The dependence of nuclear magnetic resonance (NMR) image contrast on intrinsic and pulse sequence timing parameters

In Nuclear Magnetic Resonance (NMR) the image pixel value is governed by at least three major intrinsic parameters: the spin density N (H), the spin-lattice relaxation time T1, and the spin-spin relaxation time T2. The extent to which the signal is weighted toward one or several parameters is related to the history of the spin system preceding detection. On the simplifying, though not generally warranted assumption that the spin density does not vary significantly in soft tissues, relative tissue contrast can be predicted quantitatively provided the relaxation times are known. Signal intensities and contrast were computed on the basis of the Bloch equations and experimentally determined relaxation times as a function of pulse timing parameters and the data compared with those in images recorded at 0.5T field strength. Significant deviations from the equal density hypothesis were found for gray and white substance. Notably partial saturation but also spin echo and inversion-recovery images are not in full accordance with predictions made on the basis of relaxation times alone.     

Near-IR cavity ringdown spectroscopy and kinetics of the isomers and conformers of the butyl peroxy radical

Cavity ringdown spectra of butyl peroxy radicals have been obtained for their A-X electronic transition in the near-IR. The radicals were produced by two independent chemical methods, allowing unambiguous assignment of the spectra of the four butyl peroxy isomers with probable conformer assignments also possible for a number of cases. Using the analyzed spectra semiquantatively, isomer specific rate constants for butyl peroxy self-reaction were measured, as was the relative reactivity of the various sorts of H atoms in butane to Cl atom attack.     

Hydrogen incorporation in diamond: the vacancy-hydrogen complex

We report the identification of the vacancy-hydrogen complex in single crystal diamond synthesized by chemical vapor deposition. The S=1 defect is observed by electron paramagnetic resonance in the negative charge state. The hydrogen atom is bonded to one of the carbon atoms neighboring the vacancy. Unlike the analogous defect in silicon, no symmetry lowering reconstruction occurs between the three remaining carbon dangling orbitals. The very small measured hydrogen hyperfine interaction is explained by dipolar coupling between the hydrogen and the unpaired electron probability density delocalized on the three equivalent carbon neighbors.     

Probing particle synthesis during femtosecond laser ablation: initial phase transition kinetics

The impulsive superheating of matter by an intense, ultrashort laser pulse drives material expansion into vacuum (ablation) and an associated formation of nanoparticles. The underlying dynamics of particle formation are complex and direct experimental probes of the rapid material evolution are essential. Femtosecond lasers coupled to modern synchrotrons offer an important new opportunity to probe ejecta dynamics on an atomic lengthscale. Here, the impulsive heating of a semiconductor (silicon) by an intense femtosecond laser pulse leads to material ejection and time-resolved photoemission spectroscopy probes rapid solidification kinetics occurring within the ejecta. Transient photoemission peak-shifts indicate that material is ejected predominantly as liquid droplets and that solidification occurs rapidly (<50 ps). The solidification time suggests that vacuum ejection leads to significantly enhanced undercooling compared to what has been obtained by more conventional quenching techniques; this may be of interest in attempts to trap novel material states associated with extreme laser heating. Finally, a low fraction of vapor particles in the ejecta supports a view that the size-distribution of ejected particles is set by an initial fragmentation process rather than by vapor condensation. PACS 82.60.Qr; 87.64.Lg; 62.50.+p     

Hydrogen incorporation in diamond: the nitrogen-vacancy-hydrogen complex

We report the identification of the nitrogen-vacancy-hydrogen complex in a freestanding nitrogen-doped isotopically engineered single crystal diamond synthesized by chemical vapor deposition. The hydrogen atom is located in the vacancy of a nearest-neighbor nitrogen-vacancy defect and appears to be bonded to the nitrogen atom maintaining the trigonal symmetry of the center. The defect is observed by electron paramagnetic resonance in the negative charge state in samples containing a suitable electron donor (e.g., substitutional nitrogen N(0)(S)).     

The heat of fusion of poly(ethylene terephthalate)

By use of the Clapeyron equation for the dependence of the melting point on pressure, the heat of fusion was found to be 32.5 cal/g, in good agreement with values determined by other methods. An equation for the dependence of the melting point on the degree of polymerization gave a heat of fusion of 27.6 cal/g when applied to hydroxyl-terminated oligomers. This simple relation applied all the way down to the smallest member of the series, di(hydroxy ethyl) terephthalate.     

Cubic irreducible graphs for the projective plane

A characterization of all cubic finite graphs that do not embed in the real projective plane P is given in the sense that Kuratowski characterized all non-planar finite graphs. Specifically it is shown that there exist exactly 6 cubic irreducible graphs for P.