摘要:

Fluorescence intensity enhancement has been studied for several coverages (10-3to 1 monolayer) of Rhodamine 6G separated from a silver island film by quartz spacer layers of varying thickness. When the dye film is brought into the proximity of the island film support, a first enhancement maximum is observed at ≈ 60 nm spacer thickness. For smaller distances, decrease in the enhancement is followed by a second maximum at 5–10 nm interlayer thickness. The magnitude of the latter maximum exhibits a pronounced dependence on dye coverage. The far-field enhancement maximum is due to an increase in fluorescence quantum yield induced by the presence of the island film substrate, combined with a resonant excitation phenomenon due to collective action of the islands. This effect is analysed in terms of stratified medium theory, in which the island film is represented by a suitable model for the effective dielectric constant. The short distance enhancement results from competition between the strongly amplified local field around the spheroidal metal particle and the radiationless loss of excitation energy by transfer to non-radiating particle surface plasmons.

ISSN:0026-8976    

DOI:10.1080/00268979300102851

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The spin-lattice relaxation of methyl and methylene13C and1H of ethanols CH3CH2OD and CD3CH2OD diluted with CD3CD2OD has been studied by the inversion-recovery method. Recovery of the proton spectral lines is described by a single exponential function with the same rate constant for all transitions. Intramolecular, intergroup and intermolecular contributions to the proton longitudinal relaxation have been determined using the concentration dependence of the relaxation rate of diluted samples. Methylene13C spectral lines are recovered non-exponentially with the different rates. This behaviour is caused by the interference or cross-correlation effects between the dipolar interaction of13C with adjacent protons and the interaction of the magnetic moment of13C with the anisotropic electronic shielding. The solutions of Redfield equations have been used for the interpretation of the time evolution of the13C multiplet spectral lines. The autocorrelation terms of the relaxation matrix have been calculated from contributions to the proton relaxation and the relaxation rate of13C during proton decoupling. Cross-correlation terms and cross-correlation times for methylene13C have been calculated from initial slopes of experimental relaxation curves, and defined more precisely by their theoretical adjustment. The correlation times indicate that the rotation of the ethanol methylene group is anisotropic.

ISSN:0026-8976    

DOI:10.1080/00268979300102861

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

In this paper we describe further developments in our method for studying interaction energies. The main feature of the scheme lies in the localization of the canonical orbitals in both the occupied and the virtual spaces. A modified Boys' procedure, which takes into account the spatial symmetry of the canonical orbitals in the localization, makes it possible to produce well-separated, transferable localized orbitals. Several basis sets are applied, the supersystems of the Ne2, Ar2, Be2and Mg2diatomics are investigated.

ISSN:0026-8976    

DOI:10.1080/00268979300102871

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The expression for the transverse magnetization for the standard four pulse sequence employed in the construction of a double-quantum filter is derived for arbitrary phase angles. By use of this expression, it is demonstrated that the NMR signal intensity of the corresponding double-quantum filter is maximized when the phases and phase cycling of the pulses employed are determined in accordance with the coherence-transfer pathway formalism. Furthermore, the filtering action of the double-quantum filter is insensitive to perturbations in the phase angles of up to 15° around these values. For these variations in phase angles, the transverse magnetization of the double-quantum filter signal decreases by at most 10%. This decrease in signal intensity is due solely to the functional dependence of the transverse magnetization on the phase angles and not due to the breakthrough of other coherences.

ISSN:0026-8976    

DOI:10.1080/00268979300102881

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Monte Carlo simulations are used to investigate the orientational properties of the nematic phase of a liquid crystal model consistent of spherocylinders with a strong longitudinal central dipole moment. The results are contrasted with those for the non-polar case and for real strongly polar liquid crystals. The singlet orientational distribution function is analysed in the light of approximations commonly made to analyse scattering experiments. The validity of various theoretical approximations for the dielectric constant is discussed.

