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The pressure-induced amorphization and subsequent recrystallization of SnI4 have been investigated using first principles molecular dynamics calculations together with high-pressure 119Sn nuclear resonant inelastic x-ray scattering measurements. Above ~8 GPa, we observe a transformation from an ambient crystalline phase to an intermediate crystal structure and a subsequent recrystallization into a cubic phase at ~64 GPa. The crystalline-to-amorphous transition was identified on the basis of elastic compatibility criteria. The measured tin vibrational density of states shows large amplitude librations of SnI4 under ambient conditions. Although high pressure structures of SnI4 were thought to be determined by random packing of equal-sized spheres, we detected electron charge transfer in each phase. This charge transfer results in a crystal structure packing determined by larger than expected iodine atoms. [ABSTRACT FROM AUTHOR]

The pressure-induced amorphization and subsequent recrystallization of SnI4 have been investigated using first principles molecular dynamics calculations together with high-pressure 119Sn nuclear resonant inelastic x-ray scattering measurements. Above ~8 GPa, we observe a transformation from an ambient crystalline phase to an intermediate crystal structure and a subsequent recrystallization into a cubic phase at ~64 GPa. The crystalline-to-amorphous transition was identified on the basis of elastic compatibility criteria. The measured tin vibrational density of states shows large amplitude librations of SnI4 under ambient conditions. Although high pressure structures of SnI4 were thought to be determined by random packing of equal-sized spheres, we detected electron charge transfer in each phase. This charge transfer results in a crystal structure packing determined by larger than expected iodine atoms. [ABSTRACT FROM AUTHOR]

High-pressure, variable temperature infrared spectroscopy and first-principles calculations on the methane filled ice structure (MH-III) at high pressures are used to investigate the vibrational dynamics related to pressure induced modifications in hydrogen bonding. Infrared spectroscopy of isotopically dilute solutions of H2O in D2O is employed together with first-principles calculations to characterize proton dynamics with the pressure induced shortening of hydrogen bonds. A Fermi resonance is identified and shown to dominate the infrared spectrum in the pressure region between 10 and 30 GPa. Significant differences in the effects of the Fermi resonance observed between 10 and 300 K arise from the double-well potential energy surface of the hydrogen bond and quantum effects associated with the proton dynamics. [ABSTRACT FROM AUTHOR]

We present a study of the structure of a fully deuterated methane hydrate under the geological conditions found in the world’s oceans. In situ high-resolution neutron diffraction experiments have been performed at temperatures of 220, 275, and 280 K and a pressure of 100 bar, corresponding to the conditions at 1000 m water depth. The data were analyzed with a combination of Rietveld refinement and maximum entropy methods. From the Rietveld refinement, precise atomic parameters of the host lattice could be determined, indicating increasing distortions of the structure of the cages at elevated temperatures and pressures. Debye–Waller factors of the encaged CD4 molecules have been found to exceed the values of the Debye–Waller factors of the D2O molecules considerably. In the large cage of structure type I the thermal center-of-mass displacements of the guests are 5–10 times larger than those of the water molecules. From the maximum entropy analysis maps of the scattering length density have been obtained, showing details of the vibrational amplitudes of the atoms in methane hydrate. The Debye–Waller factors of all molecules have been found to deviate considerably from a simple spherical geometry. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have studied the dynamical properties of doubly and singly occupied structure II N[SUB2] clathrate hydrates by performing a series of classical molecular dynamics calculations. The intermolecular vibrational density of states of the guest molecules show the most explicit changes when going from single to double occupancies. Instead of the narrow peak for single occupancy (at 20 cm[SUP-1]), a broad range of frequencies occurs between 0 and 200 cm[SUP-1] with four modes, which have been assigned to the possible vibrations in the cage. This spectrum provides a unique fingerprint of double occupancy. For the host lattice, the libron band and the optic phonon branch show lower frequencies as compared to single fillings. In contrast, the acoustic phonon branch shifts to lower frequencies for a single filling of the large cases. We find that there is a large vibrational guest-host coupling for both single and double occupancies, although the frequency range is broader in the latter case. There is a significant translational diffusion of the N[SUB2] molecules in the doubly occupied cages. Occasionally both molecules simultaneously migrate from one local energy minimum to another. An orientational order of the N[SUB2] molecules occurs only within the small cages. However, the relative orientation of the two molecules in the doubly occupied cages is preserved in time. [ABSTRACT FROM AUTHOR]

We have performed molecular dynamics calculations to obtain a number of properties of a doubly occupied structure II N[sub 2] clathrate hydrate, in particular to study its behavior under higher pressures. The calculated neutron diffraction pattern is in agreement with the experimental result. The effect of the presence of the filling of the small cages and of the large cages (in either single or double occupancy) on the calculated pattern is demonstrated and discussed. The calculated Raman spectra show that the average vibrational frequency of the N[sub 2] molecules decreases in going from the singly occupied small cages to the doubly occupied large cages and then to the singly occupied large cages, respectively. The frequency distributions are explained in terms of radial distribution functions. When applying large pressures at low temperatures, a clathrate-amorphous transition occurs for a partially doubly occupied clathrate. The transition occurs at about the same pressure as for single occupations, but the densification is larger for the latter. In both cases, the transition is reversible. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ice I transforms to a high-density amorphous phase when pressed to 10 kbar at 77 K. Similar transformations in structure I and structure II clathrate hydrates have been studied by pressing samples in a piston-cylinder apparatus and by molecular dynamics simulations. The simulations were also carried out on structure I and II empty lattices. The hydrates and the empty lattices were found to transform to high-density phases under pressure at 77 K. The high-density phases of the empty lattices could be recovered at zero pressure, as is possible in the case of high-density amorphous phase of ice. However, it was not possible to recover the high-density phases of the hydrates at zero pressure. Instead, they reverted back to their original crystalline structures when the pressure was released. The molecular dynamics results suggest that under pressure the water molecules in the hydrates collapse around the guest molecules, and the repulsive forces between the guest and the water molecules are mainly responsible for the reversible transition to the original structure when the pressure is released. [ABSTRACT FROM AUTHOR]

