Search Results (24 total)

No monochromatic (Δω_x/ω_x<1%), high peak brightness [>10²⁰   photons/(mm²×mrad²×s×0.1%   bandwidth)], tunable light sources currently exist above 100 keV. Important applications that would benefit from such new hard x-ray and γ-ray sources include the following: nuclear resonance fluorescence spectroscopy and isotopic imaging, time-resolved positron annihilation spectroscopy, and MeV flash radiography. In this paper, the peak brightness of Compton scattering light sources is derived for head-on collisions and found to scale quadratically with the normalized energy, γ; inversely with the electron beam duration, Δτ, and the square of its normalized emittance, ε; and linearly with the bunch charge, eN_e, and the number of photons in the laser pulse, N_γ:   B-^ _x∝γ²N_eN_γ/ε²Δτ. This γ² scaling shows that for low normalized emittance electron beams (1 nC, 1  mm·mrad, <1  ps, >100  MeV), and tabletop laser systems (1–10   J, 5 ps) the x-ray peak brightness can exceed 10²³   photons/(mm²×mrad²×s×0.1%   bandwidth) near ℏω_x=1   MeV; this is confirmed by three-dimensional codes that have been benchmarked against Compton scattering experiments performed at Lawrence Livermore National Laboratory. The interaction geometry under consideration is head-on collisions, where the x-ray flash duration is shown to be equal to that of the electron bunch, and which produce the highest peak brightness for compressed electron beams. Important nonlinear effects, including spectral broadening, are also taken into account in our analysis; they show that there is an optimum laser pulse duration in this geometry, of the order of a few picoseconds, in sharp contrast with the initial approach to laser-driven Compton scattering sources where femtosecond laser systems were thought to be mandatory. The analytical expression for the peak on-axis brightness derived here is a powerful tool to efficiently explore the 12-dimensional parameter space corresponding to the phase spaces of both the electron and incident laser beams and to determine optimum conditions for producing high-brightness x rays.

Advanced high-brightness beam applications such as inverse-Compton scattering (ICS) depend on achieving of ultrasmall spot sizes in high current beams. Modern injectors and compressors enable the production of high-brightness beams having needed short bunch lengths and small emittances. Along with these beam properties comes the need to produce tighter foci, using stronger, shorter focal length optics. An approach to creating such strong focusing systems using high-field, small-bore permanent-magnet quadrupoles (PMQs) is reported here. A final-focus system employing three PMQs, each composed of 16 neodymium iron boride sectors in a Halbach geometry has been installed in the PLEIADES ICS experiment. The field gradient in these PMQs is 560   T/m, the highest ever reported in a magnetic optics system. As the magnets are of a fixed field strength, the focusing system is tuned by adjusting the position of the three magnets along the beam line axis, in analogy to familiar camera optics. This paper discusses the details of the focusing system, simulation, design, fabrication, and experimental procedure in creating ultrasmall beams at PLEIADES.

Velocity bunching has been recently proposed as a tool for compressing electron beam pulses in modern high brightness photoinjector sources. This tool is familiar from earlier schemes implemented for bunching dc electron sources, but presents peculiar challenges when applied to high current, low emittance beams from photoinjectors. The main difficulty foreseen is control of emittance oscillations in the beam in this scheme, which can be naturally considered as an extension of the emittance compensation process at moderate energies. This paper presents two scenarios in which velocity bunching, combined with emittance control, is to play a role in nascent projects. The first is termed ballistic bunching, where the changing of relative particle velocities and positions occur in distinct regions, a short high gradient linac, and a drift length. This scenario is discussed in the context of the proposed ORION photoinjector. Simulations are used to explore the relationship between the degree of bunching, and the emittance compensation process. Experimental measurements performed at the UCLA Neptune Laboratory of the surprisingly robust bunching process, as well as accompanying deleterious transverse effects, are presented. An unanticipated mechanism for emittance growth in bends for highly momentum chirped beam was identified and studied in these experiments. The second scenario may be designated as phase space rotation, and corresponds closely to the recent proposal of Ferrario and Serafini. Its implementation for the compression of the electron beam pulse length in the PLEIADES inverse Compton scattering (ICS) experiment at LLNL is discussed. It is shown in simulations that optimum compression may be obtained by manipulation of the phases in low gradient traveling wave accelerator sections. Measurements of the bunching and emittance control achieved in such an implementation at PLEIADES, as well as aspects of the use of velocity-bunched beam directly in ICS experiments, are presented.

