Search

We present here detailed simulations of the interaction of energetic C10 and C5 clusters at the energies of 1, 2, and 4 MeV per carbon atom with an amorphous carbon target. The spatial evolution of the cluster components is simulated accounting for both scattering and Coulomb explosion. The former is calculated by means of the Monte Carlo method while the latter is computed by means of molecular dynamics. The charge state of the individual cluster components is calculated as a function of penetration depth, and is determined by the competition between electron ionization and recombination. The results of calculations of the effect of the neighbouring cluster components on the suppression of the values of the charge state are presented and compared to the experimental values of Brunelle et al. Charge state suppression calculations for the 2 MeV/C clusters for both C10 and C5 agree well with the experimental results for penetration depths of less than about 500 and 250 Å respectively, assuming the intracluster Coulomb potential is screened by four target valence electrons. At 4 MeV/C the results are similar although less screening is required; a possible explanation is the inability of the plasma to completely screen the higher velocity projectiles. The 1 MeV/C calculated results however differ in their behaviour from the 2 and 4 MeV/C cases.

We present here detailed simulations of the interaction of energetic C10 and C5 clusters at the energies of 1, 2, and 4 MeV per carbon atom with an amorphous carbon target. The spatial evolution of the cluster components is simulated accounting for both scattering and Coulomb explosion. The former is calculated by means of the Monte Carlo method while the latter is computed by means of molecular dynamics. The charge state of the individual cluster components is calculated as a function of penetration depth, and is determined by the competition between electron ionization and recombination. The results of calculations of the effect of the neighbouring cluster components on the suppression of the values of the charge state are presented and compared to the experimental values of Brunelle et al. Charge state suppression calculations for the 2 MeV/C clusters for both C10 and C5 agree well with the experimental results for penetration depths of less than about 500 and 250 Å respectively, assuming the intracluster Coulomb potential is screened by four target valence electrons. At 4 MeV/C the results are similar although less screening is required; a possible explanation is the inability of the plasma to completely screen the higher velocity projectiles. The 1 MeV/C calculated results however differ in their behaviour from the 2 and 4 MeV/C cases.

Energy and angular distributions of negative ions (Au–, Au2-, Au3-, and Au5-) emitted from gold target bombarded by Au, Au4, and Au9 projectiles at 200 keV/atom were measured with a multipixel position sensitive detector. The angular distributions are symmetrical with respect to the normal to the target surface and forward peaked. They depend on the type of emitted ions, on the emission energy, and on the projectile size. More forward directed emission is observed with Au9 projectiles. The secondary ion energy distributions obtained with Au and Au4 projectiles are well reproduced by a sum of linear collision cascades and thermal spike processes. However, in the case of Au9 projectiles the energy distributions are better described by using a simple spike model with two different average temperature regimes: the first one corresponds to high emission energy occurring in the early stage of the whole process, and the second to the low energy component.

Energy and angular distributions of negative ions (Au–, Au2-, Au3-, and Au5-) emitted from gold target bombarded by Au, Au4, and Au9 projectiles at 200 keV/atom were measured with a multipixel position sensitive detector. The angular distributions are symmetrical with respect to the normal to the target surface and forward peaked. They depend on the type of emitted ions, on the emission energy, and on the projectile size. More forward directed emission is observed with Au9 projectiles. The secondary ion energy distributions obtained with Au and Au4 projectiles are well reproduced by a sum of linear collision cascades and thermal spike processes. However, in the case of Au9 projectiles the energy distributions are better described by using a simple spike model with two different average temperature regimes: the first one corresponds to high emission energy occurring in the early stage of the whole process, and the second to the low energy component.

Energy and angular distributions of negative ions (Au–, Au2-, Au3-, and Au5-) emitted from gold target bombarded by Au, Au4, and Au9 projectiles at 200 keV/atom were measured with a multipixel position sensitive detector. The angular distributions are symmetrical with respect to the normal to the target surface and forward peaked. They depend on the type of emitted ions, on the emission energy, and on the projectile size. More forward directed emission is observed with Au9 projectiles. The secondary ion energy distributions obtained with Au and Au4 projectiles are well reproduced by a sum of linear collision cascades and thermal spike processes. However, in the case of Au9 projectiles the energy distributions are better described by using a simple spike model with two different average temperature regimes: the first one corresponds to high emission energy occurring in the early stage of the whole process, and the second to the low energy component.

