolution X-ray diffraction. Whereas one might have expected the irradiations to induce important structural transformations, e.g. a phase transition from an icosahedral to a rhombohedral phase, only minor structural modifications are observed in the icosahedral phase and none in the α-phase. The defects created are not phasonic. Our results suggest a remarkable structural stability of these phases with respect to heavy-ion irradiation.

Electronic excitation induced sputtering of insulating and semiconducting oxides by high energy heavy ions

We have measured the electronic sputtering yields of eight samples of insulating and semiconductor oxides by high energy heavy ions for systematic investigation of the electronic excitation effect on atomic displacement, applying a carbon-film collector method. We have found that the sputtering yields, which include the oxygen contribution, increase super-linearly with the electronic stopping power S e and that they are larger by 30–2000 than the calculated yields based on the elastic collision cascade. In the plot of the sputtering yields at a given electronic stopping power versus the band gap E g, the upper limits of the yields are suggested and the maximum yields follow E g 4 dependence.