We present a detailed comparison of the measured characteristics of Thomson backscattered x rays produced at the Picosecond Laser-Electron Interaction for the Dynamic Evaluation of Structures facility at Lawrence Livermore National Laboratory to predicted results from a newly developed, fully three-dimensional time and frequency-domain code. Based on the relativistic differential cross section, this code has the capability to calculate time and space dependent spectra of the x-ray photons produced from linear Thomson scattering for both bandwidth-limited and chirped incident laser pulses. Spectral broadening of the scattered x-ray pulse resulting from the incident laser bandwidth, perpendicular wave vector components in the laser focus, and the transverse and longitudinal phase spaces of the electron beam are included. Electron beam energy, energy spread, and transverse phase space measurements of the electron beam at the interaction point are presented, and the corresponding predicted x-ray characteristics are determined. In addition, time-integrated measurements of the x rays produced from the interaction are presented and shown to agree well with the simulations.

We report detailed measurements of the transverse phase space distortions induced by magnetic chicane compression of a high brightness, relativistic electron beam to subpicosecond length. A strong bifurcation in the phase space is observed when the beam is strongly compressed. This effect is analyzed using several computational models and is correlated to the folding of longitudinal phase space. The impact of these results on current research in collective beam effects in bending systems and implications for future short wavelength free-electron lasers and linear colliders are discussed.

The measurement of emittance in space-charge dominated, high brightness beam systems is investigated from conceptual, computational, and experimental viewpoints. As the self-field-induced collective motion in the low energy, high brightness beams emitted from photoinjector rf guns are more important in determining the macroscopic beam evolution than thermal spreads in transverse velocity; traditional methods for phase space diagnosis fail in these systems. We discuss the role of space charge forces in a traditional measurement of transverse emittance, the quadrupole scan. The mitigation of these effects by use of multislit- or pepper-pot-based techniques is explained. The results of a direct experimental comparison between quadrupole scanning and slit-based determination of the emittance of a 5 MeV high brightness electron beam are presented. These data are interpreted with the aid of both envelope and multiparticle simulation codes. It is shown that the ratio of the beam’s β function to its transverse plasma wavelength plays a central role in the quadrupole scan results. Methods of determining the presence of systematic errors in quadrupole scan data are discussed.

The transverse dynamics of space-charge dominated beams are investigated both analytically and computationally, in order to understand the mechanisms for emittance oscillations and growth due to nonlinear space-charge fields. This work explores the role of space-charge dominated equilibrium and its relationship to phase space wave breaking, which is responsible for the irreversible emittance growth in these systems. The physics of both coasting and accelerating beams are examined in order to illuminate the most effective approaches to beam handling during the emittance compensation process as well as during subsequent beam transport. These results are discussed within the context of recent ultrahigh brightness rf photoinjector designs.

Photoemission studies of cleaved InP(110) surfaces with multilayers of physisorbed O₂ at 25 K demonstrate photoinduced oxidation. The photoemission results show a 4–5-eV chemical shift in the P 2p core level, and the bonding configuration of the oxide is similar to that of InPO₄. The amount of oxide formed depends on photon exposure and the quantity of oxygen condensed on the surface. During initial oxide growth, the interaction between photogenerated conduction-band electrons from the substrate and physisorbed oxygen molecules controls the reaction and yields an effective reaction cross section of ∼6×10^-15 cm²/photon for 170-eV photons. The growth rate for the thicker oxide is much lower because reaction is limited by diffusion through the oxide layer. This second stage of oxide growth is characterized by dissociation of molecular O₂ due to direct photoexcitation and dissociative secondary-electron attachment. Finally, the cross section for photon-stimulated desorption of O₂ by 170-eV photons is measured to be 4×10^-17 cm²/photon.

