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Tsang, T.

Publications and source records attributed to Tsang, T..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

Trace element partitioning between ionic crystal and liquid

The partitioning of trace elements between ionic crystals and the melt has been correlated with lattice energy of the host. The solid-liquid partition coefficient has been expressed in terms of the difference in relative ionic radius of the trace element and the homogeneous and heterogeneous strain of the host lattice. Predictions based on this model appear to be in general agreement with data for alkali nitrates and for rare-earth elements in natural garnet phenocrysts.

Tsang, T.↗
NASA NTRS1977

Velocity autocorrelation functions of Lennard-Jones fluids

By a self-consistent procedure, the velocity autocorrelation functions of both liquid and gaseous argon have been calculated without introducing any arbitrary parameters. The results are in satisfactory agreement with computer experiments. The correlation functions are primarily determined by the nearest-neighbor coordination. Because of the strong hard-core repulsion, the behaviors are more vibratory and damp out quickly at high densities and are more diffusive at lower densities.

Tsang, T.↗
NASA NTRS1976

Proton magnetic relaxation and internal rotations in tetramethylammonium cadmium chloride

Nuclear magnetic resonance (NMR) and relaxation studies of the proton spin-lattice relaxation time (PSLRT) and proton second moment (PSM) are reported. Tetramethylammonium cadmium chloride (TMCC) was selected as a diamagnetic member of the isomorphic series, and hence proton data relate directly to the motion of the tetramethylammonium ion in the absence of paramagnetic ions. In the model adopted, the correlation time for hindered motion of one of the methyl groups differs from that of the other three groups in the low-temperature phase below 104 K. PSLRT and PSM values agree closely with experimental data with this model. Crystallographic phase transitions in TMCC occur at 104 K and 119 K according to the PSLRT measurements. Dipolar interactions between adjacent protons account for the PSLR rates below 104 K.

Tsang, T.↗
NASA NTRS1976

Widths of atomic 4s and 4p vacancy states, Z between 46 and 50

X-ray photoelectron and Auger spectra involving N sub 1, N sub 2, and N sub 3 vacancy states of Pd, Ag, Cd, In, and Sn were measured and compared with results of free-atom calculations. As previously observed in Cu and Zn Auger spectra that involve 3d-band electrons, free-atom characteristics are found, with regard to widths and structure, in the Ag and Cd M sub 4-N sub 4,5 N sub 4,5 and M sub 5-N sub 4,5 N sub 4,5 Auger spectra that arise from transitions of 4d-band electrons. Theoretical N sub 1 widths computed with calculated free-atom Auger energies agree well with measurements. Theory, however, predicts wider N sub 2 than N sub 3 vacancy states (as observed for Xe), while the measured N sub 2 and N sub 3 widths are nearly equal to each other and to the average of the calculated N sub 2 and N sub 3 widths. The calculations are made difficult by the exceedingly short lifetime of some 4 p vacancies and by the extreme sensitivity of super-Coster-Kronig rates, which dominate the deexcitation to the transition energy and to the fine details of the atomic potential.

Chen, M. H.↗
NASA NTRS1976

On the ion-bombardment reduction mechanism

Recent laboratory studies of solar-wind reduction mechanisms on the lunar surface are discussed, emphasizing the effects of the electronic configurations of transition metals and the effects of covalent bonding. A series of experiments is described which involved argon ion bombardment of extremely thin targets consisting of simple halides and cyanides of first-row transition metals. Experimental results are summarized which imply that the cyanides generally have much higher reduction efficiencies than the halides. It is suggested that although reduction and sputtering take place concurrently under ion bombardment, they may actually be two independent processes.

Yin, L.↗
NASA NTRS1975

Widths of atomic 4s and 4p vacancy states, 46 less than or equal to Z less than or equal to 50

Auger and X-ray photoelectron spectra involving N1, N2, and N3 vacancy states of Pd, Ag, Cd, In, and Sn were measured and compared with results of free atom calculations. As previously observed in Cu and Zn Auger spectra that involve 3d-band electrons, free-atom characteristics with regard to widths and structure were found in the Ag and Cd M4-N4,5N4,5 and M5-N4,5N4,5 Auger spectra that arise from transitions of 4d-band electrons. Theoretical N1 widths computed with calculated free-atom Auger energies agree well with measurements. Theory however predicts wider N2 than N3 vacancy states (as observed for Xe), while the measured N2 and N3 widths are nearly equal to each other and to the average of the calculated N2 and N3 widths. The calculations are made difficult by the exceedingly short lifetime of some 4p vacancies and by the extreme sensitivity of super-Coster-Kronig rates, which dominate the deexcitation, to the transition energy and to the fine details of the atomic potential.

