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41.
This paper extends the analysis of previously reported data, concerning the field evaporation of rhodium atoms that escape as Rh+ and are then post-ionised to Rh2+. A more general formula is derived for obtaining values of critical distance xcr from correlated measurements of temperature, field (F), activation energy (Q), and onset appearance energy. A zkT-type correction is included. If prior to evaporation the surface atom is vibrating in an effectively parabolic potential well, and a Gomer-type escape mechanism operates, then experimental plots of versus and of xcr versus should be linear. Such plots may be used to obtain estimates of zero-Q evaporation field Fe, electrical bonding distance a, and vibrational force-constant k and frequency v. These formulae and methods are applicable to a range of materials and evaporation situations. For rhodium this paper establishes that escape takes place via a Gomer-type escape mechanism, and deduces the following values: Fe = 61 ± 20 V/nm; a = 0.13 ± 0.035 nm; v = (1.4+0.5?0.3) × 1012Hz. 相似文献
42.
Eric G. Forbes 《Historia Mathematica》1978,5(2):167-181
Gauss's interest in astronomy dates from his student-days in Göttingen, and was stimulated by his reading of Franz Xavier von Zach's Monatliche Correspondenz… where he first read about Giuseppe Piazzi's discovery of the minor planet Ceres on 1 January 1801. He quickly produced a theory of orbital motion which enabled that faint star-like object to be rediscovered by von Zach and others after it emerged from the rays of the Sun. Von Zach continued to supply him with the observations of contemporary European astronomers from which he was able to improve his theory to such an extent that he could detect the effects of planetary perturbations in distorting the orbit from an elliptical form. To cope with the complexities which these introduced into the calculations of Ceres and more especially the other minor planet Pallas, discovered by Wilhelm Olbers in 1802, Gauss developed a new and more rigorous numerical approach by making use of his mathematical theory of interpolation and his method of least-squares analysis, which was embodied in his famous Theoria motus of 1809. His laborious researches on the theory of Pallas's motion, in which he enlisted the help of several former students, provided the framework of a new mathematical formulation of the problem whose solution can now be easily effected thanks to modern computational techniques.Up to the time of his appointment as Director of the Göttingen Observatory in 1807, Gauss had little opportunity for engaging himself in practical astronomical work. His first systematic observations were concerned with re-establishing the latitude of of that observatory, which had been well-determined by Tobias Mayer more than fifty years earlier. However, he found a small but not negligible discrepancy between results obtained independently from stellar and solar observations, as well as irregularities among later measurements of polar altitudes (made at the new observatory completed in 1816), which he was never able to explain, despite repeated attempts to do so using different instruments and observational techniques. Similar anomalies were also detected by a number of other astronomers at around this time. These may have been associated--at any rate, partially--with the phenomenon identified later in the century as a “variation of latitude” due to minor periodic fluctuations in the Earth's axis of rotation produced by meteorological and geological factors. 相似文献
43.
K.C. Joshi R.K. Bansal T. Sharma R.D.H. Murray I.T. Forbes A.F. Cameron A. Maltz 《Tetrahedron》1979,35(11):1449-1453
The structure of two new diterpenoids (1 and 3), both of which contain the unique feature of a 7-membered hemiacetal ring B, have been established from chemical and spectroscopic (especially NMR) evidence. Single crystal X-ray crystallographic analysis of 1 has confirmed the structure and allowed the relative stereochemistry to be determined. 相似文献
44.
45.
Forbes MW Volmer DA Francis GJ Böhme DK 《Journal of the American Society for Mass Spectrometry》2005,16(5):779-791
The gas phase stabilities of Group I metal complexes of the polyether ionophore antibiotics lasalocid and monensin were investigated by collision induced dissociation mass spectrometry. Electrospray ionization was used with a triple quadrupole mass spectrometer for the determination of threshold dissociation energies upon application of increasing collision energies. Various data analysis techniques for the determination of dissociation energies are discussed to assess the most suitable method for determining the stabilities of the ionophore-metal complexes studied here. In all cases only the relative stabilities of different complexes may be obtained by the method presented in this study, which does not assess absolute gas phase dissociation energies. Correction factors have been applied, however, to account for the energy conversion during collisions of different metal complexes and the varying degrees of freedom of different sized ligands, allowing for the comparison of the stabilities of different ionophores with like-metals. The measured threshold dissociation energies were compared with respect to the ionic radius of the metal cation, revealing a maximum stability for the K+ complexes of both lasalocid and monensin. A striking decrease in the stabilities of the Rb+ and Cs+ complexes was observed and is believed to be related to a decreasing degree of coordination that the ionophores can accomplish with the larger metals. 相似文献
46.
Irradiation of toluence or n-hexanne solutions of dicyclopentadienyldicarbonyldinickel complexes at λ ? 350 nm gives good yields of bridged dinuclear complexes (RCCR′)(CpNi)2. Their bonding and mass spectra are discussed. 相似文献
47.
48.
Oren P. Anderson Stephen A. Fieldhouse Charles E. Forbes Martyn C.R. Symons 《Journal of organometallic chemistry》1976,110(2):247-255
Exposure of (CO)5MnPbPh3 to 60Co γ-ray at 77 K gave one major paramagnetic species detectable by ESR spectroscopy. This exhibited an anisotropic hyperfine interaction with 55Mn, near free-spin g-values, and a small, almost isotropic coupling to 207Pb. The form of the A(55Mn) and g-tensor components suggest an orbital of dz2 symmetry on manganese for the unpaired electron, but this cannot be directed along the MnPb bond since the 207Pb hyperfine coupling indicates a very low spin-density on lead. We suggest that the centre is formed by electron addition to manganese to give a formal d7 centre, with concomitant loss of one equatorial carbonyl ligand. We defind z as the direction of the lost ligand. A second centre, detected at high gain, having a large hyperfine coupling to 207Pb and a 31 G coupling to 55Mn is tentatively identified as the parent cation.In marked contrast, the molecule (CO)4CoPbPh3 gave a single centre having comparable 59Co hyperfine and g-tensor components, but also a very large hyperfine coupling to 207Pb (ca. 3300 G). Thus, in this case, an electron gain centre (d9) has been formed, the electron being accomodated in the highest MO having a large dz2 component on cobalt (z being now the CoPb direction).Reasons for the adoption of these different structures are discussed. 相似文献
49.
Lammert R.M. Cockerill T.M. Forbes D.V. Coleman J.J. 《Photonics Technology Letters, IEEE》1994,6(10):1167-1169
Selective-area metalorganic chemical vapor deposition is used to fabricate a dual-channel strained-layer InGaAs-GaAs-AlGaAs WDM source with integrated coupler. Threshold currents of 11.5 mA were obtained for 1100 μm long uncoated channels operating cw at room temperature. Both channels can be coupled into a single mode fiber without the need for an external coupler 相似文献
50.