Detailed simulations of the interaction of energetic C-60 beams with amorphous targets are presented here. The spatial evolution of the cluster components is calculated accounting for multiple scattering and Coulomb explosion by means of Monte Carlo and molecular dynamics, respectively. The charge states of the individual cluster components (atoms, atomic ions, fragment cluster ions) as a function of penetration depth are also calculated in tandem with the above calculations by means of the Monte Carlo method. The relative importance of scattering versus Coulomb repulsion is studied as a function of the C-60 cluster energy. The effect of the neighboring cluster constituents on the average charge state of the cluster atoms is calculated as a function of the depth of penetration for a C-60 cluster of 40 MeV. The calculation accounts for the increase in ionization energy of the atom due to the other constituents. Relative track radii are calculated as a function of penetration depth and good agreement with the experimental results is obtained for the interaction of a 30 MeV carbon cluster with silicon. Track splitting observed well into the target as measured by Dunlop in yttrium iron garnet is obtained in the simulations described here for the case of amorphous carbon, provided the Coulomb repulsion is screened by the four valence electrons. Collective energy deposition enhancement is calculated for the 720 MeV cluster. Here the cluster constituents are nearly fully ionized, thereby minimizing the ambiguity related to the value of the ionic charge in the calculation.

Detailed simulations of the interaction of energetic C-60 beams with amorphous targets are presented here. The spatial evolution of the cluster components is calculated accounting for multiple scattering and Coulomb explosion by means of Monte Carlo and molecular dynamics, respectively. The charge states of the individual cluster components (atoms, atomic ions, fragment cluster ions) as a function of penetration depth are also calculated in tandem with the above calculations by means of the Monte Carlo method. The relative importance of scattering versus Coulomb repulsion is studied as a function of the C-60 cluster energy. The effect of the neighboring cluster constituents on the average charge state of the cluster atoms is calculated as a function of the depth of penetration for a C-60 cluster of 40 MeV. The calculation accounts for the increase in ionization energy of the atom due to the other constituents. Relative track radii are calculated as a function of penetration depth and good agreement with the experimental results is obtained for the interaction of a 30 MeV carbon cluster with silicon. Track splitting observed well into the target as measured by Dunlop in yttrium iron garnet is obtained in the simulations described here for the case of amorphous carbon, provided the Coulomb repulsion is screened by the four valence electrons. Collective energy deposition enhancement is calculated for the 720 MeV cluster. Here the cluster constituents are nearly fully ionized, thereby minimizing the ambiguity related to the value of the ionic charge in the calculation.

Following great success in the creation of the Keck Observatory, scientists at the California Institute of Technology and the University of California have begun to explore the scientific and technical prospects for a much larger telescope. The Keck telescopes will remain the largest telescopes in the world for a number of years, with many decades of forefront research ahead after that. Though these telescopes have produced dramatic discoveries, it is already clear that even larger telescopes must be built if we are to address some of the most profound questions about our universe. The time required to build a larger telescope is approximately ten years, and the California community is presently well-positioned to begin its design and construction. The same scientists who conceived, led the design, and guided the construction of the Keck Observatory have been intensely engaged in a study of the prospects for an extremely large telescope. Building on our experience with the Keck Observatory, we have concluded that the large telescope is feasible and is within the bounds set by present-day technology. Our reference telescope has a diameter of 30 meters, the largest size we believe can be built with acceptable risk. The project is currently designated the California Extremely Large Telescope (CELT).

We present an efficient algorithm able to predict the trajectories of individual cluster constituents as they penetrate relatively thick amorphous targets. Our algorithm properly treats both the intracluster Coulomb repulsion and the collisions between cluster constituents and target atoms. We have compared our simulation predictions to experimental measurements of the distribution of lateral exit velocities, and demonstrated that the in-target Coulomb explosion of 2MeV/atom carbon clusters in carbon foils must be shielded with a screening length of less than 2.5 Å. We also present a simple phenomenological model for the suppression of the exit-side charge of ions in clusters which depends on the enhanced ionization potential that an electron near an ion feels due to the ion’s charged comoving neighbors. By using our simulation algorithm we have predicted the exit correlations of the cluster constituents and verified that the charge suppression model fits the observed charge suppression of ions in clusters to within the experimental uncertainties.

We present an efficient algorithm able to predict the trajectories of individual cluster constituents as they penetrate relatively thick amorphous targets. Our algorithm properly treats both the intracluster Coulomb repulsion and the collisions between cluster constituents and target atoms. We have compared our simulation predictions to experimental measurements of the distribution of lateral exit velocities, and demonstrated that the in-target Coulomb explosion of 2MeV/atom carbon clusters in carbon foils must be shielded with a screening length of less than 2.5 Å. We also present a simple phenomenological model for the suppression of the exit-side charge of ions in clusters which depends on the enhanced ionization potential that an electron near an ion feels due to the ion’s charged comoving neighbors. By using our simulation algorithm we have predicted the exit correlations of the cluster constituents and verified that the charge suppression model fits the observed charge suppression of ions in clusters to within the experimental uncertainties.

We report the use of delta-doped charge-coupled devices (CCDs) for direct detection of electrons in the 50–1500 eV energy range. We show that modification of the CCD back surface by molecular beam epitaxy can greatly improve sensitivity to low-energy electrons by introducing an atomically abrupt dopant profile to eliminate the dead layer. Using delta-doped CCDs, we have extended the energy threshold for detection of electrons by over an order of magnitude. We have also measured high gain in response to low-energy electrons using delta-doped CCDs. The effect of multiple electron hole pair production on the observed signals is discussed. Electrons have been directly imaged with a delta-doped CCD in the 250–750 eV range.

