X‐ray microscopy at the advanced photon source

In modern third generation synchrotron sources, undulators have become the principal source of X-rays and today a brilliance close to 10²¹ photons/sec?mm²?mrad?0.1%BW is routinely attained for photon energies of 10 keV. However, generating brilliant beams of photons with energies of 50 keV and above leads to conflicting choices for the undulator parameters as the following analysis shows.     

Smith-Purcell free electron laser based on the semi-elliptical resonator

A novel Smith-Purcell(S-P) free electron laser composed of an electron gun,a semi-elliptical resonator,a metallic reflecting grating and a collector,is presented for the first time.This paper studies the characteristics of this device by theoretical analysis and particle-in-cell simulation method.Results indicate that tunable coherent S-P radiation with a high output peak power at millimeter wavelengths can be generated by adjusting the length of the grating period,or adjusting the voltage of the electron beam.The present scheme has the following advantages:the semi-elliptical resonator can reflect all radiation with the emission angle θ and random azimuthal angles,back onto the electron beam with same-phase and causes the electrons to be modulated,so the output power and efficiency are improved.     

Compositional change in human enamel irradiated with MIR free electron laser

The purpose of this study was to investigate compositional changes in human enamel irradiated with the free electron laser (FEL).The exposure on dental enamel at the wavelength of 9.64μm was observed with the Beijing free electron laser.The distribution of elements in the irradiated or non-irradiated enamel was measured by scanning electron microscope (SEM) with energy-dispersive spectroscopy and synchrotron radiation X-ray fluorescence(SRXRF) in Beijing Synchrotron Radiation Facility (BSRF).The results showed that the P/Ca ratio in the ablation region of enamel at the maximum wavelength of infrared absorption of enalmel at the maximum wavelength of infrared absorption was obviously smaller than that at the non-maximum wavelength.In the ablation region the ratios of P/Ca and Ca/Sr were smaller than those in the non-ablation region.The distribution of P,Ca and Sr in the ablation region were heterogeneous due to the element change caused by FEL irradiation.     

Smith–Purcell free electron laser based on a semi-conical resonator

We investigate a Smith–Purcell free electron laser composed of an electron gun, a semi-conical resonator, a metallic grating and collector. The semi-conical resonator could reflect all Smith–Purcell radiation with emission angle θ, and with random azimuthal angles, back onto the electron beam and causes the electrons to be modulated. Tunable coherent far-infrared Smith–Purcell radiation with a high output peak power at millimeter wavelengths can be generated.     

Harmonic lasing of X-ray free electron laser：on the way to smaller and cheaper

By utilizing higher harmonics of undulator radiation, harmonic lasing is helpful in the development of compact X-ray free electron lasers (FELs), i.e. reducing their cost and size. Harmonic lasing of FELs has been experimentally demonstrated in the low-gain FEL oscillators from terahertz, infrared to ultraviolet spectral range. Based on the current status and future directions of short-wavelength FELs worldwide, this paper reviews the progress on harmonic lasing of X-ray FELs, mainly concentrating on the recently proposed harmonic lasing of X-ray FEL oscillators and further ideas on harmonic lasing of single pass X-ray FEL amplifiers.     

NSLS‐II: A proposed new synchrotron at Brookhaven National Laboratory

A workshop on high-resolution X-ray emission spectroscopy was held in conjunction with the joint NSLS/CFN 2006 Users' Meeting. With the availability of high-brightness X-ray sources, and the development of improved optics and detectors for high-resolution (millivolt to volt) X-ray detection, X-ray emission spectroscopy (XES) has become one of the more rapidly growing areas of condensed matter synchrotron science. Most of the applications to date have focused on using XES for problems in condensed matter physics. The goal of this workshop was to illustrate the potential of XES for addressing important questions in chemistry and biology.     

X‐ray sources for studies of ultrafast processes

Radio signals from Jupiter were first detected in 1955 in the radio range at a frequency of 22.2 MHz. The emissions were sporadic in character, and were confined to frequencies below 40 MHz. These decametric (DAM) emissions have been interpreted as coherent cyclotron radiation from electrons in the tens of keV range. The innermost jovian moon Io, which orbits Jupiter in only 1.8 days, appears to modulate the emission: both the intensity and the probability of the occurrence of bursts increase when Io is at certain locations in its orbit with respect to Jupiter and the observer. The emissions originate in Jupiter's aurora, being produced by electrons that travel along magnetic field lines. Particles that enter the atmosphere may locally excite atoms and molecules, which upon de-excitation are visible as aurora at UV and infrared wavelengths (sometimes also at X-ray wavelengths). A fraction of the electrons is reflected back along the field lines, and produces DAM emissions.     

