Showing papers by "David L. Andrews published in 2000"

Orientation factors in three-centre energy pooling

Abstract: At high levels of optical illumination, multichromophore arrays can exhibit a novel form of resonance energy transfer involving the coupling of two donors and one acceptor chromophore. This three-centre energy pooling is the subject of an investigation into its dependence on the mutual positions and orientations of the participating chromophores. Employing results obtained through quantum electrodynamical calculation, comparisons are made between pairwise resonance energy transfer and the novel three-body analogue, with counterparts to the conventional orientation factor identified. The interplay of such factors is analysed in the context of molecular architectures based on linear configurations. The relative dominance of cooperative and accretive mechanisms for energy pooling is addressed and the relative alignments and magnitudes of the donor and acceptor transition moments and polarisabilities are shown to have a profound effect on achievable pooling efficiency.

Rayleigh Scattering and Raman Effect, Theory

Abstract: Rayleigh and Raman scattering are elastic and inelastic forms of light scattering, usually by molecules. From a common ground in electrodynamic theory, the mechanisms are explained and shown to provide the basis for using Raman effect measurements as a spectroscopic tool.

An accretive mechanism for blue-shifted fluorescence in strongly pumped systems:resonance energy transfer with Raman emission

Abstract: Since Kaiser and Garrett's pioneering work on two-photon absorption 40 years ago, observations of blue-shifted fluorescence have been widely used as a marker and measure of non-linear excitation. In doped crystals or in molecular multi-chromophore systems, where resonance energy transfer conveys excitation from the sites of initial photoabsorption to others which yield the fluorescence, this process can play a possible intermediary role in the generation of two-photon fluorescence. Recent work has revealed other competing mechanisms, involving energy transfer between three fluorophore sites, that should be equally or more significant in strongly pumped systems depending on the conditions. It is the purpose of this paper to identify fully the characteristics of one such process involving the coupling of fluorescence energy transfer with Raman emission, and to determine the precise nature of the conditions under which it may be observed.

Quantum-electrodynamical treatment of second-harmonic generation through phase-conjugate six-wave mixing: Temporal analysis

Abstract: It is shown how the effects of molecular reorientation may be incorporated in a fully quantized quantumelectrodynamical treatment of a high-order nonlinear optical effect. Specifically, a general temporal theory is developed to account for the second-harmonic intensity produced through phase-conjugate six-wave mixing. The theory permits elucidation of the intensity of the second-harmonic radiation for arbitrary arrangements of the generating laser beams and molecular geometry. Several models are considered: a one-dimensional model, linear geometry, and a planar geometry. A comparison is made between the results associated with these models and with those obtained from ultrafast experiments on dilute solutions of substituted stilbenes. We find that the off-axial components of the molecular polarizabilities are necessary to properly describe the orientational dynamics of such molecules.

Comment on `Non-equilibrium thermodynamics of light absorption'

Abstract: A recent paper by Meszena and Westerhoff (1999 J. Phys. A: Math. Gen. 32 301) has aimed to address what is referred to as a principal question of biological thermodynamics, the possibility of describing photosynthesis in terms of non-equilibrium thermodynamics. The issue is associated with a misrepresentation of the fundamental photophysics involved, and as a result the analysis is invalid.

Electro-optic response in isotropic media

Abstract: A formal proof has recently been given to show that an electro-optic response of chiral molecules in isotropic media is forbidden by time reversal invariance. A claim to the contrary by Beljonne et al. for the occurrence of such an effect in second order, when dephasing terms are included, is examined critically using a quantum electrodynamics (QED) formulation.

Mechanisms for three-centre energy pooling in lanthanide doped crystals

Abstract: The migration of excitation energy within dielectric crystals doped with lanthanide ions is a rich area for the study of mechanisms involving both conventional pairwise resonance energy transfer and more complex many-body effects. Within the framework of molecular quantum electrodynamics, we present a delineation of systems undergoing simultaneous transfer from two pre-excited donor ions to a single ground-state acceptor, termed three-centre energy pooling. Calculations lead to the identification of two competing mechanisms of transfer co-existing in each system-one cooperative pathway where the excitation is transferred directly to the acceptor from both donors, alongside an accretive mechanism where the excitation energy of one donor is accrued by a second, with net transfer to the acceptor ensuing.

Second harmonic generation and the dynamics of optical excitation in small particles

Abstract: In several studies of second harmonic emission from isotropic suspensions it has been shown that the unusually strong signal detected from some particles, notably purple membrane material, is attributable to optical coherence within the separate particles of the suspension. As such, the emission displays an amalgam of the characteristics associated with full coherence (second harmonic generation) and incoherence (hyper-Rayleigh scattering). The principle of local additivity for the hyperpolarisabilities associated with different optical centres or chromophores within such systems leads to intriguing possibilities for materials strongly pumped by an ultrafast source. The key feature is the relationship between the hyperpolarisabilities of optical centres in their ground and electronic excited states under conditions of resonance with the applied radiation, a result which is derived from a full quantum electrodynamical treatment of the interaction. As a consequence of this relationship, the effective second order susceptibility of each domain or particle proves to be very strongly influenced by the instantaneous degree of optical excitation, and the harmonic signal acquires a temporal signature which faithfully registers the dynamics of optical excitation and decay. Thus, where a significant degree of optical excitation is established in such a system by a primary laser source, studying the characteristics of the second harmonic generated by a probe beam offers the means for direct measurement of the excited state population dynamics. It is shown that the effect is strongly dependent on the nature of the excited state damping, whose correct representation in the expressions for hyperpolarisability is crucial for analysis of the results.

Photoactive arrays at high levels of optical excitation: the effects of chromophore geometry and polarisability

Abstract: At high levels of optical illumination, multichromophore arrays can exhibit a novel form of resonance energy transfer involving the coupling of two excited donor chromophores (A and A') and one neighbouring acceptor chromophore (B) in its electronic ground state This trichromophore effect is the subject of an investigation into the dependence on the mutual positions and orientations of the participating chromophores Employing methods similar to those well-known in bimolecular photophysics, comparisons are made between pairwise resonance energy transfer and the novel three-body analogue, with counterparts to the conventional orientation factor identified By the scrutiny of molecular architectures based on linear configurations (either A-B-A' or A-A'-B), the interplay of the orientation factors is analysed The relative dominance of cooperative and accretive mechanisms over energy pooling rates is comprehensively analysed, only special cases having been discussed in initial studies The relative alignments and magnitudes of the donor and acceptor transition moments and polarisabilities are also shown to have a profound effect on achievable pooling efficiency, with reference to particular alignments and intermolecular vectors The delineation presented should give direction to suitable methods of investigation into the performance and geometric optimisation of such arrays at high levels of illumination