Using monochromatized synchrotron radiation, the gas phase photoelectron spectra of the Hg 5d and valence levels of Hg(CH3)2 have been obtained between 21 and 100 eV photon energies. Experimental valence band branching ratios have been compared with theoretical branching ratios from MS-Xα calculations. Our results strongly suggest that the ordering of valence levels is 2a‘2<3a1<2e’<2e‘, rather than 2a‘2<3a1<2e‘<2e’ from previous theoretical calculations. For the 3a’1 orbital, two shape resonances are predicted at 30 and 50 eV kinetic energy. Only the one at 30 eV is observed. Interchannel coupling with Hg 5d photoemission is important for the outermost 2a‘2 and 3a’1 orbitals. Shape resonances and interchannel coupling strongly suggest that He ii:He i cross section ratios cannot generally be used to infer orbital character. The Hg 5d intensity ratio in Hg(CH3)2 differs from that of atomic Hg in the low photon energy region. This effect is probably due to the differences in the outgoing f wave potential between Hg and Hg(CH3)2. [ABSTRACT FROM AUTHOR]

Using monochromatized synchrotron radiation, gas phase photoelectron spectra of CF3I have been obtained between 21 and 100 eV photon energies. Experimental valence band branching ratios have been compared with theoretical branching ratios from MS-Xα calculations. The generally good agreement between experiment and theory confirms the orbital assignment: 4e<4a1<1a2∼3e<2e<3a1<1e∼2a1 <1a1, in order of increasing binding energy. In contrast to CF4, MS-Xα calculations predict two shape resonances for CF3I at 15.2 eV (e channel) and 17.2 eV (a1 channel). Experimentally, weak shape resonances are observed on e orbitals at ∼14 eV kinetic energy. The cross sections are very useful for correlating orbitals in CF3I with those of CF4. For example, the cross sections suggest that the 3e orbital in CF3I correlates with the 1t1 orbital rather than the 4t2 orbital in CF4. After the onset of the I 4d level at ∼57 eV, intershell coupling becomes important in enhancing valence band levels having high I 5p character such as the 4e orbital. The I 4d branching ratio from 66 to 110 eV is similar to the Xe 4d branching ratio in atomic Xe, indicating atomic-like behavior for the I 4d subshell. [ABSTRACT FROM AUTHOR]

Presents a study that examined several porous silicon, siloxene, heat-treated siloxene and layered polysilane samples. Use of K- and L-edge x-ray photoabsoption, photoemission and powder x-ray diffraction; X-ray absorption pattern of layered polysilane; Comparison between silicon K-edge absorption spectra for electrochemical and chemical porous-silicon with crystalline silicon.

Using monochromatized synchrotron radiation, the gas phase photoelectron spectra of the first five valence orbitals of XeF2 have been obtained from 21–50 eV photon energy. From these spectra, the branching ratios and partial cross sections have been calculated. Theoretical branching ratios and partial cross sections have been obtained from MS-Xα calculations and compared with experiment. The generally good agreement between experiment and theory confirms the DV-Xα and MS-Xα orbital assignments: 5πu <10σg <3πg <4πu <6σu, in order of increasing binding energy. Somewhat surprisingly because of the lack of antibonding virtual orbitals above threshold and the long Xe–F bond length (2.00 Å), a number of strong resonances are predicted and observed. For example, resonances are predicted at 4.8 (πg channel), 11.6 (πu, δu channels), and 14.3 eV (σu channel) kinetic energies. The observed resonance positions are usually in reasonable agreement with these. The nature of these resonances are discussed, and compared to similar features in O2, F2, and CO2. [ABSTRACT FROM AUTHOR]

Using monochromatized synchrotron radiation and angle integrated photoelectron spectroscopy, the Si 2p, Si LVV Auger, and F 2s cross sections have been obtained for SiF4 molecules in the gas phase between 116 and 150 eV photon energies. These cross sections and the Si 2p β value have also been calculated using the MS-Xα method. There is qualitative agreement between theory and experiment. Together with recent experimental and theoretical Si 2p cross sections from SiCl4 and valence band cross sections on SiF4, the two major resonances at 5 and 22 eV kinetic energies are assigned as t2 resonances. The valence band branching ratios are not affected by interchannel coupling above the Si 2p threshold. [ABSTRACT FROM AUTHOR]

Using monochromatized synchrotron radiation, the gas phase photoelectron spectra of the five valence orbitals of CF4 and SiF4 have been obtained from 21 to 100 eV photon energies. Branching ratios (BR) for both molecules, and partial cross sections (σ) for CF4, have been obtained from these spectra. The agreement between our σ values for CF4 and previous results is excellent. Theoretical branching ratios, partial cross sections, and β values have been obtained from MS Xα calculations, and compared with experiment. The generally good agreement between experiment and theory for SiF4 confirms the orbital assignment: 1t1<5t2<1e<4t2<5a1, in order of increasing binding energy. In contrast to the CF4 results which show little structure, five shape resonances are predicted for SiF4, at 3, 7, 13, 23, and 35 eV kinetic energies. The predicted resonance positions and intensities are in good agreement with experiment (e.g., for the 23 and 35 eV resonances), but the low energy resonance positions and intensities are usually not well reproduced experimentally. The radically different resonance features in CF4 and SiF4 are discussed. [ABSTRACT FROM AUTHOR]