Computed adsorptive-energy distribution in the monolayer (Caedmon)

A computer-based method is developed to perform the numerical analysis of the measured adsorption isotherm in order to determine the quantitative distribution of potential energies displayed by a solid substrate for a physically-adsorbed gas. The characteristic adsorptive energies of the basal planes of graphite and of boron nitride for argon and nitrogen are determined as examples of the use of the method. As more such values are put on record the method becomes suitable for the qualitative analysis of substrates.     

Magnetic fields associated with a nuclear magnetic resonance medical imaging system

Measurements were made of magnetic and electric field levels in and around a nuclear magnetic resonance imaging system undergoing a clinical trial. Magnetic field levels ranged from 0.04 tesla (T) in the imaging volume down to about 0.0006 T at the end of the patient table. The peak radio-frequency magnetic field level was 15 amperes per meter (A/m) in the imaging volume, while the rms value was 4.6 A/m. The specific absorption rate resulting from the radio-frequency magnetic field was calculated to be no more than 0.017 watts per kilogram (W/kg). The radio-frequency electric field was detectable only within a few centimeters of the coil assembly, and does not significantly contribute to the specific absorption rate. These exposure levels were much lower than existing guidelines for clinical NMR procedures.     

Charged multiplicities in K−p interactions at 100 GeV/c

The charged multiplicity distribution is presented for K^?p interactions produced in the hydrogen bubble chamber, BEBC, using an r.f. separated, tagged K^? beam of 110 GeV/c momentum. A comparison with K⁺p, πp and pp data at lower energies shows that the main features of the multiplicity distributions depend on energy and charge of the incident particles, but not on their strangeness. At high energies, only the energy is important.     

Time-of-flight measurements of the vibrational and rotational excitation of no by collision with 7Li+ ions at 7 eV collision energy

The energy loss of ⁷Li⁺ ions scattered from NO has been measured in a molecular beam experiment over a range of scattering angles (θ_lab = 6.0–30.0°) at E_c.m. = 7 cV. The vibrational/rotational energy transfer behaves similar to the previously reported study on ⁷Li⁺CO, suggesting that in NO the degeneracy of the electronic ground state has little effect.     

A study of the headgroup motion of sphingomyelin using 31P NMR and an analytically soluble model

A 31P NMR investigation has been carried out of the headgroup dynamics of sphingomyelin molecules in bilayers for the L alpha and L beta' phases. The resulting line shapes have been analysed in terms of a reduced-parameter model, using van Faassen's method for obtaining an analytic solution to first-order stochastic differential equations to simulate the line shapes of oriented and non-oriented samples. Our treatment results in good fits to measured data but using fewer parameters than traditional methods. Angles and correlation times (tau parallel and tau perpendicular) describing the geometry and dynamics of the headgroup are obtained by optimising the agreement between simulated and experimental data. The results are contrasted with those obtained for the lecithins DMPC and DPPC using a similar analysis. Not only are tau parallel and tau perpendicular now equal in value for the L alpha phase, but this value is also found to be nearly four times larger than the longest correlation time for the lecithins. We interpret this as evidence of inter- and intramolecular hydrogen bonding in the L(alpha) phase of sphingomyelin. In the L beta' phase of sphingomyelin, however, although tau(parallel) and tau(perpendicular) remain equal their value is now 32% smaller than that of the lecithins in the same phase. This indicates less difference between the fluidities in the headgroup region of the two phases of sphingomyelin as compared to that of the lecithins. Another significant difference between the L beta' phases is that the tilt angle for sphingomyelin is found to be nearly twice as large as for the lecithins. We argue that these combined observations point to the existence of hydrogen bonding in this phase also. Again in contrast to our previous work on lecithins, our results provide evidence of a negative diamagnetic anisotropy in sphingomyelin molecules, even in the L beta' phase. This is provided for in our model by the assumption that our unoriented samples consist of prolate ellipsoidal liposomes whose major axes are oriented parallel to the static magnetic field. The apparently different diamagnetic behaviour of sphingomyelin in the present work is due to the higher static field used rather than any intrinsic difference in this respect between sphingomyelin and the lecithins DMPC and DPPC.     

Sawtooth faceting in silicon nanowires

We observe in situ the vapor-liquid-solid (VLS) growth of Si nanowires, in UHV-CVD using Au catalyst. The nanowire sidewalls exhibit periodic sawtooth faceting, reflecting an oscillatory growth process. We interpret the facet alternation as resulting from the interplay of the geometry and surface energies of the wire and liquid droplet. Such faceting may be present in any VLS growth system in which there are no stable orientations parallel to the growth direction. The sawtooth structure has important implications for electronic mobility and scattering in nanowire devices.     

Melting of copper and nickel at high pressure: the role of d electrons

Melting curves of Cu and Ni were measured in the laser-heated diamond cell to 97 GPa (3800 K) and 60 GPa (2970 K), respectively. The temperatures of Cu are in good agreement with recent theoretical calculations. The Cu melting slope (dT/dP) is about 2.5 times steeper than for Ni. The present results confirm the key role d-shell electrons play in determining the temperature dependence of high pressure melting curves in transition metals that have filled or partially filled d electron bands.     

Pseudomorphic growth of a single element quasiperiodic ultrathin film on a quasicrystal substrate

An ultrathin film with a periodic interlayer spacing was grown by the deposition of Cu atoms on the fivefold surface of the icosahedral Al70Pd21Mn9 quasicrystal. For coverages from 5 to 25 monolayers, a distinctive quasiperiodic low-energy electron diffraction pattern is observed. Scanning tunneling microscopy images show that the in-plane structure comprises rows having separations of S=4.5+/-0.2 A and L=7.3+/-0.3 A, whose ratio equals tau=1.618... within experimental error. The sequences of such row separations form segments of terms of the Fibonacci sequence, indicative of the formation of a pseudomorphic Cu film.