Spectra Of Bulk Research Articles

Extended-x-ray-absorption-fine-structure studies of low-Zatoms in solids and on surfaces: Studies ofSi3N4, SiO2, and oxygen on Si(111)

Extended-x-ray-absorption-fine-structure (EXAFS) studies above the nitrogen (\ensuremath{\sim}400 eV) and oxygen (\ensuremath{\sim}535 eV) $K$ absorption edges are reported for silicon nitride, silicon dioxide, and oxygen on a Si (111) surface. Measurements were carried out using soft-x-ray synchrotron radiation and employing the surface-sensitive secondary-electron-yield detection technique. EXAFS spectra of bulk ${\mathrm{Si}}_{3}$${\mathrm{N}}_{4}$ and Si${\mathrm{O}}_{2}$ are analyzed to test the reliability of theoretical phase shifts and to derive experimental ones for the N-Si and O-Si systems. It is found that nearest-neighbor distances from low-$Z$ atoms can be determined to an accuracy of \ensuremath{\lesssim}0.03 \AA{} and second-nearest-neighbor separations to \ensuremath{\lesssim}0.05 \AA{} using calculated phase shifts. The surface EXAFS spectrum of one of the initial oxidation stages (characterized by a Si $2p$ chemical shift of 2.5 eV) reveals that the O-Si bond length is slightly (0.04 \AA{}) larger than in Si${\mathrm{O}}_{2}$. Analysis of the relative EXAFS amplitudes provides information on the oxygen-bonding geometry on the Si (111) surface. The importance and advantage of polarization-dependent surface EXAFS studies are discussed.

Raman and infrared spectra of vitreousAs2O3

We report the polarized reduced Raman spectra, the infrared reflectivity, and dielectric constants of bulk ${\mathrm{As}}_{2}$${\mathrm{O}}_{3}$ glass for vibrational frequencies up to 1500 ${\mathrm{cm}}^{\ensuremath{-}1}$. Longitudinal and transverse optical modes are identified in the high-frequency region, as are some features of the second-order response. The atomic motions associated with several lines are inferred from an analogy with the spectra of the tetrahedral glass Ge${\mathrm{O}}_{2}$. Our assignments bring into question the force constants previously deduced by Papatheodorou and Solin. The spectra of bulk ${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$ glass and ${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$ glass are also given a parallel interpretation.

Mössbauer studies of the morin transition in bulk and microcrystalline α-Fe 2O 3†

Mössbauer spectra of bulk and microcrystalline hematite have been analyzed at the Morin transition for both the weak-ferromagnetic (WF) and anti-ferromagnetic (AF) phases simultaneously, increasing precision in the extracted Mössbauer parameters. The Morin transition in bulk took place in less than 0.4 K, and in the microcrystals decreased linearily with expanding lattice in agreement with its increase under external pressure. Temperature hysteresis measurements in the microcrystals gave values of the fourth-order magnetic anisotropy energy which indicated possible surface spin-pinning effects. In bulk the magnetic field decreased by 7 ± 0.5 kOe in going from the WF to the AF phase on decreasing temperature, and the isomer shift decreased by 0.014 ± 0.003 mm sec. With lattice dilation the AF quadrupolar interaction appeared to decrease, while the isomer shift change across the transition goes positive. In contrast the change in magnetic field is not simply related to this lattice dilation, indicating surface spin-pinning effects.

Transmission Raman and depolarization spectra of bulk a-Se from 13 to 300 cm −1

The Raman and depolarization spectra of bulk a-Se have been measured at 11.4 K. The 112 cm -1 mode is unambiguously identified from both its position and polarization as an Se 8 ring mode. The low frequency Raman coupling coefficient is found to be constant in the range 13<ω<35 cm −1 An upper bound of 8 Å is found for the correlation range in a-Se.

Investigation of the effect of 60Co-γ radiation on single and polycrystalline polyethylene by broad line N.M.R.

The broad line N.M.R. spectra of irradiated and unirradiated high density bulk (HD) and single crystals (SC) of polyethylene were found to give rise to two components, e.g. broad and narrow lines, attributed to crystalline and amorphous regions, respectively. An increase in the line width of the narrow component with radiation dose is more marked in bulk HD specimens, and is ascribed to crosslinking in the radiation-sensitive amorphous region; whereas no such increase is apparent for the SC specimen. Though there seems to be a slight increase in the second moment of the total absorption curve with radiation up to about 200 Mrad, both the second moment and the line width of the broad component was found to decrease with radiation dose for both the samples. This indicates an increased chain mobility in the crystalline regions and is considered to be due to the destruction of regularity in the crystal lattices by the combined processes of crosslinking (or endlinking) in the amorphous regions and the formation of lattice defects and imperfections by the ionizing radiation. A relatively faster decrease of line width with radiation dose above about 1000 Mrad may be attributed to the predominance of the latter process at high doses.

Some Magnetic Properties of Magnetite (Fe3O4) Microcrystals

Single-domain particles of Fe3O4 have been observed to exhibit superparamagnetic behavior with the Mössbauer effect. Using a simple theory for the spin relaxation, it was possible to make an approximate curve fit of the spectra, taking into account the particle-size distribution of the samples. For a sample prepared by a grinding method, the values obtained by the fitting procedure were Ku=(6.0±1.0)×104 erg/cm3 for the energy barrier and τ0=(9.5±1.5) × 10−10 sec for the frequency factor. The value of Ku was interpreted as being due to shape anisotropy; analysis of the hyperfine spectra of both bulk and fine-particle Fe3O4 at T=4.2°K supported this view. The flipping frequencies of the particle moments were altered by the application of a magnetic field, so that the average internal field in a relaxed spectrum could be determined. Other samples of Fe3O4, prepared by a precipitation method, gave the values Ku=6.0×104 and 8.0×104 erg/cm3. The latter value could be partially accounted for by the existence of a small oxygen excess in those particles prepared at temperatures close to 100°C.

Infra-red Emission Spectra of Minerals

THIS study was undertaken to explore the feasibility of measuring infra-red emission spectra of bulk, opaque minerals. As this involved the examination of the rough, irregular surfaces of bulk specimens at temperatures close to room temperature, emission methods using conventional spectrometers1 were not applicable. The emittance of specimens near room temperature is too low to operate dispersion spectrometers.