Interfaces formed at 20 K by condensing O₂, NO, and N₂O on InP(110) reveal striking differences in reactivity when they are heated, illuminated with visible light, or irradiated with soft x rays [hν=170 eV, photon exposures of (1–1000)×10¹⁴ cm^-2]. Synchrotron-radiation photoemission studies show that O₂ and N₂O desorb from the surface without reacting when annealed above ∼100 K but that oxidelike In and P bonding configurations are produced by NO reactions when heated to ∼150 K. Flood-lamp illumination induces reaction for O₂/InP and NO/InP at 20 K but not for N₂O/InP (hν=0.5–4.5 eV, photon exposure ∼10¹⁹ cm^-2). Irradiation with 170-eV photons yields surface oxides for all three oxygen-bearing condensates. Investigations of the reaction cross sections for these systems indicate that surface chemistry is mediated by the capture of photogenerated low-energy electrons. Electron-attachment processes that produce negative ions are less likely for N₂O than for NO or O₂.

We have studied the interaction of N₂O with GaAs(110) at 25 K as a function of photon beam exposure, using photoemission to detect and characterize reactions. The results show that x rays (hν=1486 and 1253 eV, photon flux 6.8×10⁹ and 2.3×10⁹ photons cm^-2 sec^-1) induce N₂O dissociation and surface oxidation while ultraviolet photons do not within our detectability limit (hν=21.2 and 40.8 eV). The primary dissociation process for physisorbed N₂O involves the attachment of low-energy secondary electrons created by photoillumination. Following electron capture, N₂O dissociation produces O^- ions that react with GaAs to yield surface Ga and As oxides. Dissociation also produces N₂ molecules that desorb without reacting but can be kinetically trapped at low temperature. The As oxides exhibit As₂O₅-like, As₂O₃-like, and intermediate AsO_x-like bonding configurations in relative amounts determined by kinetic constraints and oxygen availability. The photon-enhanced formation of a thick oxide at low temperature is limited by diffusion through the oxide layer, and the formation of O₂ molecules is observed. Warming to 300 K enhances Ga₂O₃ growth at the expense of As-O configurations.

High-resolution synchrotron-radiation photoemission studies of molecular O₂ condensed on GaAs(110) at 20 K show that oxidation is a consequence of photon irradiation. Core-level results for 2 L O₂ [1 langmuir (L)==10^-6 Torr sec] demonstrate that the topmost layer of As atoms is initially involved in a sequential, two-step reaction to produce As¹⁺- and As³⁺-like oxides. These reactions are mediated by secondary electron capture by O₂ which then dissociates to form surface oxides. As⁵⁺-like bonding configurations are formed when additional O₂ is condensed on the surface and exposed to photon irradiation. O₂-GaAs interface reactions slow as transport through the thickening oxides is impeded, and photon-induced desorption of oxygen becomes significant. Studies of Fermi-level movement into the gap as a function of O₂ exposure suggest that oxidation at 20 K produces acceptorlike states. Fermi-level evolution for n-type GaAs is strongly dependent on dopant concentration, O₂ dose, and light exposure, indicating band flattening for lightly doped samples due to surface photovoltage effects. These effects are not significant for p-type GaAs at 20 K, consistent with the formation of acceptorlike states. Together, these results show a complex dependence of surface chemistry on photon irradiation, but remarkably little dependence of the surface Fermi-level position on the reactions.

