Partial waves in the nonspherical case

Many quantum mechanical problems are separable in one or several of the standard classifications of general coordinate systems. The associated solution is most conveniently evaluated using expansions in Lamé wave functions. We give here some preliminary numerical results displaying the continuous distortion of an s-, p-, and d-wave spherical harmonic. The results indicate rather surprising features which may be useful in general quantum mechanical contexts. © 1996 John Wiley & Sons, Inc.     

Radiative lifetimes in Ti I

Radiative lifetimes are reported for 33 levels of Ti I. The longer-lived (τ>100 ns) levels show a systematic difference from previously reported values. The new results reduce the uncertainty in absolute normalisation of the Oxford relativef-values to less then 2.5%.     

Spectroscopy of nonspherical atomic bubbles in solid helium

We analyze the effect of the host crystal symmetry on the optical spectra of the 6P(1/2)-6S(1/2) and 6P(3/2)-6S(1/2) transitions of atomic Cs in solid (4)He matrices. In particular, we address the deformation of the bubble structures formed by Cs in such quantum crystals. We show that the anisotropy of the stiffness tensor leads to static quadrupolar bubble shape deformations in hexagonally close-packed (hcp) crystals, while the corresponding deformations in the body-centered cubic (bcc) phase of the matrix have a hexadecupolar symmetry. A comparison of the measured excitation spectra with our model calculations allow us to infer quantitative values of the deformation parameters.     

On the surface transition in small nonspherical particles

We discuss a theoretical model for a surface transition in a nonspherical geometry. We study the stationary states of coexistence within a droplet of given volume. We show that finite range interactions with the matrix act to stabilize a finite non-spherical surface layer which is thickest in zones of highest curvature. At a certain temperature, the core tends to a spherical shape, becomes unstable and coexistence is no longer possible.     

Critical adsorption on nonspherical colloidal particles

We consider a nonspherical colloidal particle immersed in a fluid close to its critical point. The temperature dependence of the corresponding order parameter profile is calculated explicitly. We perform a systematic expansion of the order parameter profile in powers of the local curvatures of the surface of the colloidal particle. This curvature expansion reduces to the short distance expansion of the order parameter profile in the case that the solvent is at the critical composition.     

Polymer depletion profiles around nonspherical colloidal particles

We study the effect of chain self-avoidance on the polymer density profiles that are induced by a single colloidal particle of nonspherical shape such as an ellipsoid, a dumbbell, or a lens in a solution of nonadsorbing polymers. For colloid sizes sigma much smaller than the size R(x) of the polymers, we observe a pronounced difference between ideal and self-avoiding chains. In the case of ideal polymers, the surfaces of constant density always have the same character as the surface of the particle, e.g., are oblate for an oblate ellipsoid. In the self-avoiding case, however, the character changes with increasing distance r from the particle, and an oblate particle induces prolate surfaces of constant density if sigma相似文献   

Convectively assembled nonspherical mushroom cap-based colloidal crystals

Monolayers from mushroom cap-shaped polymer colloids were fabricated by a vertical substrate deposition technique. As confirmed by SEM and autocorrelation analysis, the monolayers show long-range hexagonal packing with particle orientational freedom restricted to either "heads up" or "heads down" alignment with respect to the substrate. The monolayers are modeled as a 2D diffraction grating and were studied with selected area laser diffraction. The stacking of ordered monolayers into the third dimension was achieved via layer-by-layer deposition. Convective assembly is shown as a viable approach to the large-scale crystallization of monodisperse nonspherical colloids.     

Controlled synthesis of nonspherical microparticles using microfluidics

The controlled synthesis of nonspherical microparticles using microfluidics processing is described. Polymer droplets, formed by shearing a photopolymer using a continuous water phase at a T-junction, were constrained to adopt nonspherical shapes by confining them using appropriate microchannel geometries. Plugs were obtained by shearing the polymer phase at low shear rates, while disks were obtained by flattening droplets using a channel of low height. The nonspherical shapes formed were permanently preserved by photopolymerizing the constrained droplets in situ using ultraviolet light. Monodisperse plugs and disks of different lengths and diameters were obtained by varying the flow rates of the two phases.     

Carotenoid triplet state lifetimes

Carotene and xanthophyll triplet lifetimes are found to depend on the concentration of the parent molecule. These results account for some of the variations in carotenoid triplet lifetimes reported previously. The rate constants obtained for ground state quenching correlate with the number of conjugated double bonds, the longer chain systems having higher quenching rate constants.     

