In near-field scanning optical microscopy (NSOM), the measured fluorescence lifetime of a single dye molecule can be shortened or lengthened, sensitively dependent on the relative position between the molecule and aluminum coated fiber tip. The modified lifetimes and other emission characteristics are simulated by solving Maxwell equations with the finite-difference time-domain (FDTD) method. The 2D computation reveals insight into the lifetime behaviors and provides guidance for nonperturbative spectroscopic measurements with NSOM. This new methodology is capable of predicting molecular emission properties in front of a metal/dielectric interface of arbitrary geometry.
The second point demonstrated by the present experiments is that the determination of transitionmetal ordering is not difficult using only powder X-ray diffraction techniques. In the present calculation, the fixed atomic parameters and B factors of V3S 4 were used, because the number of reflections sufficient to measure the accurate intensity was limited to seven, and was thus much less than the number of parameters to be determined. Even so, the reliability of the NOA model can obviously be recognized in the differences in R values (Table 1), and in the match of F o and F c, especially for reflections 002, 101, 202 and 013, the structure factors of which are dominantly contributed to by the difference in the atomic scattering factors allocated to M(1) and M(2). This easy recognition of metal ordering suggests the possibility of investigating the behaviour of metal ordering 'at high temperatures'. This may elucidate the question of whether vacancy ordering and different metal ordering are coupled or are independent, because the in situ X-ray diffraction experiments at high temperature have so far concerned only vacancy ordering (e.g. Nakazawa, Saeki & Nakahira, 1975; Nakazawa, 1979; Wada, 1979a,b). The use of synchrotron-radiation X-rays may be particularly helpful in this context, because of tunability for a given wavelength and the intense flux.
AbstractThe structure of dicobalt octacarbonyl, Co2(CO)8, has been redetermined at low temperature. Cell dimensions at 100 K are: a = 6.503 ( theoretical results predicting metal-metal bonding through the bridging ligand, d-Orbital populations derived from the refinement results agree well with theoretical values. The splitting of the 'eg-type' levels is evident when the refinement is performed in a coordinate system fitted to the local pseudo octahedral symmetry.
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