High-resolution core-level and valence-band photoemission studies of cleaved CoSi₂(111), FeSi₂(001), and MoSi₂(001) show that a Si monolayer terminates the exposed surface. Comparison with results for CoSi₂(111) surfaces prepared by annealing reveals significant differences in surface morphology and Si 2p binding energies. Titanium-atom deposition onto the cleaved surfaces leads to the disruption of the Si monolayer and the appearance of two Ti-induced features that correspond to TiSi-like bonding configurations and Si atoms in solution in the Ti overlayer. Quantitative differences in the distribution of Si in solution for Ti/MoSi₂(001) compared to Ti/CoSi₂(111) and Ti/FeSi₂(001) are related to substrate silicide stability. Analysis of the Ti 3p core-level evolution shows equivalent changes for Ti/CoSi₂(111) and Ti/MoSi₂(001), supporting the conclusion that TiSi-like bonding is produced at low Ti coverage. Valence-band spectra show the convergence to Ti metal at high coverage. Analysis of the Si 2p emission for (18 Å Ti)/CoSi₂(111) annealed to ∼200 °C shows increased amounts of TiSi as silicide growth is kinetically enhanced.

Studies of O₂ interaction with GaAs(110) at 20 K show dynamic conversion of multilayers of physisorbed O₂ into As₂O₃-like oxides due to the synchrotron radiation beam used to acquire photoemission data (hυ=90 eV, photon flux ∼2×10¹³ cm^-2 sec^-1). A lower coordination of As and O is observed and is the precursor to As₂O₃ at the GaAs surface. As₂O₅-like bonding configurations are also produced when the amount of condensed O₂ is increased but this As₂O₅ is metastable with respect to high-energy photon irradiation. These low-temperature results show the interplay between photoinduced surface chemistry and kinetic constraints on oxygen diffusion over very short distances.

Abrupt interfaces with no observed substrate disruption are produced by a novel method of metal-semiconductor junction formation. This method involves the condensation of a thin Xe buffer layer on cleaved surfaces to isolate the semiconductor from impinging metal atoms. This Xe buffer layer provides a surface upon which the metal atoms diffuse, nucleate, and grow into metallic clusters. These clusters are then brought into contact with the substrate when the Xe is thermally desorbed. The result is an abrupt, nondisrupted, nearly ideal interface. Photoemission studies of Al, Ag, Au, Ga, Ti, and Co clusters grown on n- and p-type GaAs(110) show unique Fermi-level positions ∼0.3 and 1.0 eV below the conduction-band minimum, respectively, that are nearly metal and coverage independent. We find no evidence that metal-induced gap states or conventional defect levels are important in determining the Fermi-level position in the gap, but photoemission results indicate surface unrelaxation around the clusters. This unrelaxation results in the reappearance of states in the gap. High-resolution electron-microscopy results for Au(clusters)/GaAs(110) show intimate contact with no intermixing at the interface, with sintering of Au clusters to form an interconnected network of metal islands at high coverages. Comparisons of these results with those for interfaces formed by atom deposition at 60 and 300 K emphasize the novel properties of the cluster interface.

Experimental results have shown that the position of the surface Fermi level E_F for GaAs(110) depends critically on the bulk dopant concentration N and the temperature at which the measurements are made T, when adatoms of a wide variety of metals are deposited. For a specific N in the low dopant regime (∼10¹⁷ cm^-3), coverage-dependent results show that E_F remains close to the band extrema for both n- and p-type GaAs until the onset of metallicity. It then moves symmetrically into the gap, exhibiting a distinctive step in all cases. For higher dopant levels (∼2×10¹⁸ cm^-3), E_F movement is induced before the metallicity limit, and the step is reduced or lost altogether. Temperature-dependent studies for 20≤T≤300 K for a fixed number of adatoms Θ demonstrate that E_F can be moved reversibly into the gap, provided there are no morphological changes. These experimental results demonstrate that E_F can be uniquely determined only when N, T, and Θ are specified. Moreover, experimental results show that the net amount of charge transfer between the bulk and an adatom varies as a function of these three parameters. This paper presents a model, the dynamic-coupling model (DCM), that describes these N-, T-, and Θ-dependent results. The model makes it possible to predict a maximum amount of band bending that will be observed for n- or p-type GaAs at any N, T, or Θ. The DCM shows that the formation of a barrier after atom deposition is a self-regulated process that can limit further charge transfer and that wave-function coupling through the barrier contains the needed dependence on N and T. .AE