Although cluster-ion interactions may superficially appear to resemble those in nuclear physics, there are important differences. First, at the energies of interest the deBroglie wavelength of an atom in the cluster is so short compared to atomic dimensions that the atom’s trajectory in a solid is classical. Thus, one may use the Born-Oppenheimer approximation in which the particles all obey Newton’s equations of motion and only the interaction potentials reflect the quantum mechanical character of the system. Thus, we can treat many atomic interaction processes in the semi-classical limit; in this paper I shall, for example, indicate how in this way one can include the effect of atomic excitation in collisions. Second, the cross sections involved are much larger than those for nuclear interactions, i.e., the mean free paths of the “reaction products” are so short that large collective effects always occur. This means that the calculations must deal with large numbers of interacting particles. For the case of cluster ions at MeV energies this has required us to develop new computational strategies, e.g., the use of massively parallel computers. By using such simulations strategically we are able both to optimize the design of experiments and to interpret their results less ambiguously.

We report the use of delta-doped charge-coupled devices (CCDs) for direct detection of electrons in the 50–1500 eV energy range. We show that modification of the CCD back surface by molecular beam epitaxy can greatly improve sensitivity to low-energy electrons by introducing an atomically abrupt dopant profile to eliminate the dead layer. Using delta-doped CCDs, we have extended the energy threshold for detection of electrons by over an order of magnitude. We have also measured high gain in response to low-energy electrons using delta-doped CCDs. The effect of multiple electron hole pair production on the observed signals is discussed. Electrons have been directly imaged with a delta-doped CCD in the 250–750 eV range.

We match the predictions of molecular-dynamics simulations of 1.2 keV and 2.0 keV 7Li+ scattered from Al(100) to observed total Li atom spectra measured by time-of-flight spectroscopy. In doing so we determine the relevant parameters in a simple distance of closest approach model for the probability of production of single and double vacancies in the Li 1s shell during hard Li-Al collisions. In the standard Fano-Lichten model of vacancy production, vacancies are produced with unit probability if the collision is hard enough to force the collision partners past some critical distance of closest approach. We find that such an assumption is insufficient to fit our simulations to experimental observations, and that we must allow for a gradual turning on of the vacancy production probability as the distance of closest approach decreases. The resulting model may be useful in modeling atomic excitation effects in simulations of other ion-impact processes.

Current-driven vortex dynamics of type-II superconductors in the weak-pinning limit is investigated by quantitatively studying the current-dependent vortex dissipation of an untwinned YBa2Cu3O7 single crystal. For applied current densities (J) substantially larger than the critical current density (Jc), non-linear resistive peaks appear below the thermodynamic first-order vortex-lattice melting transition temperature (Tm), in contrast to the resistive hysteresis in the low-current limit (J < Jc). These resistive peaks are quantitatively analysed in terms of the current-driven coherent and plastic motion of vortex bundles in the vortex-solid phase, and the non-linear current - voltage characteristics are found to be consistent with the collective flux-creep model. The effects of high-density random point defects on the vortex dynamics are also investigated via proton irradiation of the same single crystal. Neither resistive hysteresis at low currents nor peak effects at high currents are found after the irradiation. Furthermore, the current-voltage characteristics within the instrumental resolution become completely ohmic over a wide range of currents and temperatures, despite theoretical predictions of much larger Jc-values for the given experimental variables. This finding suggests that the vortex-glass phase, a theoretically proposed low-temperature vortex state which is stabilized by point disorder and has a vanishing resistivity, may become unstable under applied currents significantly smaller than the theoretically predicted Jc. More investigation appears necessary in order to resolve this puzzling issue.

Delta-doped CCDs have achieved stable quantum efficiency, at the theoretical limit imposed by reflection from the Si surface in the near UV and visible. In this approach, an epitaxial silicon layer is grown on a fully-processed CCD using molecular beam epitaxy. During the silicon growth on the CCD, 30 percent of a monolayer of boron atoms are deposited nominally within a single atomic layer, resulting in the effective elimination of the backside potential well. In this paper, we will briefly discuss delta-doped CCDs and their application of low-energy electron detection. We show that modification of the surface this way can greatly improve sensitivity to low-energy detection. We show that modification of the surface this way can greatly improve sensitivity to low-energy electrons. Measurements comparing the response of delta-doped CCDs with untreated CCDs were made in the 50 eV-1.5 keV energy range.For electrons with energies below 300 eV, the signal from untreated CCDs was below the detection limit for our apparatus, and data are presented only for the response of delta-doped CCDs at these energies. The effects of multiple electron hole pair production and backscattering on the observed signals are discussed.

