Abstract: A quantitative HPLC method was applied to determine the amounts of C-60 and C-70 present in extracts of soot produced in the electric arc reactor and in flames. The combustion method was found to yield a higher C-70/C-60 ratio (0.67) compared with the evaporation experiment where the C-70/C-60 ratio amounts to 0.27.

Abstract: Laser microprobe mass spectrometry (LMMS) aims at the identification of local organic and inorganic constituents at the surface of solids, The low mass resolution capabilities of the initially used time-of-flight (TOF) mass spectrometers have often proved to be insufficient for identification, Therefore, high mass resolution Fourier transform (ET) LMMS was developed, Neo-ceramic powders with oxide or carbide coatings were analyzed by both FT LMMS and TOF LMMS, The data are useful to compare the analytical information gained from both methods, Analytical results of these samples by electron microprobe x-ray analysis (EPXMA) and secondary ion mass spectrometry (SIMS) are discussed to assess the place of FT LMMS and TOF LMMS in the spectrum of microanalytical techniques.

Abstract: We present a theoretical study of the electronic structure of a heavily Si delta-doped layer in a GaAs/AlxGa1-xAs/GaAs quantum barrier. In this class of structures the effect of DX centers on the electronic properties can be tuned by changing the AlxGa1-xAs barrier width and/or the Al concentration, which leads to a lowering of the DX level with respect to the Fermi energy without disturbing the wave functions much. A self-consistent approach is developed in which the effective confinement potential and the Fermi energy of the system, the energies, the wave functions, and the electron densities of the discrete subbands have been obtained as a function of both the material parameters of the samples and the experimental conditions. The effect of DX centers on such structures at nonzero temperature and under an external pressure is investigated for three different models: (1) the DX(nc)(0) model with no correlation effects, (2) the d(+)/DX(0) model, and (3) the d(+)/DX(-) model with inclusion of correlation effects. In the actual calculation, influences of the background accepters, the discontinuity of the effective mass of the electrons at the interfaces of the different materials, band nonparabolicity, and the exchange-correlation energy of the electrons have been taken into account. We have found that (1) introducing a quantum barrier into delta-doped GaAs makes it possible to control the energy gaps between different electronic; subbands; (2) the electron wave functions are mon spread out when the repellent effect of the barriers is increased as compared to those in delta-doped GaAs; (3) increasing the quantum-barrier height and/or the application of hydrostatic pressure are helpful to experimentally observe the effect of the DX centers through a decrease of the total free-electron density; and (4) the correlation effects of the charged impurities are important for the systems under study.

Abstract: Starting from a model of microscopic interactions between C-70 molecules, we have developed a theory which describes the orientational dynamics and its coupling to lattice displacements in the rhombohedral phase of C-70 fullerite. The Landau free energy is calculated. We obtain a first-order phase transition to a monoclinic structure with the space group P2(1)/m. The transition is driven by the condensation of orientational quadrupoles at the F point of the Brillouin zone of the rhombohedral lattice. We find no evidence that the monoclinic structure is connected with the freezing in of orientations around the fivefold molecular axis. We calculate the lattice strains that are associated with the transition to the monoclinic structure. The theory is compared with a range of experimental data on the phase transition.