Search Results (58 total)

We report the metallization of the Si(001)2×1 surface at elevated temperatures using angle-resolved photoemission spectroscopy (ARPES) and near-edge x-ray absorption fine structure (NEXAFS). A metallic state (S_m) over the E_F, which corresponds to the empty (π^∗) state of the 2×1 asymmetric dimer model, increases in the ARPES spectra, while the π^∗ state decreases in the NEXAFS spectra with increasing temperature. Since S_m is observed even at 400 K, the structural phase transition at ∼900 K [Phys. Rev. Lett. 91, 126103 (2003); Phys. Rev. Lett. 77, 3869 (1996)] is not related to the metallization. Thermal excitation seems to be too small to detect in ARPES in initial stage of the metallization and cannot account for the different behavior of S_m and the filled surface state of the up-dimer upon oxidation. We suggest, based on the existence of S_m even at 400 K and the oxidation behavior, that the metallization is attributed to thermal adatoms.

Operation of the Smith-Purcell backward wave oscillator requires a flat electron beam. Without the electron beam focusing, the requirement leads to a very stringent criterion on vertical emittance. In this paper, we discuss a way to relax the criterion by introducing an external focusing.

We describe the radiation properties of an x-ray free-electron laser (FEL) oscillator, beginning with its start-up from noise through saturation. We first decompose the initially chaotic undulator radiation into the growing longitudinal modes of the composite system consisting of the electron beam, the undulator, and the Bragg mirror resonator cavity. Because the radiation initially comprises several modes whose growth rates are comparable, we find that only after many oscillator passes is the output pulse dominantly characterized by the lowest-order Gaussian mode. We verify our analytic results with a novel, reduced one-dimensional FEL code (derived in the text), and with two-dimensional FEL simulations. Understanding the full longitudinal structure during the initial amplification will be critical in assessing the tolerances on the electron beam, undulator, and optical cavity required for robust operation.

An x-ray free-electron laser oscillator proposed recently for hard x rays [K. Kim, Y. Shvyd’ko, and S. Reiche, Phys. Rev. Lett. 100, 244802 (2008)] can be made tunable by using an x-ray cavity composed of four crystals, instead of two. The tunability of x-ray energy will significantly enhance the usefulness of an x-ray free-electron laser oscillator. We present a detailed analysis of the four-crystal optical cavity and choice of crystals for several applications: inelastic x-ray scattering, nuclear resonant scattering, bulk-sensitive hard x-ray photoemission spectroscopy, other high-energy-resolution (≲1  meV) spectroscopic probes, and for imaging with hard x rays at near-atomic resolution (≃1  nm).

A cross-relaxation energy-transfer scheme using the 5d state of Tm³⁺ as the donor and the 4f¹³ states of Tm³⁺ as the acceptor is proposed. The scheme is tested in the host KY₃F₁₀ doped with several concentrations of Tm³⁺ as a potential vacuum ultraviolet (VUV) excited blue phosphor of quantum yield greater than unity. Emission and diffuse reflection spectra along with studies of the time evolution of the 5d and 4f populations of Tm³⁺ in KY₃F₁₀ crystals and powders under UV and VUV excitations are reported and analyzed. The results show that the proposed quantum cutting mechanism occurs but the ¹I₆, ¹D₂, and ¹G₄ acceptor levels cross relax rapidly to lower-lying levels by an additional cross-relaxation energy transfer and that this effectively quenches the blue emission. Based on the temperature dependence of the spectra, an interesting heat-assisted relaxation process involving intersystem crossing is observed above 300 K.

We propose a compact Smith-Purcell radiation device that can potentially generate high average power THz radiation with high conversion efficiency. The source is based on a train of short electron bunches from an rf photoemission gun at an energy of a few MeV. Particle tracking simulation and analysis show that, with a beam current of 1 mA, it is feasible to generate hundreds of watts of narrow-band THz radiation at a repetition rate of 1 MHz.

