Joel D. Eaves

Theory of coherent resonance energy transfer

A theory of coherent resonance energy transfer is developed combining the polaron transformation and a time-local quantum master equation formulation, which is valid for arbitrary spectral densities including common modes. The theory contains inhomogeneous terms accounting for nonequilibrium initial preparation effects and elucidates how quantum coherence and nonequilibrium effects manifest themselves in the coherent energy transfer dynamics beyond the weak resonance coupling limit of the Forster and Dexter (FD) theory. Numerical tests show that quantum coherence can cause significant changes in steady state donor/acceptor populations from those predicted by the FD theory and illustrate delicate cooperation of nonequilibrium and quantum coherence effects on the transient population dynamics.

Isotropic and anisotropic Raman scattering from molecular liquids measured by spatially masked optical Kerr effect spectroscopy

Spatially masked optical Kerr effect (SM-OKE) spectroscopy is a nonresonant femtosecond pump–probe technique capable of measuring isotropic contributions to the transient birefringence of molecular liquids. In conjunction with traditional optical-heterodyne-detected optical Kerr effect spectroscopy, polarization-selective SM-OKE measurements are used to experimentally measure the anisotropic and isotropic third-order nonlinear response of CS2, acetonitrile, methanol, and water. These two responses, which allow the intermolecular dynamics to be separated by symmetry, form a complete and independent basis for describing the polarization dependence of nonresonant third-order experiments. The Fourier transform spectral densities of these responses are presented for each liquid and are interpreted in terms of the molecular and interaction-induced contributions to the many-body polarizability. The molecular contributions are suppressed in the isotropic response for all liquids, while the line shape in the inter...

Polarization-selective femtosecond Raman spectroscopy of low-frequency motions in hydrated protein films

Abstract We investigated the vibrational dynamics of proteins in amorphous hydrated films of lysozyme and myoglobin using polarization-selective time-domain Raman spectroscopy. The anisotropic spectra for these proteins all have a broad peak due to librational motion of side chains at 90 cm −1 and a background that may arise from bound water. The isotropic spectrum of lysozyme is similar to that of myoglobin, and has peaks at 240 and 500 cm −1 that are likely due to secondary structure fluctuations. These results suggest that low-frequency deformations of the protein molecule may contribute to the solvation dynamics of proteins in aqueous solution.

Spatial dimension and the dynamics of supercooled liquids

Inspired by recent theories that apply ideas from critical phenomena to the glass transition, we have simulated an atomistic model of a supercooled liquid in three and four spatial dimensions. At the appropriate temperatures and density, dynamic density correlation functions in three and four spatial dimensions correspond nearly exactly. Dynamic heterogeneity, quantified through the breakdown of the Stokes–Einstein relationship, is weaker in four dimensions than in three. We discuss this in the context of recent theories for dynamical heterogeneity. Because dimensionality is a crucially important variable, our work adds a stringent test for emerging theories of glassy dynamics.

The Tunable Hydrophobic Effect on Electrically Doped Graphene

Using molecular dynamics simulations, we study the hydrophobic effect on electrically doped single layer graphene. With doping levels measured in volts, large changes in contact angle occur for modest voltages applied to the sheet. The effect can be understood as a renormalization of the surface tension between graphene and water in the presence of an electric field generated by the dopant charge, an entirely collective effect termed electrowetting. Because the electronic density of states scales linearly in the vicinity of the Fermi energy, the cosine of the contact angle scales quartically with the applied voltage rather than quadratically, as it would for a two-dimensional metal or in multiple layer graphene. While electrowetting explains the phenomenon, it does not account for the slight asymmetry observed in the hydrophobic response between n- and p-doping.