Synchrotron-radiation photoemission studies of the formation of Al/GaAs(110) interfaces have been performed as a function of substrate temperature for 60≤T≤300 K for n- and p-type doped samples. The results show temperature-dependent changes in surface Fermi-level position, surface morphology, and the distribution of released substrate atoms in the overlayer. Detailed examination shows a separation in energy of ∼1.0 eV for the Al 2p binding energy for n- and p-type GaAs at low coverage. This equals the difference in band bending for the two substrates and demonstrates that the adatom energy reference is an intrinsic level of the semiconductor, not the Fermi level. Substrate band bending approaches its final value when E_F becomes the energy reference for the overlayer, and this occurs at the onset of metallic overlayer behavior. Temperature-dependent band bending observed below ∼1 monolayer can be understood in terms of the coupling of an adsorbate energy level to the semiconductor via steady-state tunneling and thermionic emission. The high-coverage results at all temperatures are consistent with metallicity.

This paper describes how temperature regulates the movement of the surface Fermi level for lightly doped GaAs(110) onto which submonolayer amounts of Ti and Ag have been deposited. Synchrotron radiation photoemission spectra show that E_F can be moved from near the band edges to ∼600 meV into the gap for p- and n-type GaAs by changing temperature from 20 to 300 K. Band bending changes are shown to be reversible when there are no morphology changes (Ti), but are not completely reversible when clustering occurs during the thermal cycle (Ag). These results demonstrate that the existence of gap states is not sufficient in itself to induce full band bending because charge exchange with those states is needed. They are discussed in the context of our dynamic-coupling model which shows how surface-bulk coupling is controlled by the bulk dopant concentration and temperature.

Ti/GaAs(110) interfaces formed at 60 and 300 K by atom-by-atom deposition and by the deposition of preformed metallic clusters exhibit very different Schottky-barrier behavior and morphologies. For atom deposition, temperature-dependent Fermi-level pinning is similar to that observed for nonreactive metals, despite the fact that the deposition process leads to the disruption of ∼3 monolayers of GaAs at all temperatures. Although disruption is apparent even at 0.02 Å, atom deposition at 60 K on n-type GaAs shows almost no Fermi-level movement until ∼2 Å when E_F moves rapidly toward midgap. Our results show that the adatoms do not act as nonreactive donors at low temperature, that thermal changes do not quench surface chemistry, and that surface reaction alone does not lead to midgap pinning. In contrast, for interfaces formed by preformed Ti cluster deposition, unique pinning positions are observed far from the point expected for known defects or metal-induced gap states. The instability of the Ti(cluster)/GaAs system is demonstrated by warming to 300 K. We postulate that kinetically restricted reactions occur beneath the metallic clusters and the midgap pinning requires disruption in the presence of a metallic overlayer.

The coverage-dependent Fermi-level movement for Ag, Co, and Ti interfaces formed at 60 K on n-type and p-type GaAs is shown to be symmetric but dependent on the bulk dopant concentration. Photoemission results show that E_F remains close to the band edges until ∼1 monolayer for doping of 1×10¹⁷ cm^-3 while E_F movement is induced by far fewer adatoms for doping of 2×10¹⁸ cm^-3 with overshooting for p-type GaAs. Remarkable surface chemical and structural insensitivity is reflected by similar band-bending trends for adatoms which exhibit very different reactivities and amounts of substrate disruption. We conclude that E_F movement is controlled by the coupling between adatom-induced states and those of the substrate at low temperature, with strong dependence on the bulk doping of the semiconductor.