We show that a free-electron laser oscillator generating x rays with wavelengths of about 1 Å is feasible using ultralow emittance electron beams of a multi-GeV energy-recovery linac, combined with a low-loss crystal cavity. The device will produce x-ray pulses with 10⁹ photons at a repetition rate of 1–100 MHz. The pulses are temporarily and transversely coherent, with a rms bandwidth of about 2 meV, and rms pulse length of about 1 ps.

We have investigated adsorption of acetic acid on Si(100)-2×1 at room temperature using high-resolution photoemission spectroscopy and near edge x-ray adsorption fine structure (NEXAFS) measurements in the partial electron yield mode. At room temperature, an acetic-acid molecule is found to chemisorb on Si(100)-2×1 surface through the formation of the OH dissociation structure. NEXAFS was conducted to characterize the adsorption geometry of acetic acid on Si(100). The π^* orbital of the CO bond shows a good angle dependence in carbon K-edge NEXAFS spectra, and we estimate the adsorption angle between chemisorbed acetic acid of CO bond and the Si(100) surface normal as ∼41°±2° using an analytical solution of NEXAFS intensity.

A critical process in high-brightness photoinjectors is emittance compensation, which brings under control the correlated transverse emittance growth due to the linear space-charge force. Although emittance compensation has been used and studied for almost two decades, the exact criteria to achieve emittance compensation is not as clear as it should be. In this paper, a perturbative analysis of slice envelopes and emittance evolution close to any reference envelope is developed, via which space-charge and chromatic effects are investigated. A new criterion for emittance compensation is found, which is complementary to the well-known matching condition for the invariant envelope and agrees very well with simulations.

A Smith-Purcell device can operate as a backward-wave oscillator for intense, narrow-bandwidth, continuous wave radiation at terahertz wavelengths. We determine the requirements on electron beam current and emittance for the system to oscillate based on a three-dimensional extension of our previous two-dimensional analysis. It is found that specially designed electron beams are required with a current that exceeds a certain threshold value and a flat transverse profile that allows the beam to travel very close to the grating surface. Two methods for producing electron beams with the required characteristics are discussed.

We grow Fe films on (4×2)-GaAs(100) at room temperature and low substrate temperature (∼130 K) and study their chemical structure by high-resolution photoelectron spectroscopy using synchrotron radiation. We find direct spectroscopic evidence for the effective suppression of the outdiffusion of both Ga and As from the substrate by the low temperature growth. Further, the Fe film is found to be stable even after warming up to 400 K. The thermal stability of the Fe film has kinetic origin; the diffusion of both Ga and As from the substrate through the already existing Fe film is expected to proceed via bulk diffusion that is much less efficient and thus a rate limiting process.

High-gain free-electron lasers (FELs) are being developed as extremely bright sources for a next-generation x-ray facility. In this paper, we review the basic theory of the start-up, the exponential growth, and the saturation of the high-gain process, emphasizing the self-amplified spontaneous emission. The radiation characteristics of an x-ray FEL, including its transverse coherence, temporal characteristics, and harmonic content, are discussed. FEL performance in the presence of machine errors and undulator wakefields is examined. Various enhancement schemes through seeding and beam manipulations are summarized.

The ability to generate small transverse emittance is perhaps the main limiting factor for the performance of high-gain x-ray free-electron lasers (FELs). Noting that beams from an rf photocathode gun can have energy spread much smaller than required for efficient FEL interaction, we present a method to produce normalized transverse emittance at or below about 0.1   μm, which will lead to a significantly shorter length undulator as well as a lower electron beam energy for an x-ray FEL project. The beam manipulation consists of producing an unequal partition of the initially equal emittances into two dissimilar emittances by a flat-beam technique and exchanging the larger transverse emittance with a smaller longitudinal emittance. We study various issues involved in the manipulation. In particular, a new emittance exchange optics we found enables an exact emittance exchange necessary for this scheme.

