Terrence Jach

A UHV compatible two-crystal monochromator for synchrotron radiation

Abstract For obvious reasons, monochromatic synchrotron radiation studies at soft X-ray energies require that the monochromator be ultra-high vacuum compatible. We will describe the design, testing and performance of a linkage based, two-crystal, non-dispersive monochromator for use at synchrotrns. The use of mechanical linkage reducesthe degrees of freedom and minimized the number of motion feedthroughs into the vacuum. The linkage chosen maintains a constant output beam position, and assures that the second crystal is always properly positioned. Pertinent design features include direct angular encoding and a feedback-controlled correction system to eliminate the effects of mechanical imperfections.

A high energy resolution X-ray spectroscopy synchrotron radiation beamline for the energy range 800–5000 eV

A beamline for X-ray spectroscopy of atomic and molecular gases and condensed matter has been designed and installed at the National Synchrotron Light Source. The beamline is UHV compatible to allow windowless operation for improved flux at low photon energies. A double axis crystal monochromator is employed with a collimating premirror and a focusing postmirror. Paris of beryl, quartz or silicon crystals define an energy band width of <0.4 eV at an arbitrary energy above 0.8 keV. The premirror acts as a tuneable low-pass filter to minimize heat loading on the first monochromator crystal. At the present operating parameters of NSLS, a flux of 109–1013 photons/s of highly monochromatic X-rays can be focused onto a 1 mm diameter spot. Initial experimental results are presented.

Pin diode detectors for synchrotron X-rays

Abstract PIN diodes offer a number of advantages over ion chambers and other X-ray detectors in size, signal-to-noise ratio, collection efficiency, dynamic range, and ultrahigh vacuum compatibility. We have measured the response of several commercially available PIN diodes in the X-ray region of 3.5–11 keV on the Naval Research Laboratorys Materials Analysis Beam Line (X-23B) at the National Synchrotron Light Source [J.P. Kirkland, D.J. Nagel and P.L. Cowan, Nucl. Instr. and Meth. 208 (1983) 49]. We analyze the results of these measurements with respect to the construction of these devices, and discuss various synchrotron applications.

Grazing-incidence x-ray photoemission spectroscopy investigation of oxidized GaAs(100): A novel approach to nondestructive depth profiling

When a beam of x rays (∼1 keV) impinges on a flat surface at grazing angles (≲3°) the x rays undergo total external reflection. Under these conditions, the penetration depth of the x rays can become comparable to photoelectron escape depths and the photoelectron yields from the surface are enhanced. As the incidence angle of the x rays is increased, the x‐ray penetration depth increases and the photoelectron yields contain a larger contribution from deeper layers within the sample. By exploiting this depth‐dependence of photoelectron yields as a function of incident x‐ray beam angle, it is possible to obtain information about the depth distribution of the photoelectron emitting atoms. We have used this novel application of grazing‐incidence x‐ray photoelectron spectroscopy (GIXPS) to study the ultraviolet oxidized GaAs(100) surface. This oxide/GaAs surface is particularly well suited for study with GIXPS because a variety of oxide phases are formed during oxidation and questions concerning stratification ...

PIN diodes as detectors in the energy region 500 eV–10 keV

Abstract PIN diodes offer several advantages over ion chambers in signal-to-noise ratio, size, and ultra-high vacuum compatibility. We have evaluated several commercially available PIN diodes, suitable for use in the X-ray region 500 eV ⩽ E ⩽ 10 keV, using both brehmsstrahlung and characteristic lines from X-ray tubes between 1.29 and 8.9 keV. We present data on response functions, noise levels, and linearity of response.