We match the predictions of molecular-dynamics simulations of 1.2 keV and 2.0 keV 7Li+ scattered from Al(100) to observed total Li atom spectra measured by time-of-flight spectroscopy. In doing so we determine the relevant parameters in a simple distance of closest approach model for the probability of production of single and double vacancies in the Li 1s shell during hard Li-Al collisions. In the standard Fano-Lichten model of vacancy production, vacancies are produced with unit probability if the collision is hard enough to force the collision partners past some critical distance of closest approach. We find that such an assumption is insufficient to fit our simulations to experimental observations, and that we must allow for a gradual turning on of the vacancy production probability as the distance of closest approach decreases. The resulting model may be useful in modeling atomic excitation effects in simulations of other ion-impact processes.

Current-driven vortex dynamics of type-II superconductors in the weak-pinning limit is investigated by quantitatively studying the current-dependent vortex dissipation of an untwinned YBa2Cu3O7 single crystal. For applied current densities (J) substantially larger than the critical current density (Jc), non-linear resistive peaks appear below the thermodynamic first-order vortex-lattice melting transition temperature (Tm), in contrast to the resistive hysteresis in the low-current limit (J < Jc). These resistive peaks are quantitatively analysed in terms of the current-driven coherent and plastic motion of vortex bundles in the vortex-solid phase, and the non-linear current - voltage characteristics are found to be consistent with the collective flux-creep model. The effects of high-density random point defects on the vortex dynamics are also investigated via proton irradiation of the same single crystal. Neither resistive hysteresis at low currents nor peak effects at high currents are found after the irradiation. Furthermore, the current-voltage characteristics within the instrumental resolution become completely ohmic over a wide range of currents and temperatures, despite theoretical predictions of much larger Jc-values for the given experimental variables. This finding suggests that the vortex-glass phase, a theoretically proposed low-temperature vortex state which is stabilized by point disorder and has a vanishing resistivity, may become unstable under applied currents significantly smaller than the theoretically predicted Jc. More investigation appears necessary in order to resolve this puzzling issue.

Single crystals of magnesium oxide containing nanoprecipitates of sodium were bombarded with swift ions (∼GeV-Pb, U) or cluster beams (∼20 MeV-C60) to study the phase change induced by electronic processes at high stopping power (≳10 keV/nm). The sodium precipitates and the defect creation were characterized by optical absorption and transmission electron microscopy. The ion or cluster bombardment leads to an evolution of the Na precipitate concentration but the size distribution remains unchanged. The decrease in Na metallic concentration is attributed to mixing effects at the interfaces between Na clusters and MgO. In addition, optical-absorption measurements show a broadening of the absorption band associated with electron plasma oscillations in Na clusters. This effect is due to a decrease of the electron mean free path, which could be induced by defect creation in the metal. All these results show an influence of high electronic stopping power in materials known to be very resistant to irradiation with weak ionizing projectiles. The dependence of these effects on electronic stopping power and on various solid-state parameters is discussed.

Be diffusivity data in the bulk metallic glass forming alloy Zr46.7Ti8.3Cu7.5Ni10Be27.5 are reported for temperatures between 530 and 710 K, extending 85 K into the supercooled liquid state of the alloy. At the glass transition temperature Tg, a change in temperature dependence of the data is observed, and above Tg the diffusivity increases more quickly with temperature than below. The data in the supercooled liquid can be described by a modified Arrhenius expression based on a diffusion mechanism suggested earlier. The comparison with viscosity data in the supercooled liquid state of Zr46.7Ti8.3Cu7.5Ni10Be27.5 reveals a breakdown of the Stokes–Einstein relation, indicating a cooperative diffusion mechanism in the supercooled liquid state of Zr46.7Ti8.3Cu7.5Ni10Be27.5.

We numerically integrate the one-dimensional, cylindrically symmetric Ginzburg-Landau equations to calculate the spatial variation of the order parameter and supercurrents for a vortex trapped by a cylindrical defect. We use the resulting field distributions to estimate the pinning energy, and make use of the vortex/two-dimensional boson analogy to calculate the depinning temperature. The microscopic behavior oi the fields depends on the size, and the conductivity of the cylindrical defect appears to be important for the pinning.

The self diffusivity of Be in the bulk metallic amorphous Zr_(41.2)Ti_(13.8)Cu_(12.5)Ni_(10)Be_(22.5) alloy is investigated by high energy elastic backscattering of ^4He^(2+) ions in the temperature range between 433K and 663K. Below the glass transition temperature, the data suggest a single atomic jump diffusion behavior of Be with a migration enthalpy of 1.05eV/atom and a pre-exponential factor of 1.8·10^(-11)m^2/s. Above the glass transition, the diffusivity of Be increases by two orders of magnitude over a temperature interval of 40K. By taking into account the change in configurational entropy due to the glass transition, we can explain the mechanism for diffusion of Be in the supercooled liquid state of Zr_(41.2)Ti_(13.0)Ni _(10)Cu_(12.5)Be_(22.5) by single atomic jumps in a slowly changing configuration of neighboring atoms. The corresponding migration enthalpy is the same as in the solid glass, and the number of neighboring atoms which influence the diffusion of Be is about 22.