Hsiungchen, M.↗
NASA NTRS1975

Velocity autocorrelation functions in model liquid metals

Starting from interatomic potentials and static radial distribution functions, a self-consistent iteration scheme has been used to calculate velocity autocorrelation functions in liquid metals. The interatomic forces are treated directly. The calculation bypasses the details of the many-body dynamics and it is not necessary to introduce any additional parameters. Several simplifications may be used without introducing appreciable deviations. The results are in good agreement with computer experiments on liquid sodium at 383 K, suggesting that the velocity autocorrelation function may be a simpler quantity than previously supposed.

Tsang, T.↗
NASA NTRS1975

Electron spectroscopic studies related to solar-wind darkening of the lunar surface

Improved instrumentation in the X-ray photoelectron spectrometer allowed the first direct observation of the change of valence state of iron in the silicate Fayalite under helium-ion bombardment. From the observed ion dosage necessary to reduce the iron in the topmost atomic layers to the metallic state it is estimated that similar reduction on the lunar surface under solar-wind proton bombardment would occur between 700 to 1000 years. Within such a time scale, it seems likely that all surface iron in the iron-containing lunar fines should have been reduced to the metallic state. Implications of these experimental observations in terms of various proposed darkening mechanisms are discussed.

Yin, L. I.↗
NASA NTRS1975

ESCA studies on solar-wind reduction mechanisms

In a continuing study of solar-wind darkening on the lunar surface, ESCA techniques have been used to examine surface reduction and optical darkening by ion bombardment. Experimental results on transition metal fluorides have shown that ion-bombardment reduction is (a) accompanied by the loss of anion species, (b) relatively independent of the incident ion mass, and (c) not simply related to physical sputtering itself. The reduction mechanism may be more chemical in nature and the rate may be determined primarily by the transition metal ion

Yin, L.↗
NASA NTRS1972

L-S coupling interpretation of high-resolution LMM Auger spectra of Cu and Zn.

High-resolution LMM Auger spectra of Cu and Zn obtained by X-ray excitation are shown to contain fine structures which have not been reported previously. An attempt has been made to classify these fine structures in terms of pure L-S coupling of the final vacancies of the atom. The agreement between the calculated and the observed energy separations of the fine-structure lines is good within the accuracy of the Hartree-Fock approximation.

Yin, L.↗
NASA NTRS1972

Investigation of low-Z Coster-Kronig transitions by means of Auger and photoelectron spectroscopy

Experimental intensity ratios of Auger transitions for Co, Ni, Cu, and Zn as well as the relative L sub 2 and L sub 3 level widths of Cu and Zn, derived from their photoelectron spectra, are presented. Evidence is presented that a great deal of vacancy reorganization took place following photoionization and prior to Auger emission. These reorganizations are assumed to be due to Coster-Kronig transitions f sub 23. These results are compared with theoretical calculations and agree with predicted discontinuity at Z = 30 where f sub 23 transitions become energetically impossible.

Yin, L. I.↗
NASA NTRS1972

Coupled nuclear spin relaxation and internal rotations in magnesium fluosilicate hexahydrate.

Both proton and fluorine nuclear spin-lattice relaxations have been studied by the 180- to 90-deg pulse method in magnesium fluosilicate hexahydrate at 25 and 13 MHz over the temperature range from 170 to 350 K. Observed nonexponential behavior of the nuclear magnetic relaxation is explained by internal rotations of the doubly charged negative fluosilicate ions and doubly charged positive magnesium hexahydrate ions.

Utton, D. B.↗
NASA NTRS1972

Nuclear magnetic resonance of Al-27 in topaz, Al2SiO4/F, OH/2.

The Al-27 nuclear quadrupolar coupling constant and asymmetry parameter (eta) in topaz have been determined to be 1.67 (plus or minus 0.03) MHz and 0.38 plus or minus 0.05, respectively. These values and the orientations of the principal axes are consistent with the Fe(3+) paramagnetic resonance data and with the symmetry of the AlO4F2 octahedron.

Tsang, T.↗