Photoemission has been used to examine the formation of metal overlayers on sputter-annealed ZnSe(100)-c(2×2) surfaces for transition metals (Ti,Co), noble and near-noble metals (Cu,Ag,Au,Pd), a lanthanide (Ce), and a simple metal (Al). Analysis of Zn 3d core-level emission reflects metal-substrate interaction as well as substrate band bending as a function of metal coverage. Large differences in chemical behavior have been found. In particular, Ag and Au adatoms induce negligible Zn 3d core-level broadening, which is consistent with no substrate disruption. For the other metals, a second Zn 3d component reflects substrate disruption, and two reaction-induced components develop for Ti and Ce. The development of the Schottky barrier is compared with work-function variations. Very different Schottky-barrier heights have been found, ranging from 0.5 eV for Ce to 1.5 eV for Au and Pd. Neither the defect model nor the metal-induced gap-states model is able to describe all the experimental Schottky-barrier data. We do find a general correlation between the barrier height and the metal work function.

Photoemission and low-energy electron diffraction studies of Au/n-type ZnSe(100) and Co/n-type ZnSe(100) interface formation show that room-temperature growth does not produce long-range order, despite the excellence lattice match of bcc Co, fcc Au, and ZnSe. Nonetheless, Au forms an abrupt interface without substrate disruption and grows in an approximately layer-by-layer fashion. In contrast, Co disrupts the substrate and the released semiconductor atoms dissolve in the Co matrix or surface segregate. Both metals induce slow changes in the Fermi-level position and give markedly different final positions, 1.29±0.10 eV and 1.72±0.10 eV above the valence-band maximum after 14 Å deposition for Au and Co, respectively. Interlayer studies show that, when Co adatoms are deposited onto 4 or 8 Å of Au on ZnSe, there is Co indiffusion to disrupt the buried interface, even though bulk Co and Au are immiscible. This Au-Co intermixing and ZnSe disruption induce changes in the direction of Fermi-level movement so that its final position lies between those of Au/ZnSe and Co/ZnSe. The deposition of Au over 4 or 8 Å of Co on ZnSe alters the distribution of Zn atoms in the overlayer and again the Fermi-level evolution is altered. These results indicate that the Schottky-barrier height can be controlled through a proper choice of metals and interlayer thicknesses and that it depends on the metal adatoms, not the distribution of Zn or Se in the overlayer.

X-ray and ultraviolet photoemission results for ZnSe(100)-c(2×2) show that the variation in relative intensity of the Zn 3d surface and bulk components can be described for a wide range of kinetic energies using the ‘‘universal’’ inelastic mean free path. In contrast, the effective mean free path for low-energy photoelectrons is much less than expected when disordered interfaces are considered. These changes are associated with disorder at the interface and additional scattering for low-energy electrons. High-energy electrons are much less affected.

Ultraviolet- and x-ray-photoemission and low-energy electron-diffraction studies during room-temperature growth of Ge/ZnSe(100) and Si/ZnSe(100) heterojunctions show that these interfaces are abrupt but disordered. Heterojunction valence-band discontinuities were found to be 1.65±0.1 and 1.55±0.1 eV for Ge/ZnSe and Si/ZnSe, respectively, as determined from the valence-band maxima for ZnSe(100) and the growing overlayers of Ge or Si, with corrections made for band bending. Complementary calculations of the difference in core-level binding-energy positions for the substrate and the overlayer confirmed these results. The final Fermi-level-pinning positions were 1.75±0.1 and 1.85±0.1 eV above the valence-band maximum of ZnSe.

High-resolution x-ray photoemission spectroscopy and Ar-ion bombardment have been used to study temperature-dependent chemical reaction and species redistribution for Ti/GaAs(100). Our results show that Ti deposited at room temperature disrupts the GaAs substrate by reacting with As and releases Ga into the overlayer. As is found to accumulate near the buried interface in the form of a Ti-As compound. Ga is depleted from, but accumulates beyond, this reacted region. Sputter-depth profiles indicate that high-temperature annealing causes Ti diffusion into the GaAs substrate and enhanced reaction with As. Ga expulsion from the forming Ti-As compound becomes more severe when the amount of Ti-As increases. Heating promotes segregation of ejected Ga atoms to the vacuum surface, but has little influence on As segregation.