Be diffusivity data in the bulk metallic glass forming alloy Zr46.7Ti8.3Cu7.5Ni10Be27.5 are reported for temperatures between 530 and 710 K, extending 85 K into the supercooled liquid state of the alloy. At the glass transition temperature Tg, a change in temperature dependence of the data is observed, and above Tg the diffusivity increases more quickly with temperature than below. The data in the supercooled liquid can be described by a modified Arrhenius expression based on a diffusion mechanism suggested earlier. The comparison with viscosity data in the supercooled liquid state of Zr46.7Ti8.3Cu7.5Ni10Be27.5 reveals a breakdown of the Stokes–Einstein relation, indicating a cooperative diffusion mechanism in the supercooled liquid state of Zr46.7Ti8.3Cu7.5Ni10Be27.5.

Silicon CCDs have limited sensitivity to particles and photons with short penetration depth, due to the surface depletion caused by the inherent positive charge in the native oxide. Because of surface depletion, internally-generated electrons are trapped near the irradiated surface and therefore cannot be transported to the detection circuitry. This deleterious surface potential can be eliminated by low-temperature molecular beam epitaxial (MBE) growth of a delta-doped layer on the Si surface. This effect has been demonstrated through achievement of 100% internal quantum efficiency for UV photons detected with delta-doped CCDs. In this paper, we will discuss the modification of the band bending near the CCD surface by low-temperature MBE and report the application of delta-doped CCDs to low-energy electron detection. We show that modification of the surface can greatly improve sensitivity to low-energy electrons. Measurements comparing the response of delta-doped CCDs with untreated CCDs were made in the 50 eV-1.5 keV energy range. For electrons with energies below 300 eV, the signal from untreated CCDs was below the detection limit for our apparatus, and data are presented only for the response of delta-doped CCDs at these energies. The effects of multiple electron hole pair (EHP) production and backscattering on the observed signals are discussed.

Single crystals of magnesium oxide containing nanoprecipitates of sodium were bombarded with swift ions (∼GeV-Pb, U) or cluster beams (∼20 MeV-C60) to study the phase change induced by electronic processes at high stopping power (≳10 keV/nm). The sodium precipitates and the defect creation were characterized by optical absorption and transmission electron microscopy. The ion or cluster bombardment leads to an evolution of the Na precipitate concentration but the size distribution remains unchanged. The decrease in Na metallic concentration is attributed to mixing effects at the interfaces between Na clusters and MgO. In addition, optical-absorption measurements show a broadening of the absorption band associated with electron plasma oscillations in Na clusters. This effect is due to a decrease of the electron mean free path, which could be induced by defect creation in the metal. All these results show an influence of high electronic stopping power in materials known to be very resistant to irradiation with weak ionizing projectiles. The dependence of these effects on electronic stopping power and on various solid-state parameters is discussed.

We numerically integrate the one-dimensional, cylindrically symmetric Ginzburg-Landau equations to calculate the spatial variation of the order parameter and supercurrents for a vortex trapped by a cylindrical defect. We use the resulting field distributions to estimate the pinning energy, and make use of the vortex/two-dimensional boson analogy to calculate the depinning temperature. The microscopic behavior oi the fields depends on the size, and the conductivity of the cylindrical defect appears to be important for the pinning.

The self-diffusivity of Be in the bulk metallic amorphous alloy Zr41.2Ti13.8Cu12.5Ni10Be22.5 is investigated. Below the glass transition temperature the data suggest a single atomic jump diffusion behavior of Be. By taking into account the change in configurational entropy due to the glass transition, we explain the mechanism for diffusion of Be in the supercooled liquid state (i.e., above the glass transition) of Zr41.2Ti13.8Ni10Cu12.5Be22.5 by single atomic jumps in a slowly changing configuration of neighboring atoms.

The crystal structure of Lu_2(SiO_4)O:Ce (“LSO”) has two trivalent cation sites which may be occupied by Ce^(3+) to form luminescence centers. Previous investigations revealed the existence of two distinct sets of Ce^(3+) excitation and emission spectra and suggested that the differences in the spectra are due to differences in the crystal fields at the two lattice sites that shift the 5d levels of Ce^(3+). In the present report, we re-examine this issue and present new evidence which suggests a different interpretation. In particular, spectra measured at 13 K suggest that both lattice sites give rise to indistinguishable excitation and emission spectra while the second set of observed spectra arises from Ce^(3+) located in interstitial sites. The evidence for the interstitial sites is a disappearance of the doublet structure in the emission spectra and a large variation in the population of the sites as a function of total cerium concentration. In addition, the results indicate that the scintillation properties of LSO are influenced by the relative population of Ce in interstitial sites compared to lattice sites as well as by differences in the transfer efficiencies to different sites.