Theoretical determination of electronic lifetimes in metals

For a variety of different fields in condensed matter physics it is important to understand the dynamics of excited electrons of bulk and surface states. In this article the results of first-principles computations for the lifetime of excited electrons in various metals are presented. In addition to crystalline systems also calculations of the electronic lifetimes in surface states of these materials are discussed. Preceding is a section in which the general theory is presented needed to calculate the lifetime of excited one-particle states. The method of choice is many-body perturbation theory using the GW approximation for the electronic self-energy. The key equations are summarized and a physical interpretation is given. This part is supplemented by the appendix in which the equations actually used in the calculations are presented in more detail.     

Heat transfer to nonspherical particles in a rarefied plasma flow

The interaction of a nonspherical metallic or nonmetallic particle with a rarefied thermal plasma flow is considered. Heat transfer to a particle of arbitrary shape with an extremely thin plasma sheath due to, respectively, gas molecules, electrons, and ions is described. Analytical expressions are derived for charge and heat fluxes in the particular case of a spheroidal metallic or nonmetallic particle in a subsonic plasma flow. It has been shown that the intensity of heat exchange is greatly influenced by gas ionization, charge transfer processes, and particle shape, velocity, and orientation in the plasma flow.     

Synthesis of nonspherical colloidal particles with anisotropic properties

We describe a promising and flexible technique for fabricating uniform nonspherical particles with anisotropic phase and surface properties. Our approach is based on the seeded polymerization technique in which monomer-swollen particles are polymerized. The polymerization causes a phase separation to occur, giving rise to two-phase nonspherical particles. We show that the elastic contraction of the swollen polymer particles induced by elevated polymerization temperatures plays an important role in the phase separation. Moreover, chemical anisotropy of nonspherical particles can be obtained by using immiscible polymer pairs and by employing surface treatments. Furthermore, we are able to produce amphiphilic dumbbell particles consisting of two different bulbs: hydrophilic poly (ethylene imine)-coated polystyrene and hydrophobic polystyrene. Controlled geometries of these amphiphilic nonspherical particles will allow a wide range of potential applications, such as engineered colloid surfactants.     

Preferential trapping of C60 in nanomesh voids

Surface nanotemplate-assisted molecular assembly offers great potential in the "bottom-up" construction of addressable molecular architectures for device miniaturization. Here, we report the fabrication of an extended 2D C60 nanomesh featuring a well-ordered nanocavity array by controlling the binary molecular phases of C60 and pentacene on Ag(111). Using low-temperature scanning tunneling microscopy, we demonstrate that the C60 nanomesh can serve as an effective template to selectively accommodate guest C60 molecules at the cavity sites, thereby leading to the formation of an ordered 2D C60 array.     

Impact of cholesterol on voids in phospholipid membranes

Free volume pockets or voids are important to many biological processes in cell membranes. Free volume fluctuations are a prerequisite for diffusion of lipids and other macromolecules in lipid bilayers. Permeation of small solutes across a membrane, as well as diffusion of solutes in the membrane interior are further examples of phenomena where voids and their properties play a central role. Cholesterol has been suggested to change the structure and function of membranes by altering their free volume properties. We study the effect of cholesterol on the properties of voids in dipalmitoylphosphatidylcholine (DPPC) bilayers by means of atomistic molecular dynamics simulations. We find that an increasing cholesterol concentration reduces the total amount of free volume in a bilayer. The effect of cholesterol on individual voids is most prominent in the region where the steroid ring structures of cholesterol molecules are located. Here a growing cholesterol content reduces the number of voids, completely removing voids of the size of a cholesterol molecule. The voids also become more elongated. The broad orientational distribution of voids observed in pure DPPC is, with a 30% molar concentration of cholesterol, replaced by a distribution where orientation along the bilayer normal is favored. Our results suggest that instead of being uniformly distributed to the whole bilayer, these effects are localized to the close vicinity of cholesterol molecules.     