Performance of a high‐energy‐resolution, tender x‐ray synchrotron radiation beamline (invited)

Beamline X‐24A at the National Synchrotron Light Source was designed for optimal performance in the x‐ray spectral region 500–5000 eV. This choice of energy range placed a number of constraints on the beamline design, requiring a crystal monochromator in a windowless UHV environment. Although this increased the complexity of the design, there were compelling scientific reasons for our desire to work in this range. In addition to tunability over the selected energy range, a primary goal was to obtain the highest possible energy resolution in the primary beam. We have achieved incident energy resolution significantly better than the typical core‐level lifetime broadening for this energy range. This has permitted studies of processes that are not broadened by lifetimes, such as resonant scattering and back‐reflection x‐ray standing‐wave effects. In addition to high resolution, it was designed to collect and focus as much flux as possible from the bending magnet source.

Grazing angle x‐ray photoemission system for depth‐dependent analysis

We have developed an x‐ray photoelectron spectrometer system which combines an adjustable grazing incidence angle source with reflected beam detection. When operated about the critical angle, this combination permits a variation of the x‐ray penetration depth which can be monitored by means of the reflectivity. At angles of incidence less than the critical angle, the sampling depth of the photoemission is diminished, but the photoemission from the surface is enhanced due to the constructive interference of the incident and reflected x‐ray beams. When used with Mg Kα radiation (Eγ=1253.6 eV), the spectrometry system obtains useful distributions of chemical species in surface layers of 10–40 A thickness. We present data showing the depth dependence obtained with the spectrometer of different oxides in a sulfide‐treated, oxidized GaAs (100) surface.

Grazing incidence X-ray photoemission and its implementation on synchrotron light source X-ray beamlines

Abstract Grazing incidence X-ray photoemission spectroscopy provides a method of obtaining information about surface chemical composition as well as the variation of composition with depth. Photoemission spectra are taken as X-rays are directed onto a surface over a range of incidence angles near the critical angle for total external reflection. The technique is particularly suited to the study of surface layers in the thickness range 10–40 A, using X-rays in the energy range of 1–2 keV. We have implemented the technique in a geometry that minimizes distortion of the spectral lineshape by keeping a fixed relationship between the sample and the electron spectrometer. We present data taken in the laboratory that illustrate its application to the study of oxide films on Ge. The counting time for a spectrum would be shortened considerably by implementing the method on a soft X-ray beamline at a synchrotron light source. We present a method for doing so that retains the advantages of a fixed geometry between the sample and the electron spectrometer.

Polycapillary X-Ray Optic Spectral Gain and Transmission

Polycapillary x-ray optics are gaining increased attention as their transmission characteristics and manufacturing difficulties are better understood. This paper reports experimental results obtained with an optic that was designed for use with both Mo Kα (17.5 keV) and Cu Kα (8 keV). The spectral gain function from 4 to 30 keV was determined from measurements made with an Si(Li) detector, and the results were compared with Monte Carlo simulations. The spatial variation in the spectral transmission was examined with both an Si(Li) detector and a magnifying x-ray CCD camera. The results show that even though the individual capillaries are tapered, their performance is well described by a simple parabolic relationship between the radius of curvature and the maximum energy transmitted derived for capillaries of constant diameter. Copyright

X-ray emission spectroscopy of nitrogen-rich compounds.

Nonresonant X-ray emission spectroscopy was used to compare the nitrogen-rich compounds ammonium nitrate, trinitrotoluene, and cyclotrimethylene-trinitramine. They are representative of crystalline and molecular structures of special importance in industrial and military applications. The spectral signature of each substance was analyzed and correlated with features in the electronic structure of the systems. This analysis was accomplished by means of theoretical simulations of the emission spectra and a detailed examination of the molecular orbitals and densities of states. We find that the two theoretical methods used (frozen-orbital density functional theory and real-space Greens function simulations) account semiquantitatively for the observed spectra and are able to predict features arising from distinct chemical complexes. A comparison of the calculations and the data provides insight into the electronic contributions of specific molecular orbitals, as well as the features due to bandlike behavior. With some additional refinements, these methods could be used as an alternative to reference compounds.