Synthesis and Properties of Dipyridylcyclopentenes

A short and general route to the substituted dipyridylcyclopentenes was explored and several new compounds belonging to this new group of diarylethenes were synthesized. The study of their photochromic and thermochromic properties shows that the rate of the thermal ring opening is strongly dependent on the polarity of the solvent.

Thermal transport in thin films measured by time-resolved, grazing incidence x-ray diffraction

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98715/1/JApplPhys_110_102203.pd

Experience with a fluorescence‐based beam position monitor at the APS

We have recently redesigned the first crystal mount of our cryogenically cooled monochromator to reduce its sensitivity to pressure fluctuations in the cryogenic lines feeding the Si (111) crystal. With the use of a fluorescence‐based X‐ray beam position monitor (BPM) placed 19 m away from the monochromator, much operational experience has been gained on the sensitivity of the beam position and intensity to small changes in the cooling system. In this presentation, we will describe our X‐ray BPM design and performance and will provide examples of changes that have made the beam position more stable on our beamline. One such change for example has been the top‐up operation of the Advanced Photon Source (APS), which has reduced the thermal drifts associated with the ring current decay. © 2004 American Institute of PhysicsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87659/2/679_1.pd

Structural optimization of reversible dibromomaleimide peptide stapling

Methods to constrain peptides in a bioactive α‐helical conformation for inhibition of protein‐protein interactions represent an ongoing area of investigation in chemical biology. Recently, the first example of a reversible “stapling” methodology was described which exploits native cysteine or homocysteine residues spaced at the i and i + 4 positions in a peptide sequence together with the thiol selective reactivity of dibromomaleimides (a previous study). This manuscript reports on the optimization of the maleimide based constraint, focusing on the kinetics of macrocyclization and the extent to which helicity is promoted with different thiol containing amino acids. The study identified an optimal stapling combination of X1 = L‐Cys and X5 = L‐hCys in the context of the model peptide Ac‐X1AAAX5‐NH2, which should prove useful in implementing the dibromomaleimide stapling strategy in peptidomimetic ligand discovery programmes

Diffractometer‐Control Software For Bragg‐Rod Measurements

We present Generalized Diffractometer Control (gdc), a diffractometer‐control software package developed specifically for high‐precision measurements of Bragg rods; we discuss its features and analyze its performance in data collection. gdc, implemented at several APS beamlines, controls a six‐circle diffractometer in either Eulerian or kappa geometry, yet does not assume a mechanically ideal diffractometer; instead, the measured directions of the diffractometer axes (and the direction of the incident beam) are input parameters. The Labview‐based program features a graphical interface, making it straightforward to find all the commands and operations. Other features include optimized scans along Bragg rods, straightforward background subtraction, and extensive sets of pseudomotors. © 2004 American Institute of PhysicsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87660/2/1221_1.pd

Order and phase nucleation in nonequilibrium nanocomposite Fe-Pt thin films with perpendicular magnetic anisotropy

We report on the time evolution of mass transport upon annealing nonequilibrium Fe-Pt nanocomposite films, leading to nucleation of L1(0) chemically ordered phase. The nonequilibrium nanocomposite films were fabricated by applying Fe(+) ion implantation to epitaxial Pt films grown on (001) MgO substrates, yielding Fe nanoclusters embedded in a Pt matrix at a tailored penetration depth. Time-resolved x-ray diffraction studies were carried out using synchrotron radiation, allowing determination of the activation energy for nucleation of the FePt L1(0) phase within the segregated nanoclusters during annealing. The growth of the segregated L1(0) ordered phase was modeled using ideal grain-size law and found to be dominated by strain-driven surface nucleation. The activation energies were found to correlate with the nanocluster size. Magnetic characterization of selected annealed samples indicates perpendicular magnetic anisotropy with high coercive field coincident with high value of the chemical order parameter of the ordered phase within the magnetic nanoclusters

Sculpting the shape of semiconductor heteroepitaxial islands: from dots to rods

In the Ge on Si model heteroepitaxial system, metal patterns on the silicon surface provide unprecedented control over the morphology of highly ordered Ge islands. Island shape including nanorods and truncated pyramids is set by the metal species and substrate orientation. Analysis of island faceting elucidates the prominent role of the metal in promoting growth of preferred facet orientations while investigations of island composition and structure reveal the importance of Si-Ge intermixing in island evolution. These effects reflect a remarkable combination of metal-mediated growth phenomena that may be exploited to tailor the functionality of island arrays in heteroepitaxial systems.Comment: accepted- Physical Review Letters; 12 pages, 4 figure

Manifestation of postcollision interaction in Krypton LMN Auger spectrafollowing K-shell photoionization

We report on an experimental and theoretical study of postcollision interaction (PCI) effects on L2−M4,5N2,3 Auger electrons measured above the Kr K-edge in which L2 vacancies are primarily generated by KL2 x-ray emission. Such cascade processes, in which a deep inner-shell vacancy decays first by x-ray emission followed by Auger electron emission, is a strong decay mode in heavy atoms. The L2−M4,5N2,3 Auger electron peak is observed to become increasingly asymmetric with a shifting peak maximum as the absorbed x-ray energy approaches the K-shell ionization threshold. This is attributed to PCI energy exchanges of the Auger electron with the 1s photoelectron. To model the PCI effects, we have applied a semiclassical approach modified to account for the combined lifetimes of the K and L2 hole states. In addition, our analysis treats several closely spaced Auger transitions with final ionic states having different terms and angular momenta