Multiple interfaces in diffusional phase transitions in binary mesogen-non-mesogen mixtures undergoing metastable phase separations

Theory and simulations of simultaneous chemical demixing and phase ordering are performed for a mixed order parameter system with an isotropic-isotropic (I-I) phase separation that is metastable with respect to an isotropic-nematic (I-N) phase ordering transition. Under certain conditions, the disordered phase transforms into an ordered phase via the motion of a double front containing a metastable phase produced by I-I demixing, a thermodynamically driven mechanism not previously reported. Different kinetic regimes are found depending on the location of the initial conditions in the thermodynamic phase diagram and the ratio between diffusional and nematic phase ordering mobilities. For a diffusional process, depending if the temperature is above or below the critical co-dissolution point, an inflection point or a phase separation takes place in the depletion layer. This phase separation leads to the formation of a second interface where the separation of the two metastable isotropic phases grows monotonically with time. The observed deviations from the typical Fickian concentration profiles are associated with strong positive deviations of the mixture from ideality due to couplings between concentration and nematic ordering. Although systems of interest include liquid crystalline nanocomposites, this novel mechanism may apply to any mixture that can undergo an order-disorder transition and demix.Comment: 24 pages, 8 figures, final accepted authors version, published version in http://pre.aps.org/abstract/PRE/v86/i1/e01160

Coherent Control of Molecular Processes in the Condensed Phase: Theory and Applications

Coherent control schemes are interference-based procedures for the control of quantum mechanical systems informed by a careful analysis of the physical conditions on interference. In this thesis, the coherent control approach is applied to experimental phase control in the condensed phase. Specifically, coherent control schemes are proposed as mechanisms for experimental manifestations of phase interference. First, a simulation of a ``black box'' feedback control experiment using realistic pulse shaping is shown to automatically find a specific, previously devised coherent control scheme. Qualitative features of experiments performed in this way are reproduced. Second, an analysis of interference in one-photon phase control yields an upper bound on the duration of phase effects. One-photon phase control was previously shown to be possible only over the transient (as opposed to steady-state) value of observables. The bound described here quantifies this statement and provide a rigorous definition of ``transient'' control. We propose that phase control beyond this bound, as reported in a number of weak-field experimental and computational studies, is the result of previously unaccounted for multiphoton effects. Third, we analyze a recent experiment showing phase control of the macroscopic current emanating from living brain cells photostimulated by two-photon absorption. We propose mechanisms for the phase dependence of the current that operate over fifteen orders of magnitude in time, from the femtosecond dynamics of light-matter interactions to the macroscopic dynamics of neuron current. Significantly, coherent control is extended to the case of systems repeatedly interacting with pulses of light; we show that macroscopic control is obtained, in this case, from the multi-pulse accumulation of small phase effects. Research into multiphoton control led to the development of a novel algorithm to compute higher order perturbation theory integrals, presented in the final chapter of this thesis. The Fourier-Laplace Inversion of the Perturbation Theory (FLIPT) method is a highly efficient, numerically exact, ``black box'' integration algorithm. The FLIPT method exploits the specific structure of the multidimensional perturbation theory integrals to obtain an exponential improvement in performance over standard quadrature.Ph.D

QP Partitioning for Radiationless Transitions

This work presents a new implementation of the QP algorithm, a computer method to diagonalize the extremely large matrices arising in multimode vibronic problems. Benchmark calculations are included, showing the accuracy of the program. The QP algorithm is extended to treat multiple electronic surfaces for competitive control and this is demonstrated with an Hamiltonian including three electronic states, a model of the benzene radical cation. Finally, the evolution of zeroth-order states in a simple two electronic states, two dimensional model with a conical intersection is explored, towards building a time-dependent view of overlapping resonances coherent control.MAS

Does landscape composition affect pest abundance and their control by natural enemies? A review

International audienceLandscape management could contribute to sustainable pest control. Landscape composition, in particular, could either directly impact a pest abundance by affecting its dispersal, mortality or reproduction, or indirectly by affecting its natural enemies. We performed an analysis of the scientific literature to assess how the proportion of different land covers at the landscape level is related to the abundance of pests or to their control by natural enemies. Of 72 independent case studies, 45 reported an effect of landscape composition. Results confirmed the suspected suppressive effect of landscape scale amounts of semi-natural areas on in-field pests: landscapes with higher proportions of semi-natural areas exhibited lower pest abundance or higher pest control in fields. Contrarily, there was no clear direction in relationships between pests and pest control and landscape when the latter was described as the overall proportion of cultivated area or as that of crops host to particular pests. The analysis of original articles indicates that this lack of direction may be due to the diversity of land use intensity in the studied landscapes and to a too rough categorizing of land covers. This pleads for a better consideration of the functionality of crops and of their management in landscapes. (C) 2011 Elsevier B.V. All rights reserved

Ultrafast Computational Screening of Molecules with Inverted Singlet-Triplet Energy Gaps Using the Pariser-Parr-Pople Semiempirical Quantum Chemistry Method

Molecules with an inverted energy gap between their first singlet and triplet excited states have promising applications in the next generation of organic light-emitting diode (OLED) materials. Unfortunately, such molecules are rare, and only a handful of examples are currently known. High-throughput virtual screening could assist in finding novel classes of these molecules, but current efforts are hampered by the high computational cost of the required quantum chemical methods. We present a method based on the semiempirical Pariser-Parr-Pople theory augmented by perturbation theory and show that it reproduces inverted gaps at a fraction of the cost of currently employed excited-state calculations. Our study paves the way for ultrahigh-throughput virtual screening and inverse design to accelerate the discovery and development of this new generation of OLED materials.ISSN:1089-5639ISSN:1520-521

Data supporting article "Ultrafast computational screening of molecules with inverted singlet-triplet energy gaps using the Pariser-Parr-Pople semi-empirical quantum chemistry method"

This repository contains the run of the Snakemake workflow used to generate all the data and produce the manuscript for the paper "Ultrafast computational screening of molecules with inverted singlet-triplet energy gaps using semi-empirical quantum chemistry