Ultrafast time‐resolved X‐ray absorption spectroscopy of chemical systems

The semiconductor industry continues in its relentless march to miniaturization [1 See, for instance http://www.itrs.net/Links/2007ITRS/2007_Chapters/2007_Lithography.pdf  [Google Scholar]]. Every four years or so, the dimensions of the features on an integrated circuit are halved, yielding an increase in density and functionality of the electronic “chip.” The economic advantages of more devices per unit area outweigh increases in fabrication costs and performance limitations, pushing the industry to seek ever-smaller patterns. At the time of writing (April 2008) advanced devices are patterned with the smallest features hovering around 45 nm, and the next generation of ～32 nm devices is on the horizon. What is perhaps most remarkable is that this level of nanopatterning is achieved with optical imaging tools and processes that use an actinic wavelength of 193 nm, the ArF laser emission line. As taught in any elementary physics textbook, the wavelength of light ultimately limits the achievable optical resolution [2 Hecht, E. and Zajac, A. 2002. Optics. Addison Wesley, : 471–474.  [Google Scholar]]. So how can we pattern 32 nm features using 193 nm radiation?     

Techniques for inelastic X‐ray scattering with μeV‐resolution

A new spectroscopic technique is introduced that allows tuning of a eVwide beam of synchrotron radiation over a range of a few meV. It relies on nuclear resonant scattering that is subject to the Doppler effect in high speed rotary motion. Two mechanisms are discussed how to extract the resonantly scattered radiation out of the broad band of synchrotron radiation: (a) grazing incidence reflection from a rotating disk in combination with a polarization filtering technique and (b) deflection of resonantly scattered radition via the recently discovered Nuclear Lighthouse Effect. Implications for inelastic Xray scattering and elastic nuclear resonant scattering are discussed.     

Symmetry role on the preedge X—ray absorption fine structure at the metal K edge

The preedge features in a system with “even”symmetry,apart from quadrupolar transition contribution,are mainly dipolar in character,associated with the existence of unoccupied states made up of mixed cation-4p with higher-neighboring cation-3d orbitals,and reflect the density of states due to the medium-range order of the system.In “odd“symmetry materials these preedge features ate the result of a transition from the ls to a final density of states of p symmetry due to an unsymmetrical mixing of the ligand wave functions with the central cation 3d orbitals,similar to atetrahedral configuration.These results are validated for Fe as a photoabsorber by comparing XAS spectra of Fe2SiO4(fayalite) to ab initio full multiple scattering calculations at the fe K edge,but pertain to all systems containing sixfold-coordinated cations.     

National school on Neutron and X‐ray scattering alumni presentations

The 4th joint Stanford–Berkeley summer school on synchrotron radiation and its applications in physical science was held June 12–17, 2005, at the Stanford Linear Accelerator Center (SLAC). The Stanford–Berkeley summer school is jointly organized by Stanford University, University of California Berkeley, Lawrence Berkeley National Laboratory (LBNL), and the Stanford Synchrotron Radiation Laboratory (SSRL). Since 2001, Anders Nilsson (Stanford/SSRL) and Dave Attwood (UC Berkeley) have been the organizers of this annual weeklong summer school, which alternates each year between Stanford and Berkeley. The summer school provides lecture programs on synchrotron radiation and its broad range of scientific applications in the physical science as well as visits to the Stanford Synchrotron Radiation Laboratory and the Advanced Light Source (ALS), where the students also have the opportunity to experience a beam line.     

Pulsed photoconductivity in diamond upon quasi-continuous laser excitation at 222 nm at the formation of an electron–hole liquid

An order-of-magnitude enhancement of the pulsed photocurrent in a polycrystalline diamond sample synthesized by chemical vapor deposition is observed under the conditions of formation of an electron–hole liquid. Nonequilibrium charge carriers are excited by laser pulses at a wavelength of 222 nm with FWHM pulse duration of 18 ns and peak intensity above 2.5 MW/cm² upon cooling the sample to 90 K. For peak intensities of laser excitation lower than 1 MW/cm², sample cooling from 300 to 90 K leads to a decrease in pulsed photocurrent by about a factor of 5. The observed increase in pulsed photocurrent is attributed to the formation of the electron–hole liquid.