Roger Carr

A tilted precursor for CO dissociation on the Fe(100) surface

Near edge X-ray absorption fine structure (NEXAFS) has been used to study the molecular orientation of the α3 state of CO on the Fe(100) surface. It is found that the molecule is tilted by 45° ± 10° with respect to the surface normal, allowing direct interaction of the oxygen end of the molecule with the iron surface. The C-O bond is found to be elongated by 0.07 ± 0.02 A in the α3 state, relative to the other molecularly adsorbed CO states on this surface.

Correlation between thermal desorption spectroscopy and optical second harmonic generation for monitoring surface coverages

Abstract Optical second harmonic generation was used to monitor CO coverages on Cu(100) and Ni(111) in an ultrahigh vacuum chamber. In both cases, the adsorption kinetics were found to obey the simple Langmuir model. In monitoring CO coverages remained on Cu(100) and Ni(111) during a thermal desorption process, optical second harmonic generation correlates well with thermal desorption spectroscopy.

Determination of chemisorption geometries for complex molecules by using near-edge x-ray absorption fine structure: Ethylene on Ni(100)

We have derived theoretical expressions for the incident beam angular dependence of the absorption cross section for ethylene chemisorbed on single crystal surfaces with linearly polarized light. The changes in the π and σ resonance intensities for C 1s near‐edge x‐ray absorption fine structure (NEXAFS) spectra can be related to the geometry of chemisorption relative to the surface. The derived equations have been used to analyze experimental data in order to determine the structure of chemisorbed ethylene on Ni(100). At low temperatures ethylene bonds with the molecular axis parallel to the surface but with the molecular plane tilted 50°. The geometry of vinyl fragments formed by annealing the crystal to 180 K was also determined. The C–C bond is bent 35° from the surface normal in this case, with the plane of the molecule tilted by 65°.

Adjustable phase insertion devices as X-ray sources

Abstract This is a presentation of the design principles for electron storage ring insertion devices whose gap is fixed but in which the rows of magnets move longitudinally with respect to each other. These devices are shown to perform in similar ways to adjustable gap insertion devices presently in use at synchrotron radiation laboratories, but to have significant advantages for new designs. The magnetic field, trajectories of electrons, and spectra of X-rays are calculated, and design considerations are discussed.

Ethylidyne formation rates on the Pt(111) surface

Abstract The isothermal rates of ethylidyne formation from adsorbed ethylene have been characterized on a Pt(111) surface from 230 to 280 K using transient near edge X-ray absorption fine structure (T-NEXAFS). These studies establish the feasibility of T-NEXAFS as a method for measuring surface reaction rates in the 10 −2 to 10 −5 monolayer/s range. Since direct measurements of surface concentrations are used to establish the rates of the surface reaction no desorbing products are required for characterization of the isothermal reaction rates. The ethylidyne formation rates are first order in ethylene coverage. A substantial isotope effect is observed, perhydroethylene reacts about twice as fast as perdeuteroethylene. The activation energies are 14.4±0.7 kcal/mol (perhydro) and 16.7±1.0 kcal/mol (perdeutero) for the alehydrogenation of adsorbed ethylene.

Performance of the elliptically polarizing undulator on SPEAR

This is a description of the first tests of the Elliptically Polarizing Undulator (EPU) on the SPEAR storage ring at SSRL. The EPU is the first device of its type; it is capable of producing plane polarized light in the vertical and horizontal, and right and left circularly polarized light in the 500-1000 eV range. Tests of the EPU were done to characterize its effect on the electron beam in SPEAR. Even at minimum gap, motion of the EPU magnets to vary the polarization of the output radiation caused negligible changes in the tune or the steering of the electron beam, even with no compensation of the steering trim coils. We also measured the polarization of x-rays generated by the EPU using a newly developed multilayer polarimeter built to be efficient in the EPU`s energy range. The EPU produces nearly 100% plane and circularly polarized x-rays. Using left and right circularly polarized radiation, we also performed tests of magnetic circular dichroism on magnetic multilayers.

An elliptically-polarizing undulator with phase adjustable energy and polarization

Abstract We present a planar helical undulator designed to produce elliptically-polarized light. Helical magnetic fields may be produced by a variety of undulators with four parallel cassettes of magnets. In our design, all cassettes are mounted in two planes on slides so that they may be moved parallel to the electron beam. This allows us to produce X-rays of left- or right-handed elliptical or circular polarization as well as horizontal or vertical linear polarization. In model calculations, we have found that by sliding the top pair of rows with respect to the bottom pair, or the left pair with respect to the right pair, we retain the polarization setting but change the magnetic field strength, and hence the X-ray energy. This allows us to select both energy and polarization by independent phase adjustments alone, without changing the gap between the rows. Such a design may be simpler to construct than an adjustable-gap machine. We present calculations that model its operation and its effects on an electron beam.

New experiments using a soft X-ray undulator☆

Abstract We discuss some of the exciting new experimental possibilities offered by soft X-ray undulators as a radiation source. In addition to the general discussion we present two specific experiments on isolated molecules for which the undulator source is extremely important. The first one of these experiments is a coincidence experiment between Auger electrons and ions generated in the events following the absorption of a soft X-ray photon by an isolated molecule. This gives a detailed picture of the involvement of individual valence electrons into the molecular bond. The second experiment is a study of the electronic decay of a resonant core electron excitation into a bound molecular orbital. The decay of these states yields new information about the localization of the molecular valence orbitals around the atom where the core hole was created.

Future possibilities of the Linac Coherent Light Source

A study of the potential for the development of the Linac Coherent Light Source (LCLS) beyond the specifications of the baseline design is presented. These future developments include delivery of X-ray pulses in the 1 fs regime, extension of the spectral range, increase of the FEL power, exploitation of the spontaneous emission, and a more flexible time structure. As this potential is exploited, the LCLS can maintain its role as a world-leading instrument for many years beyond its commissioning in 2008 and initial operation as the worlds first X-ray free-electron laser.

Exotic sources of x-rays for iodine K-edge angiography

Abstract Digital subtractive angiography has been performed to image human coronary arteries using wiggler radiation from electron storage rings. The significant medical promise of this procedure motivates the development of smaller and less costly X-ray sources. Several exotic sources are candidates for consideration, using effects such as Cherenkov, channeling, coherent bremsstrahlung, laser backscattering, microundulator, parametric, Smith-Purcell, and transition radiation. In this work we present an analysis of these effects as possible sources of intense X-rays at the iodine K-edge (33.169 keV). The criteria we use are energy, efficiency, flux, optical properties, and technical realizability. For each of the techniques, we find that they suffer either from low flux, a low energy cutoff, target materials heating, too high electron beam energy requirement, optical mismatch to angiography, or a combination of these. We conclude that the foreseeable state-of-the-art favors a compact storage ring design (H. Wiedemann et al., these Proceedings (8th Nat. Conf. on Synchrotron Radiation Instrumentation, Gaithersburg, MD, USA, 1993) Nucl. Instr. and Meth. A. [1]).