Oxidation of silicon during the growth of silicon oxide by ion beam sputter deposition was studied by in situ x-ray photoelectron spectroscopy as a function of oxygen partial pressure at various deposition temperatures below 600 °C. At low temperatures, the variation of incorporated oxygen content is similar to a dissociative adsorption isotherm of O₂ on Si indicating that the surface-confined reaction of the deposited Si atoms with the adsorbed oxygen atoms is the main process. However, it shows a three-step variation with the oxygen partial pressure at high temperatures. The evolution of SiO species confirmed by the XPS indicates that an adsorption-induced surface reaction and a diffusion-induced internal reaction are the main pathways for the Si oxidation.

Angle-resolved photoemission spectroscopy (ARPES) is used to investigate whether the Ge(001) surface is metallic or not at room temperature (RT). The uppermost filled state is a surface state originating from the dangling bonds of the updimers. A metallic state, which has been previously reported to exist only near Γ, is also weakly visible at the J₂^′ point along the [010] direction at temperatures higher than RT and the state completely disappears at RT. These results indicate that the metallic state is the lowest empty surface state of the asymmetric 2×1 structure at RT. Contrary to the belief that the surface is metallic at RT, the present ARPES results strongly suggest that the Ge(001) surface is semiconducting with a gap of about 0.44 eV at RT.

We report an experimental study on terahertz (THz) emission from Ga_1−xIn_xSb with 0≤x≤1. THz emission is excited by femtosecond near-infrared laser pulses. For this material system THz emission is maximized for an In mole fraction x≈0.5. The maximum in THz emission occurs as a result of carrier compensation (N_A≈N_B) for this specific material composition. The THz emission from n-type InSb is twice as large than that from p-type GaSb. The THz emission from Ga_1−xIn_xSb is explained according to the photo-Dember model. The Ga_1−xIn_xSb material system enabled the study of the influence of carrier concentrations on the THz emission process in narrow band gap semiconductors. Our study demonstrates the existence of a compromise between the positive effect of high electron temperature provided by narrow band gap materials and the negative effect of a high intrinsic carrier concentration. This compromise dictates the extent to which the band gap in a semiconductor can be reduced in order to enhance the THz emission. This same analysis can be extended to explain why the THz emission from InSb is lower than that of InAs.

The generation of a flat electron beam directly from a photoinjector is an attractive alternative to the electron damping ring as envisioned for linear colliders. It also has potential applications to light sources such as the generation of ultrashort x-ray pulses or Smith-Purcell free electron lasers. In this paper, we report on the experimental generation of a flat beam with a measured transverse emittance ratio of 100±20 for a bunch charge of ∼0.5  nC; the smaller measured normalized root-mean-square emittance is ∼0.4      μm and is limited by the resolution of our experimental setup. The experimental data, obtained at the Fermilab/NICADD Photoinjector Laboratory, are compared with numerical simulations and the expected scaling laws.

We present an analysis of the beam dynamics in a Smith-Purcell free-electron laser (FEL). In this system, an electron beam interacts resonantly with a copropagating surface electromagnetic mode near the grating surface. The surface mode arises as a singularity in the frequency dependence of the reflection matrix. Since the surface mode is confined very close to the grating surface, the interaction is significant only if the electrons are moving very close to the grating surface. The group velocity of the surface mode resonantly interacting with a low-energy electron beam is in the direction opposite to the electron beam. The Smith-Purcell FEL is therefore a backward wave oscillator in which, if the beam current exceeds a certain threshold known as start current, the optical intensity grows to saturation even if no mirrors are employed for feedback. We derive the coupled Maxwell-Lorentz equations for describing the interaction between the surface mode and the electron beam, starting from the slowly varying approximation and the singularity in the reflection matrix. In the linear regime, we derive an analytic expression for the start current and calculate the growth rate of optical power in time. The analysis is extended to the nonlinear regime by performing a one-dimensional time-dependent numerical simulation. Results of our numerical calculation compare well with the analytic calculation in the linear regime and show saturation behavior in the nonlinear regime. We find that a significant amount of power grows in the surface mode due to this interaction. Several ways to outcouple this power to freely propagating modes are discussed.