Deformation and bursting of nonspherical polysiloxane microcapsules in a spinning-drop apparatus

We analyze the deformation and bursting process of nonspherical organosiloxane capsules in centrifugal fields. Measurements were performed in a commercial spinning-drop tensiometer at different values of tube rotation. A theoretical analysis of the mechanics of initially ellipsoidal elastic shells subjected to centrifugal forces is developed where the deformation of the capsule is predicted as a function of the initial geometry and membrane elastic properties. For different types of organosiloxane membranes the Poisson number varies between 0 and 0.9. This phenomenon points to a considerable reduction of the membrane thickness at the onset of mechanical stress. Membrane-breaking processes always initiated at one of the pole ends of the capsules. Such rupture processes can be interpreted in terms of the derived theoretical model.     

Inhomogeneous dephasing masks coherence lifetimes in ensemble measurements

An open question at the forefront of modern physical sciences is what role, if any, quantum effects may play in biological sensing and energy transport mechanisms. One area of such research concerns the possibility of coherent energy transport in photosynthetic systems. Spectroscopic evidence of long-lived quantum coherence in photosynthetic light-harvesting pigment protein complexes (PPCs), along with theoretical modeling of PPCs, has indicated that coherent energy transport might boost efficiency of energy transport in photosynthesis. Accurate assessment of coherence lifetimes is crucial for modeling the extent to which quantum effects participate in this energy transfer, because such quantum effects can only contribute to mechanisms proceeding on timescales over which the coherences persist. While spectroscopy is a useful way to measure coherence lifetimes, inhomogeneity in the transition energies across the measured ensemble may lead to underestimation of coherence lifetimes from spectroscopic experiments. Theoretical models of antenna complexes generally model a single system, and direct comparison of single system models to ensemble averaged experimental data may lead to systematic underestimation of coherence lifetimes, distorting much of the current discussion. In this study, we use simulations of the Fenna-Matthews-Olson complex to model single complexes as well as averaged ensembles to demonstrate and roughly quantify the effect of averaging over an inhomogeneous ensemble on measured coherence lifetimes. We choose to model the Fenna-Matthews-Olson complex because that system has been a focus for much of the recent discussion of quantum effects in biology, and use an early version of the well known environment-assisted quantum transport model to facilitate straightforward comparison between the current model and past work. Although ensemble inhomogeneity is known to lead to shorter lifetimes of observed oscillations (simply inhomogeneous spectral broadening in the time domain), this important fact has been left out of recent discussions of spectroscopic measurements of energy transport in photosynthesis. In general, these discussions have compared single-system theoretical models to whole-ensemble laboratory measurements without addressing the effect of inhomogeneous dephasing. Our work addresses this distinction between single system and ensemble averaged observations, and shows that the ensemble averaging inherent in many experiments leads to an underestimation of coherence lifetimes in individual systems.     

Lengthening of fluorescence lifetimes in self-organized metal-organic assemblies

Resonance energy transfer from tris(2,2'-bipyridyl)ruthenium (II) ([Ru(bpy)3](2+)) to nile blue A is demonstrated in aqueous solution in the presence of sodium dodecyl sulfate (SDS). At SDS concentrations below the critical micelle concentration, aggregates that permit energy transfer between these dyes at optically dilute (10 microM) concentrations with nearly 100% efficiency are formed. The disparity between the lifetimes of the donor and acceptor results in the lengthening of the photoluminescence lifetime of the sensitized emission observed from nile blue A. Time-resolved luminescence measurements confirm that the long-lived components of the emission originate from sensitized acceptor emission.     

The lifetimes of organosulphur compounds in the troposphere

Dimethyl sulphide and other reduced sulphur compounds, produced by marine biogenic activity and other processes, play a significant role in the global biogeochemical cycling of the element. The rates of their reactions with atmospheric oxidants are reviewed and their lifetimes in the troposphere due to the various reactions are computed. Sufficient data are available on the tropospheric abundance of the hydroxyl radical (OH) and on the rates of its reactions with the sulphur compounds for reasonable estimates to be made of the sulphur lifetimes with respect to OH. Summertime lifetimes of 14–87 h for (CH₃)₂S are computed at different latitudes. In the case of the tropospheric concentrations of the nitrate radical (NO₃), few data are available. There is a similar paucity of data on its rates of reactions with the sulphur compounds, and so large uncertainties exist in the computed lifetimes. These are, in any case, much longer than those due to OH. The possibility exists that iodine photochemistry, producing iodoxyl (IO) radicals, may efficiently oxidize the reduced sulphur and other organic compounds in the marine troposphere leading to lifetimes of the order of hours. Few data exist on the rates or mechanisms of these reactions and these are identified as representing the greatest uncertainties in the estimates of organosulphur lifetimes in the troposphere.