Alfred Prock

Lifetime of an emitting molecule near a partially reflecting surface

A classical treatment of energy transfer in metal insulator systems is presented. The approach involves the calculation of the radiation field of an emitting molecule near a partially reflecting surface. The modification of the image theory result produces a large correction when the molecule is near the surface. This in turn produces a correction to the calculated lifetime of the molecule as a function of distance from the surface that differs substantially from previous theoretical descriptions of this system and brings the theory into good agreement with experiment.

Comments on the classical theory of energy transfer

Energy transfer from an emitting molecule to an absorbing half−space is considered from the viewpoint of electromagnetic theory. The lifetime of a dipole emitter in the presence of a mirror is determined through a calculation of the complex Poynting vector in the dielectric surrounding the dipole. This method has the advantage over previous approaches to this problem in that the radiative and nonradiative components of the lifetime expression may be rigorously separated. The influence on emitter lifetime of a mirror of finite thickness is also described. A simple expression is derived describing the energy transfer rate in these layered systems. It is shown that nonradiative energy transfer results from coupling of the near field of the dipole to the surface plasmon modes in the metallic absorber. The Forster energy transfer rate law is discussed in the context of the present theory.

Fluorescence and energy transfer near interfaces: The complete and quantitative description of the Eu+3/mirror systems

The classical electromagnetic description of fluorescent emission and energy transfer in the Eu+3/mirror systems is shown to be in quantitative agreement with the results of eight experimental systems studied by the fatty acid monolayer assembly technique. The emitter lifetime measured as a function of distance from the mirror(s) is found to be consistent with an isotropic spatial orientation for the emitter in all cases; furthermore, it is shown to be exclusively so in most of these cases. Both the quantum yield and the radiative lifetime of the luminescent state of the Eu+3 ion are determined by theoretical fits to the data. Whereas the quantum yield spans a range of 0.69–0.86, the radiative lifetime of the electric dipole transition at 612 nm is nearly constant, as required, with a value of 803±29 μsec. Both the quantitative agreement between theory and experiment and the consistency among the eight experimental systems in predicting the radiative lifetime provide a clear demonstration of the utility o...

Comments on the classical theory of energy transfer. II. Extension to higher multipoles and anisotropic media

The classical electromagnetic theory of the fluorescence emission and energy transfer in layered systems is extended to describe magnetic dipole and electric quadrupole radiation and anisotropic media. A general formulation is developed for energy transfer from various emitter types and orientations to isotropic acceptors. The description is exact within the classical framework and requires none of the usual assumptions as to the nature of the acceptor layer. The theory is further extended to describe one‐ and two‐dimensional acceptors and electric dipole radiation in an anisotropic medium. The latter case is the actual situation in the fatty‐acid layer experiments. The coupling to the surface plasmon modes of the acceptor is discussed as it relates to the various systems described here.

Decay of an emitting dipole between two parallel mirrors

A classical treatment is presented for the modification of the emission rate of an atom between two parallel mirrors. For the case of two perfectly reflecting mirrors, the results are identical with previous treatments of this problem using a quantum mechanical approach. The purely classical treatment has the advantage of being able to treat nonperfectly reflecting mirrors.

Triplet diffusion and energy transfer in molten naphthalene

The diffusion length of triplet excitation in molten naphthalene near the melting point has been determined to be 10±2×10−3 cm. From this length and the measured lifetime, 0.140±0.06 sec, in the molten state the diffusion coefficient has been calculated to lie between 0.3×10−3 and 1.8×10−3 cm2 sec−1. This value is greater by more than an order of magnitude when compared to ’’molecular’’ diffusion coefficients. It is therefore taken as evidence for the existence of triplet excitons in molten naphthalene.

Luminescent lifetimes near multiple interfaces: A quantitative comparison of theory and experiment

Abstract Results are presented for the classical theory of an emitting dipole located between two parallel mirrors. The calculated lifetime variations are in quantitative agreement with experiment for the silver/dielectric/Eu3+/air system.

Photoacoustic study of a UV initiated solid‐state polymerization

We report a detailed photoacoustic study of a highly exothermic solid‐state reaction, the photopolymerization of diacetylene crystals. Both the amplitude and the phase shift of the photoacoustic signal are monitored during the course of the polymerization at various UV excitation wavelengths and various chopping frequencies. A one‐dimensional heat‐diffusion model employing impedance boundary conditions is shown to fit the experimental data quite adequately. We conclude from the theoretical interpretation of the results that the quantum yield for photopolymerization is very high (∼100) and that the polymer acoustically couples much more effectively to the gas than does the monomer.

Forced diffusion model for unipolar injection in low conductivity media

Abstract A unipolar charge injection model based on the Poisson equation and the one-dimensional forced diffusion equation is presented. Its application to low dielectric constant fluids provides a description of conductivity in such media. The first integral leads to the Ricatti equation which describes the electric field distribution. In the presence of image forces the boundaries of this equation must be displaced from the physical electrodes. The two boundary conditions of this system are stated in terms of injection and discharge conditions. Employment of the image potential gives rise to an expression for an injection law (relation between field and current) and its applicability is discussed along with the analytical solutions that are obtained for field profile, charge density, current and voltage. Detachment of ions bound to the electrode by image forces, whose description enters the injection law, is investigated for finite current flow in the system. The role of dielectric constant of the electrode on the shape of current—volgate curves is discussed.

Extrinsic photoconduction with reduced image forces

Unipolar charge injection from an electrode/fluid interface into a medium of nearly the same dielectric constant as the electrode is studied. For both photo‐ and dark injection, electrical image forces no longer serve as an impediment to charge detachment. The system investigated is mesotetraphenylporphin (excited in the vis‐near UV) in fluorobenzene solution against tin oxide electrodes. Despite polarity of the medium, photocurrent is extrinsic, with both dark and photocurrent injection occurring at the positive electrode. Both show saturation with applied field in accordance with a model based on forced diffusion theory. In addition, interfacial charge generation and recombination rates are extractable from the data. This method shows promise as a way to study these as a function of excited state and dielectric properties of the system.