Various projects under study require an angular-momentum-dominated electron beam generated by a photoinjector. Some of the proposals directly use the angular-momentum-dominated beams (e.g., electron cooling of heavy ions), while others require the beam to be transformed into a flat beam (e.g., possible electron injectors for light sources and linear colliders). In this paper we report our experimental study of an angular-momentum-dominated beam produced in a photoinjector, addressing the dependencies of angular momentum on initial conditions. We also briefly discuss the removal of angular momentum. The results of the experiment, carried out at the Fermilab/NICADD Photoinjector Laboratory, are found to be in good agreement with theoretical and numerical models.

The electronic structure of La_0.7Ce_0.3MnO₃ (LCeMO) thin films has been investigated using photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). The Ce 3d core-level PES and XAS spectra of LCeMO are very similar to those of CeO₂, indicating that Ce ions are close to being tetravalent. A very weak 4f resonance is observed around the Ce 4d→4f absorption edge, suggesting that the localized Ce 4f states are almost empty in the ground state. The Mn 2p XAS spectrum reveals the existence of the Mn²⁺ multiplet feature, confirming the Mn²⁺-Mn³⁺ mixed-valent states of Mn ions in LCeMO. The measured Mn 3d PES/XAS spectra for LCeMO agree reasonably well with the Mn 3d partial density of states calculated using the LSDA+U (LSDA, local spin-density approximation) method. The LSDA+U calculation predicts a half-metallic ground state for LCeMO. This study confirms that the LCeMO films are indeed electron doped.

We report on a characterization of the chaotic optical field from a high-gain, self-amplified spontaneous-emission (SASE) free-electron laser. The temporal structure of the amplitude and phase are measured in a single-shot mode, with a resolution well below the coherence length, and the statistics over multiple pulses is determined. The measurement is in excellent quantitative agreement with the prediction based on analysis of random noise, and further verifies the chaotic nature of the SASE optical field.

A study of round-to-flat configurations, and vice versa, of angular-momentum-dominated beams is presented. The beam propagation in an axial magnetic field is described in terms of the familiar Courant-Snyder formalism by using a rotating coordinate system. The discussion of the beam transformation is based on the general properties of a cylindrically symmetric beam matrix and the existence of two invariants for a symplectic transformation in 4D phase space.

We describe the status of our effort to realize a first neutrino factory and the progress made in understanding the problems associated with the collection and cooling of muons towards that end. We summarize the physics that can be done with neutrino factories as well as with intense cold beams of muons. The physics potential of muon colliders is reviewed, both as Higgs factories and compact high-energy lepton colliders. The status and time scale of our research and development effort is reviewed as well as the latest designs in cooling channels including the promise of ring coolers in achieving longitudinal and transverse cooling simultaneously. We detail the efforts being made to mount an international cooling experiment to demonstrate the ionization cooling of muons.

We have used synchrotron-based high-resolution core-level photoemission and valence-band emission measurements to characterize hydrogen chemisorption on nonpolar GaAs(110) and polar GaAs(100) and GaAs(001) surfaces. Chemisorbed atomic hydrogen forms both Ga-H and As-H bonds on all three surfaces, causing chemical shifts of core-level binding energies and changing the valence-band emission. For low hydrogen exposures arsenic desorbs from all surfaces. However, at higher exposures, the (110) surface transforms into a Ga-rich structure with traces of metallic Ga, while the (100) surface transforms into an As-rich structure. We have also observed some additional changes in the binding energy of bulk components of Ga and As core levels as a function of hydrogen exposure, which may be explained by hydrogen-induced changes in band bending.