ISSN:0026-8976    

DOI:10.1080/00268979300102891

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The structure of a confined fluid of hard circular cylinders confined to a slit is studied using a simple theory in the context of a restricted orientation model; previous work is extended to higher densities. It is shown that, as the density increases, the fluid within the central region of the slit undergoes phase transitions which depend on the ratio,l/d, of the length to the diameter of the cylinders, and these transitions are interpreted in terms of the bulk behaviour of the fluid. Whenl/dis 0·25, the bulk fluid first undergoes a first-order isotropic-nematic transition as the density increases and then makes a transition to a spatially ordered phase. On the other hand whenl/dis 0·8, the fluid undergoes a transition to a spatially ordered phase and does not pass through a nematic phase at all. These results are qualitatively in agreement with results obtained by Frenkel and Mulder from the simulation of hard ellipsoids. The limitations of the model are discussed and suggestions made for modifications to improve upon the quantitative predictions of the model at high densities.

ISSN:0026-8976    

DOI:10.1080/00268979300102901

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

From the thermodynamic viewpoint the glass transition is a second-order change: there is no latent heat involved, so the entropy varies continuously through the transition. But the transition is not a reversible process. A simulation study of the hard-sphere fluid shows that the statistical entropy, defined in terms of the configuration integral, decreases by 0·20 ± 0·03Nkwhen it forms a glass, which indicates that a system ofNhard spheres can form about exp (0·2N) glasses. For the system ofN= 256 spheres studied there are about 1022distinct glasses.

ISSN:0026-8976    

DOI:10.1080/00268979300102911

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

We report density functional theory calculations of bond-lengths, frequencies and dissociation energies for a number of open-shell diatomic molecules. Our calculations were performed with very accurate quadrature for three distinct forms of the exchange-correlation functional: S-VWN, B-P86-VWN and B-LYP. We find good agreement between experiment and theory for equilibrium bond lengths and harmonic frequencies. The density functional theory methods represent a significant improvement in cost and accuracy over the restricted and unrestricted Møller-Plesset techniques. We compare the relative performance of the different functionals and conclude that the gradient corrected functionals give significantly better frequencies and dissociation energies than the local density approximation (S-VWN). Kohn-Sham density functional theory calculations were performed in both the unrestricted Kohn-Sham and restricted Kohn-Sham formalisms.

ISSN:0026-8976    

DOI:10.1080/00268979300102921

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Trajectory calculations simulating the interaction of a classical plane electromagnetic wave with a highly rotationally excited ethane molecule in a quasi-bound state are reported. The internal motions of the molecule are markedly changed within 4 to 6 × 10-12s of commencing the illumination, with the divergence between the perturbed and unperturbed trajectories being exponential during this period. The energy of the molecule is perturbed by about 10-5cm-1.

ISSN:0026-8976    

DOI:10.1080/00268979300102931

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Spin-lattice relaxation time and self-diffusion coefficient of13CO2and C161H34have been measured on the four isotherms 35, 40, 50 and 60°C from atmospheric pressure up to the critical point of the mixture in both the coexisting vapour and liquid phases. The data have been correlated with phase compositions determined earlier in this laboratory and with calculated viscosities.13CT1data indicate a peculiar behaviour of carbon dioxide in the liquid phase, namely that the reorientational correlation time of the CO2molecule is constant in the liquid phase up to the critical region.1HT1data are in qualitative agreement with theory. The ratio of the self-diffusion coefficients in each phase is closely related to the ratios of the partial molar volumes, in agreement with arguments from statistical mechanics and irreversible thermodynamics. Calculated mutual diffusivities are found to be in excellent agreement with available experimental diffusivities. Unfortunately, because of the scarcity of experimental mutual diffusivities, no conclusions can be drawn at this point concerning the relationship between self and mutual diffusivities for this type of system.

ISSN:0026-8976    

DOI:10.1080/00268979300102941

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor