On functionalized fullerenes C 60 X n

Optimal configurations of functionalized fullerenes C ₆₀ X _n for n = 2, 4, 6, 8 have been determined by application of three topological stability measures. It has been found that the optimal configurations with higher n contain optimal configurations with lower values of n. Two stability measures prefer configurations in which functional groups Xs are crowded while the third one points to configurations with isolated pairs of Xs.     

Novel coherent two-dimensional optical spectroscopy probes of chirality exchange and fluctuations in molecules

We demonstrate how stochastic transitions between molecular configurations with opposite senses of chirality may be probed by 2D optical signals with specific pulse polarization configurations. The third-order optical response of molecular dimers (such as biphenyls) with dynamical axial chirality is calculated to order of k(2) in the wavevector of light. Spectroscopic signatures of equilibrium chirality fluctuations are predicted for three dynamical models (Ornstein-Uhlenbeck, two-state jump, and diffusion in double well) of the dihedral angle that controls the chirality.     

Improved configuration space sampling: Langevin dynamics with alternative mobility

We present a new and efficient method for determining optimal configurations of a large number (N) of interacting particles. We use a coarse-grained stochastic Langevin equation in the overdamped limit to describe the dynamics of this system and replace the standard mobility by an effective space dependent inverse Hessian correlation matrix. Due to the analogy of the drift term in the Langevin equation and the update scheme in Newton's method, we expect accelerated dynamics or improved convergence in the convex part of the potential energy surface Phi. The stochastic noise term, however, is not only essential for proper thermodynamic sampling but also allows the system to access transition states in the concave parts of Phi. We employ a Broyden-Fletcher-Goldfarb-Shannon method for updating the local mobility matrix. Quantitative analysis for one and two dimensional systems shows that the new method is indeed more efficient than standard methods with constant effective friction. Due to the construction, our effective mobility adapts high values/low friction in configurations which are less optimal and low values/high friction in configurations that are more optimal.     

Comparison of configurations of a four-column simulated moving bed process by multi-objective optimization

Configurations of a four-column simulated moving bed chromatographic process are investigated by multi-objective optimization. Various existing column configurations are compared through a multi-objective optimization problem. Furthermore, an approach based on an SMB superstructure is applied to find novel configurations which have been found to outperform the standard SMB configuration. An efficient numerical optimization technique is applied to the mathematical model of the SMB process. It has been confirmed that although the optimal configuration highly depends on the purity requirement, the superstructure approach is able to find the most efficient configuration without exploring various existing configurations.     

Tracking excited state dynamics with coherent control: automated limiting of population transfer in LDS750

Closed loop automated pulse shaping experiments are conducted to investigate population transfer in solutions of the laser dye LDS750 in acetonitrile and ethanol. Guided by a genetic algorithm, the optical phases of broadband noncollinear parametric amplifier pulses are modulated by a micromachined deformable mirror to minimize sample fluorescence. The objectives were to test if nonlinearly chirped pulses could reduce population transfer below levels attained by their linearly chirped analogues, and if so, whether the resulting pulse shapes could be rationalized in terms of the photoinduced molecular dynamics. We further aimed to discover how the optimal solutions depend on the pulse fluence, and on the nature of the solvent. Using frequency resolved optical gating, the optimal field is shown to consist of a transform limited blue portion, which promotes population to the excited state, and a negatively chirped red tail, which follows the Stokes shifting of the excited density and dumps it back down to the ground state through stimulated emission. This is verified by comparing the optimal group delay dispersion with multichannel transient absorption data collected in acetonitrile. The optimal pulse shape was not significantly affected by variation of pulse fluence or by the change of solvent for the two polar liquids investigated. These results are discussed in terms of accumulated insights concerning the photophysics of LDS750 and the capabilities of our learning feedback scheme for quantum control.     

A generalization of the method of stimulated emission pumping

Drawing on the results of an analysis of the nature of the pulse that generates complete transfer of population from one to another level in a system with a discrete spectrum, a generalization of the method of stimulated emission pumping is proposed. It is shown that a small subset of the Fourier components of the optimal pulse will, if their relative amplitudes are the same as in the optimal pulse, generate almost as efficient a population transfer, thereby generating the opportunity to prepare a system in a selected state with a selected population.     

The influence of laser field noise on controlled quantum dynamics

The influence of laser noise on the dynamics of simple quantum systems is analyzed. An anharmonic ladder is chosen for illustration and several pulses are obtained that optimize the yield of a quantum transition by constraining the laser parameters. The following models of laser noise are introduced: Amplitude white noise, phase white noise, frequency white noise and shot-to-shot static noise in the different pulse parameters. It is shown that the optimal pulses are robust to white amplitude noise, since the system acts as a dynamical filter. White phase noise affects the optimal pulses in a similar way by reducing the pulse area. This effect can be easily compensated for by pulse amplitude rescaling, up to a high level of noise. White frequency noise reduces the pulse area and induces spectral broadening, more strongly affecting the high frequency components. It can be partially compensated for by amplitude rescaling. The effects of static noise in the parameters cannot be easily corrected. It is shown that optimal pulses that drive n-photon transitions become more sensitive to noise in the amplitude and less sensitive to noise in the frequency as n increases. The effects of noise in the relative phase rapidly become constant for a large number of interfering pathways.     

Terahertz-laser control of large amplitude vibrational motion in the sub-one-cycle pulse limit

We investigate the control of state-selective population transfer in the THz spectral range generated by sub-one-cycle pulse excitation. To this end we developed a zero-net-force modification of the optimal control algorithm which allows us to extend the algorithm into the ultrashort pulse domain. By combining the analysis of the control landscapes and that of optimal control theory, we were able to formulate a general mechanism suitable for laser control by ultrashort pulses. The strategy consists of a superposition of two pi-pulses with carrier envelope phases of phi = pi/2. The first pulse is effectively in resonance with the targeted transition, while the second one, fired at around the minimum of the first pulse second lobe, removes leaking to the dipole-coupled background state. To compensate for the pulses ultrashort duration, the carrier frequencies of both pulses are red-shifted from the spectroscopic resonance.     

Electric field studies of liquid crystal droplet suspensions

The responses of freely-suspended micron-sized liquid crystal droplets subjected to an alternating electric field are presented. By examining droplets of isotropic, nematic bipolar, and nematic radial configurations, we test the effect of anchoring on the droplet response. Specifically, using birefringence and scattering dichroism we measure the relaxation of electric field-induced orientation following a field pulse. Results indicate that bipolar and radial droplets in suspension orient in the field through very different mechanisms. Bipolar droplets are observed to rotate their defect axes in the field while radial droplets orient through a nematic distortion. By varying the field pulse, we observe that droplets also respond differently to the field depending on their relative sizes. In radial droplet suspensions we quantitatively measure time scales associated with the reorientation and restructuring of the defect region.     

Energy optimization of water polyhedra. II. Classification of optimal configurations in clusters from a cube up to fullerene

Using a combinatory optimization method based on discrete models of intermolecular interaction, the classes of optimal configurations in polyhedral water clusters have been calculated. By means of calculations with various pair potentials an essential advantage in energy of discretely optimized configurations is ascertained. The effect of the dipole moment of clusters on their energy is studied.     

High-throughput dielectrophoretic manipulation of bioparticles within fluids through biocompatible three-dimensional microelectrode array

Dielectrophoresis (DEP) has been deemed as a potential and ideal solution for bioparticle manipulation. A 3-D carbon micro-electro-mechanical system (MEMS) fabricated from the latest developed carbon-MEMS approach has advantages of offering low-cost, biocompatible and high-throughput DEP manipulation for bioparticles. In this paper, a typical process for fabrication of various 3-D microelectrode configurations was demonstrated; accurate numerical analysis was presented on electric field gradient distribution and DEP force based on various microelectrode array configurations. The effects of electrode edge angle, electrode edge-to-edge spacing and electrode height on the electric field distributions were investigated, and optimal design considerations and rules were concluded through analysis of results. The outcomes demonstrate that the sharp edge electrode is more effective in DEP manipulation and both electrode edge-to-edge spacing and electrode height are critical design parameters for seeking optimal DEP manipulation. The gradient magnitude increases exponentially as the electrode spacing is reduced and the electric field extends significantly as the electrode height increases, both of which contribute to a higher throughput for DEP manipulation. These findings are consistent with experimental observations in the literature and will provide critical guidelines for optimal design of DEP devices with 3-D carbon-MEMS.     

Numerical investigation of boiling heat transfer in a quenching process of jet impingement considering solid temperature distribution

Boiling jet impingements are being widely used in various industries. Hence, a quenching jet impingement is simulated numerically. A solver code based on volume of fluid method was modified to analyze the effects of conjugation and mass transfer, and validated against an experimental study. Then, optimized cooling factor (OCF) was defined to involve temperature uniformity of the block and the cooling rate simultaneously. Subsequently, in laminar two-jet configurations, the effects of velocity inlet function, jet-to-surface and jet-to-jet spacing on standard temperature uniformity index (STUI) and OCF in a highly heated block were investigated. Heaviside function of time for the inlet velocity and periods of pulse between 0 and 0.2 were considered. Some remarkable results are achieved by the proposed configurations. In all cases with pulsating jets, improvements in STUI and OCF relative to pulse-free ones were observed; when V?=?0.4 m s^?1, OCF peaked at 2 in P?=?0.06, which was almost eight times greater than OCF of pulse-free configuration (OCF?=?0.24). As velocity decreased, the temperature uniformity improved; however, OCF showed the highest value at higher velocities occurring for lower periods of pulses. This happens because of more uniform temperature distribution in both plate sides and continual destroying film boiling layers generated on the surface. Also, in a jet-to-jet spacing of about one-third of the block length, for all plate lengths, optimal temperature uniformity with maximum OCF was obtained, due to formation of two stagnation points having the highest heat transfer rate by positioning in an ideal distance from each other.

     

Coherent control of the population transfer in complex solvated molecules at weak excitation. An experimental study

We performed a series of successful experiments for the optimization of the population transfer from the ground to the first excited state in a complex solvated molecule (rhodamine 101 in methanol) using shaped excitation pulses at very low intensities (1 absorbed photon per 100-500 molecules per pulse). We found that the population transfer can be controlled and significantly enhanced by applying excitation laser pulses with crafted pulse shapes. The optimal shape was found in feedback-controlled experiments using a genetic search algorithm. The temporal profile of the optimal excitation pulse corresponds to a comb of subpulses regularly spaced by approximately 150 fs, whereas its spectrum consists of a series of well-resolved peaks spaced apart by approximately 6.5 nm corresponding to a frequency of 220 cm(-1). This frequency matches very well with the frequency modulation of the population kinetics (period of approximately 150 fs), observed by excitation with a short (approximately 20 fs) transform-limited laser pulse directly after excitation. In addition, an antioptimization experiment was performed under the same conditions. The difference in the population of the excited state for the optimal and antioptimal pulses reaches approximately 30% even at very weak excitation. The results of optimization are reproducible and have clear physical meaning.     

Comparison of Single Pulse and Double Simultaneous Pulse Laser Induced Breakdown Spectroscopy

A simple and cost-effective variant of laser induced breakdown spectroscopy is presented that involves a double simultaneous pulse configuration employing a single laser source. Its performance is compared with conventional single pulse configuration. Double simultaneous pulses were accomplished by splitting a Nd:YAG laser (1064 nm, 6 ns, 360 mJ) beam into two components that were focused on the sample surface to produce two concurrent breakdowns. Experiment was repeated for single pulse and double simultaneous pulses under different ambient pressures. The performance was evaluated on the basis of self-absorption, signal-to-noise ratio (SNR), and relative standard deviation (RSD) of the Mg II doublet (280.2704 nm, 279.553 nm). Optically thin emission lines of better profiles with higher signal-to-noise ratio resulted from double simultaneous pulses. The lowest relative standard deviations obtained by single pulse and double simultaneous pulse configurations were 18.89% and 12.01%, respectively. In fact, double simultaneous pulses have performed better than single pulse in all respects within the studied regime.     