Observation of trapped-hole diffusion on the surfaces of CdS nanorods

In CdS nanocrystals, photoexcited holes rapidly become trapped at the particle surface. The dynamics of these trapped holes have profound consequences for the photophysics and photochemistry of these materials. Using a combination of transient absorption spectroscopy and theoretical modelling, we demonstrate that trapped holes in CdS nanorods are mobile and execute a random walk at room temperature. In CdS nanorods of non-uniform width, we observe the recombination of spatially separated electrons and trapped holes, which exhibits a t-1/2 power-law decay at long times. A one-dimensional diffusion-annihilation model describes the time-dependence of the recombination over four orders of magnitude in time, from one nanosecond to ten microseconds, with a single adjustable parameter. We propose that diffusive trapped-hole motion is a general phenomenon in CdS nanocrystals, but one that is normally obscured in structures in which the wavefunctions of the electron and trapped hole spatially overlap. This phenomenon has important implications for the oxidation photochemistry of CdS nanocrystals.

Collective aspects of singlet fission in molecular crystals

We present a model to describe collective features of singlet fission in molecular crystals and analyze it using many-body theory. The model we develop allows excitonic states to delocalize over several chromophores which is consistent with the character of the excited states in many molecular crystals, such as the acenes, where singlet fission occurs. As singlet states become more delocalized and triplet states more localized, the rate of singlet fission increases. We also determine the conditions under which the two triplets resulting from fission are correlated. Using the Bethe Ansatz and an entanglement measure for indistinguishable bipartite systems, we calculate the triplet-triplet entanglement as a function of the biexciton interaction strength. The biexciton interaction can produce bound biexciton states and provides a source of entanglement between the two triplets even when the triplets are spatially well separated. Significant entanglement between the triplet pair occurs well below the threshold for bound pair formation. Our results paint a dynamical picture that helps to explain why fission has been observed to be more efficient in molecular crystals than in their covalent dimer analogues and have consequences for photovoltaic efficiency models that assume that the two triplets can be extracted independently.

Nanoscale Probing of Dynamics in Local Molecular Environments

Vibrational spectroscopy can provide information about structure, coupling, and dynamics underlying the properties of complex molecular systems. While measurements of spectral line broadening can probe local chemical environments, the spatial averaging in conventional spectroscopies limits insight into underlying heterogeneity, in particular in disordered molecular solids. Here, using femtosecond infrared scattering scanning near-field optical microscopy (IR s-SNOM), we resolve in vibrational free-induction decay (FID) measurements a high degree of spatial heterogeneity in polytetrafluoroethylene (PTFE) as a dense molecular model system. In nanoscopic probe volumes as small as 10(3) vibrational oscillators, we approach the homogeneous response limit, with extended vibrational dephasing times of several picoseconds, that is, up to 10 times the inhomogeneous lifetime, and spatial average converging to the bulk ensemble response. We simulate the dynamics of relaxation with a finite set of local vibrational transitions subject to random modulations in frequency. The combined results suggest that the observed heterogeneity arises due to static and dynamic variations in the local molecular environment. This approach thus provides real-space and real-time visualization of the subensemble dynamics that define the properties of many functional materials.

The subdiffusive targeting problem.

Recent experiments in living cells have observed subdiffusive motion, where the mean squared displacement of a molecule grows sublinearly with time as [x (2)] approximately t(alpha). Motivated by these experiments, we develop a theory for subdiffusion-limited bimolecular enzyme kinetics. As in normal diffusion, the statistics of the reaction times depend on the ratio between the distance to the target and the reaction rate in a scale invariant manner. Contrary to some studies, we find no critical value of alpha and therefore no evidence to suggest that it has been a target of evolutionary optimization.

Atomistic Hydrodynamics and the Dynamical Hydrophobic Effect in Porous Graphene

Mirroring their role in electrical and optical physics, two-dimensional crystals are emerging as novel platforms for fluid separations and water desalination, which are hydrodynamic processes that occur in nanoscale environments. For numerical simulation to play a predictive and descriptive role, one must have theoretically sound methods that span orders of magnitude in physical scales, from the atomistic motions of particles inside the channels to the large-scale hydrodynamic gradients that drive transport. Here, we use constraint dynamics to derive a nonequilibrium molecular dynamics method for simulating steady-state mass flow of a fluid moving through the nanoscopic spaces of a porous solid. After validating our method on a model system, we use it to study the hydrophobic effect of water moving through pores of electrically doped single-layer graphene. The trend in permeability that we calculate does not follow the hydrophobicity of the membrane but is instead governed by a crossover between two competing molecular transport mechanisms.