The self-diffusivity of Be in the bulk metallic amorphous alloy Zr41.2Ti13.8Cu12.5Ni10Be22.5 is investigated. Below the glass transition temperature the data suggest a single atomic jump diffusion behavior of Be. By taking into account the change in configurational entropy due to the glass transition, we explain the mechanism for diffusion of Be in the supercooled liquid state (i.e., above the glass transition) of Zr41.2Ti13.8Ni10Cu12.5Be22.5 by single atomic jumps in a slowly changing configuration of neighboring atoms.

Compositionally modulated thin films of Cu and Y were prepared in an ultrahigh-vacuum dc ion-beam deposition chamber. The amorphization reaction was monitored by in situ x-ray-diffraction measurements. Growth of amorphous Cu1-xYx is observed at room temperature with the initial formation of a Cu-rich amorphous phase. Further annealing in the presence of unreacted Y leads to Y enrichment of the amorphous phase. Growth of crystalline CuY is observed for T=469 K. Transmission-electron-microscopy measurements provide real-space imaging of the amorphous interlayer and growth morphology. Models are developed, incorporating metastable interfacial and bulk free-energy diagrams, for the early stage of the amorphization reaction.

Photostimulated luminescence (PSL) and thermoluminescence (TL) from five Lu_(2(1-x))Ce_(2x)(SiO_4)O (LSO) crystals with different light outputs is reported. Optical irradiation into the Ce^(3+) absorption bands causes the appearance of a broad absorption band near 280 nm which is ascribed to Ce^(4+). In addition, a tail is observed extending beyond 700 nm. Optical irradiation into this tail (PSL) or heating of the crystal (TL) results in Ce^(3+) emission. It is shown that both PSL and TL are due to the same traps: In addition, an anti-correlation is found between the light output under gamma-ray irradiation and the trap concentration in the crystal. The nature of the recombination centers responsible for the low light output in some crystals is not clear. Annealing experiments suggest that the traps and the recombination centers may be related to oxygen vacancies.

The contributions to this volume are based on invited review lectures presented at a symposium on fundamental processes in sputtering (SPUT92), attended by 82 active researchers in the field . Eight contributions cover recoil processes, or `collisional sputtering', and six contributions cover electronic processes, or `electronic sputtering'. The last two contributions cover two of six application areas presented at SPUT92, and an introductory chapter was added by the editor. The book emphasizes developments over the past decade and offers comprehensive reviews on most active research areas in the field of sputtering by particle bombardment, with the emphasis on fundamental physical and chemical processes.

The scintillation decay of cerium-doped gadolinium oxyorthosilicate Gd_2(SiO_4)O:Ce is lengthened by the energy transfer from Gd to Ce. To investigate the role of the Gd in the scintillation processes, the Gd was partially replaced by optically inactive rare earth elements, Y and Lu, and the effective transfer rates from Gd to Ce were measured as a function of Gd and Ce concentrations using UV- and gamma-ray excitations. The data clearly indicate the dilution of the Gd by the Y and the Lu further lengthens the migration time through the Gd in the energy transfer process from Gd to Ce.

Compositionally modulated thin films of Cu and Y were prepared in an ultrahigh-vacuum dc ion-beam deposition chamber. The amorphization reaction was monitored by in situ x-ray-diffraction measurements. Growth of amorphous Cu1-xYx is observed at room temperature with the initial formation of a Cu-rich amorphous phase. Further annealing in the presence of unreacted Y leads to Y enrichment of the amorphous phase. Growth of crystalline CuY is observed for T=469 K. Transmission-electron-microscopy measurements provide real-space imaging of the amorphous interlayer and growth morphology. Models are developed, incorporating metastable interfacial and bulk free-energy diagrams, for the early stage of the amorphization reaction.

Secondary neutrals sputtered by 3-keV Ar+ from FeS2 and Fe were studied by use of laser postionization mass spectrometry and laser-induced fluorescence spectroscopy. The velocity distribution and yield of Fe from the FeS2 sample were measured. The velocity distribution of Fe sputtered from the sulfide is very similar to that of sputtered Fe from the pure metal with both distributions peaking at nearly the same velocity. The yield of Fe from the sulfide target, after prolonged bombardment, was at most 27% of the yield from the metal. The results are compared with Monte Carlo calculations using the TRIM (transport of ions in matter) code. The total sputtering yield of FeS2 was measured using profilometry and was compared with the same measurements of the sputtered metal targets. The surface of the FeS2 was examined by x-ray photoelectron spectroscopy before and after ion bombardment and indicates preferential sputtering of S at low primary ion doses. After prolonged bombardment, the surface becomes similar to that of FeS. The profilometry measurements and the yield measured by mass spectrometry indicate that some of the sputtered Fe is not observed in the neutral ground state; we propose that the unobserved Fe is in the form of clusters.

Core excitation in Al atoms following the impact of pure Al clusters and composite Al-Au clusters on Au surfaces has been investigated by molecular-dynamics simulation. Core excitation was found to be most probable during the early, compressional phase of cluster impacts. Our simulations predict that a significant fraction of the core-excited Al cluster atoms will decay by atomiclike Auger emission following ejection from the surface, providing a useful diagnostic tool to investigate collisions of cluster ions with surfaces.

We report systematic studies of the dc transport properties in proton-irradiated Y-Ba-Cu-O single crystals. We find that the onset of vortex dissipation in moderately irradiated samples is associated with the occurrence of a second-order vortex-solid melting transition. In addition, the decreasing zero-field transition temperature and increasing critical current density with the increasing defects reveal the effects of disorder on reducing the electron mean-free-path and on increasing the pinning density.

Using MeV ions for the modification of electronic materials offers a number of advantages: minimizing surface damage; implantation into completed devices – for example, through contacts or photoresist layers; producing controlled radiation damage for flux pinning where the ions pass completely through the sample and thus do not modify its chemical nature; and enhancing the electronic excitation of the target material versus collisional damage, as in adhesion enhancement processes. In all cases, however, one requires a detailed understanding of the new damage mechanisms that occur and how they can be modified in a controlled way by annealing. In this report I shall present examples from a number of our experiments: resistivity and index of refraction modification in semiconductors; adhesion enhancement; mixing of multilayer structures; and modification of the electronic properties of insulators and superconductors.

Two types of excitation and emission spectra with ultraviolet excitation are shown at 11 K for Lu_2(SiO_4)O:Ce (LSO). Since there are two crystallographically independent lutetium sites in LSO, the existence of two Ce activation centers (two-activation-center model) is proposed. The existence of two different decay time constants of the order of tens of nanoseconds, which are characteristic for fast Ce emission, are shown at 77 K. Although the temperature dependence of the decay time and the luminescence efficiency could not be measured for the Ce2, the fact that the gamma-ray-excited emission can be reconstructed from a weighted combination of the Ce1 and Ce2 emission spectra supports the two-activation-center model for LSO.

Secondary neutrals sputtered by 3-keV Ar+ from FeS2 and Fe were studied by use of laser postionization mass spectrometry and laser-induced fluorescence spectroscopy. The velocity distribution and yield of Fe from the FeS2 sample were measured. The velocity distribution of Fe sputtered from the sulfide is very similar to that of sputtered Fe from the pure metal with both distributions peaking at nearly the same velocity. The yield of Fe from the sulfide target, after prolonged bombardment, was at most 27% of the yield from the metal. The results are compared with Monte Carlo calculations using the TRIM (transport of ions in matter) code. The total sputtering yield of FeS2 was measured using profilometry and was compared with the same measurements of the sputtered metal targets. The surface of the FeS2 was examined by x-ray photoelectron spectroscopy before and after ion bombardment and indicates preferential sputtering of S at low primary ion doses. After prolonged bombardment, the surface becomes similar to that of FeS. The profilometry measurements and the yield measured by mass spectrometry indicate that some of the sputtered Fe is not observed in the neutral ground state; we propose that the unobserved Fe is in the form of clusters.

Core excitation in Al atoms following the impact of pure Al clusters and composite Al-Au clusters on Au surfaces has been investigated by molecular-dynamics simulation. Core excitation was found to be most probable during the early, compressional phase of cluster impacts. Our simulations predict that a significant fraction of the core-excited Al cluster atoms will decay by atomiclike Auger emission following ejection from the surface, providing a useful diagnostic tool to investigate collisions of cluster ions with surfaces.

Bombardment with beams of heavy molecular ions (cluster ions) has shown initial promise in a wide range of applications. Since little intuition has been developed about how the results are affected by the relevant experimental variables, e.g., cluster type, mass, and energy, these attempts at establishing feasibility may be quite far from being optimal in the choices of these parameters. I propose here that simulations based on molecular dynamics techniques may play a very important role in the efficient design and accurate interpretation of the next generation of such experiments. In addition to the impact this will have on applications in materials science, these calculations and the relevant experiments that follow may be important in determining the lifetimes of small grains in the interstellar medium.

Bombardment with beams of heavy molecular ions (cluster ions) has shown initial promise in a wide range of applications. Since little intuition has been developed about how the results are affected by the relevant experimental variables, e.g., cluster type, mass, and energy, these attempts at establishing feasibility may be quite far from being optimal in the choices of these parameters. I propose here that simulations based on molecular dynamics techniques may play a very important role in the efficient design and accurate interpretation of the next generation of such experiments. In addition to the impact this will have on applications in materials science, these calculations and the relevant experiments that follow may be important in determining the lifetimes of small grains in the interstellar medium.