Optimal SNR for proton electron double resonance imaging

A significant problem for whole body PEDRI is the absorbed RF power in the body. In this paper the optimal pulse and field strength are determined which maximize the signal to noise ratio for a certain RF power level. Our calculations show that the relaxation times of the free radical contrast agent are the dominating factors which determine the pulse sequence and main field strength.     

Adsorption of ascorbic acid on the C60 fullerene

Adsorption of ascorbic acid (AsA) on C60 is investigated using classical molecular mechanics and density functional theory (DFT). Classical annealing was performed to explore the space of molecular configurations of ascorbic acid adsorbed on C60, searching for optimal geometries. From the structure with the smallest total energy, 10 initial configurations were prepared by applying rotations of 90 degrees about three orthogonal axes. Each one of these configurations was optimized using DFT (for both LDA and GGA exchange-correlation functionals), and an estimate of their total and adsorption energies was found. Different configurations have minimal adsorption energies (defined here as the total energy of the adsorbate minus the total energy of the separate molecules) from -0.54 to -0.10 eV, with distinct optimal distances between the AsA and C60 centers of mass. According to a Hirshfeld population analysis, AsA is, in general, an acceptor of electrons from C60. Our results demonstrate the feasibility of noncovalent functionalization of C60 with AsA and provide minimal energy values for the several different configurations investigated. These results should be considered in reactions as a possible way to prevent against the oxidative damage and toxicity of C60. The beneficial effects of using AsA-C60 includes its action when administered together with levodopa, against the neurotoxicity generated by levodopa isolated, which opens new strategies for the Parkinson's disease treatment.     

Laser pulse control of exciton dynamics in a biological chromophore complex

Femtosecond laser pulse control of exciton dynamics in a biological chromophore complex is studied theoretically. The computations use the optimal control theory specified to open quantum systems and formulated in the framework of the rotating wave approximation. Based on the laser pulse induced formation of an excitonic wave packet the possibility to localize excitation energy at a certain chromophore within a photosynthetic antenna system (FMO complex of green bacteria) is investigated. Details of exciton dynamics driven by a polarization shaped pulse are discussed.     

Aquatic passive sampling of a short-term thiacloprid pulse with the Chemcatcher: impact of biofouling and use of a diffusion-limiting membrane on the sampling rate

We examined the performance of the Chemcatcher (University Portsmouth, UK) in two different configurations when used for the aquatic passive sampling of a 1-day pulse contamination with thiacloprid under field-relevant conditions. The configuration without diffusion-limiting membrane led to biofouling of the Empore disk receiving phase resulting in a fourfold reduction in analyte uptake compared to unfouled passive samplers. The sampling rate for the configuration with diffusion-limiting polyethersulfone membrane was also much lower than in a long-term exposure scenario, although no biofouling occurred. Both configurations of the Chemcatcher exhibited high variation in analyte uptake with up to 100% RSD. Short-term contamination events may be underestimated in passive sampling when the receiving phase is biofouled or a diffusion-limiting membrane is employed.     

Optimal control for maximally creating and maintaining a superposition state of a two-level system under the influence of Markovian decoherence

Reducing decoherence is an essential step toward realizing general-purpose quantum computers beyond the present noisy intermediate-scale quantum (NISQ) computers. To this end, dynamical decoupling (DD) approaches in which external fields are applied to qubits are often adopted. We numerically study DD using a two-level model system (qubit) under the influence of Markovian decoherence by using quantum optimal control theory with slightly modified settings, in which the physical objective is to maximally create and maintain a specified superposition state in a specified control period. An optimal pulse is numerically designed while systematically varying the values of dephasing, population decay, pulse fluence, and control period as well as using two kinds of objective functionals. The decrease in purity due to the decoherence limits the ability to maintain a coherent superposition state; we refer to the state of maximal purity that can be maintained as the saturated value. The optimally shaped pulse minimizes the negative effect of decoherence by gradually populating and continuously replenishing the state of saturated purity.