A detailed study of the influence of substrate temperature on the radiation-induced lattice strain field and crystalline-to-amorphous (c-a) phase transition in MeV oxygen ion implanted GaAs crystals has been made using channeling Rutherford backscattering spectroscopy, secondary ion mass spectrometry, and the x-ray rocking curve technique. A comparison has been made between the cases of room temperature (RT) and low temperature (LT) (about 100 K) implantation. A strong in situ dynamic annealing process is found in RT implantation at a moderate beam current, resulting in a uniform positive strain field in the implanted layer. LT implantation introduces a freeze-in effect which impedes the recombination and diffusion of initial radiation-created lattice damage and defects, and in turn drives more efficiently the c-a transition as well as strain saturation and relaxation. The results are interpreted with a spike damage model in which the defect production process is described in terms of the competition between defect generation by nuclear spikes and defects diffusion and recombination stimulated by electronic spikes. It is also suggested that the excess population of vacancies and their complexes is responsible for lattice spacing expansion in ion-implanted GaAs crystals.

A detailed study of the influence of substrate temperature on the radiation-induced lattice strain field and crystalline-to-amorphous (c-a) phase transition in MeV oxygen ion implanted GaAs crystals has been made using channeling Rutherford backscattering spectroscopy, secondary ion mass spectrometry, and the x-ray rocking curve technique. A comparison has been made between the cases of room temperature (RT) and low temperature (LT) (about 100 K) implantation. A strong in situ dynamic annealing process is found in RT implantation at a moderate beam current, resulting in a uniform positive strain field in the implanted layer. LT implantation introduces a freeze-in effect which impedes the recombination and diffusion of initial radiation-created lattice damage and defects, and in turn drives more efficiently the c-a transition as well as strain saturation and relaxation. The results are interpreted with a spike damage model in which the defect production process is described in terms of the competition between defect generation by nuclear spikes and defects diffusion and recombination stimulated by electronic spikes. It is also suggested that the excess population of vacancies and their complexes is responsible for lattice spacing expansion in ion-implanted GaAs crystals.

Energy, number-density, and confinement-time properties of 1-keV/atom Al32 and Al63 cluster impacts on Al and Au targets have been investigated by molecular-dynamics simulation. Inertial confinement of the incoming clusters leads to number-density increases of ∼2 for very short time periods (<20 fs) in the primary impact zone. Simultaneous and near-simultaneous multiple collisions occasionally increase the potential energy of a particle to more than twice that allowed in isolated two-body collisions. Up to 12% of the kinetic energy of the incoming cluster is transferred to atoms which eject early in the collision cascade.

A modified version of the multiple-interaction code SPUT2 has been used to simulate impacts of 63-atom Cu clusters on six–layer Cu targets. Simulations were carried out with cutoff times of 100 and 500 fs for an incident cluster energy of 63 keV (1 keV/atom). Significant enhancements were observed in the maximum potential and kinetic energies achieved in the early phase of the collision cascade. Some hard collisions yielded atoms with potential energies as high as 925 eV (in the CM frame). This is almost twice the energy allowed in an isolated two-body collision. The number of hard collisions per time-step vs potential energy is well-fitted with a decaying exponential, allowing extrapolation to higher energies. These results together with similar results for Al clusters impacting Au targets suggest that non-linear collisional effects cannot explain the high D–D fusion rates seen in Beuhler, Friedlander, and Friedman's recent experiment.

An experimental study of lattice disordering, the crystalline-to-amorphous (c-a) phase transition, and substrate temperature effects in MeV-ion-implanted III-V compound semiconductor crystals is presented. A comparison has been made between the GaAs and InP systems, which have been implanted with 2 MeV oxygen ions at either room temperature (RT) or near liquid nitrogen temperature (LT). A strong in situ dynamic annealing has been found in the RT implanted GaAs, and the LT implanted GaAs exhibits heterogeneous (at the end-of-range of the ions) and homogeneous (at the subsurface region) c-a phase transitions. In InP crystals, in situ annealing is much less pronounced in RT implantation, and dose-dependent damage nucleation and layer-by-layer amorphization take place. LT implantation results in lattice disordering and phase transition with a critical dose at least one order lower than that for GaAs. The mechanisms and kinetics of lattice disordering by ion irradiation are also discussed.

Energy, number-density, and confinement-time properties of 1-keV/atom Al32 and Al63 cluster impacts on Al and Au targets have been investigated by molecular-dynamics simulation. Inertial confinement of the incoming clusters leads to number-density increases of ∼2 for very short time periods (<20 fs) in the primary impact zone. Simultaneous and near-simultaneous multiple collisions occasionally increase the potential energy of a particle to more than twice that allowed in isolated two-body collisions. Up to 12% of the kinetic energy of the incoming cluster is transferred to atoms which eject early in the collision cascade.

Sputtering yields for niobium bombarded by 12-MeV protons have been measured using a technique involving prior activation of the target material. Values obtained for backward and forward sputtering are SB=(1.6±0.1)×10^-4 and SF=(4.7±0.3)×10^-4, respectively. These values, expected to be similar to fast-neutron sputtering yields, are consistent with most recently published fast-neutron data.