Molecular control of recombination dynamics in dye sensitised nanocrystalline TiO2 films

Modification of the structure of a porphyrin dye shows a significant change in the rate of charge recombination between injected electrons in the TiO2 and the oxidized dye anchored to it following optical excitation, offering an insight into fundamental understanding of processes occurring at the dye/semiconductor interface.     

Nonlinear effects in acousto-optic imaging

Acousto-optic (AO) imaging is a promising technique that is able to reveal optical properties in the millimeter range inside scattering media by tagging the photon paths with an ultrasonic beam. To increase both the contrast and the resolution of the AO images, we have explored the possibility of using the nonlinear response of the speckle modulation. Variation of the second-harmonic signal as the square of the ultrasonic amplitude has been found, and strong reduction of the tagged zone size has been demonstrated.     

Two-photon ionization spectrum of the 1Lb ← S0 transition in toluene

The multiphoton ionization (MPI) spectrum of toluene arising from the ¹B₂ (¹L_b) valence state has been investigated. The state participates as a two-photon resonance. A total of nine excited state fundamentals have been characterized, including three non-totally symmetric vibrations. The toluene MPI spectrum shows a strong resemblance to the two-photon fluorescence excitation spectrum with the strongest transitions taking place to the origin and excited state modes ν₁(a₁), ν₁₂(a₁) and ν₁₄(b)₂). The intensities of the observed fundamentals are rationalized in terms of Franck-Condon and vibronic coupling effects. A major conclusion is, that the primary mechanism for the activity of ν₁₂ is vibronic coupling.     

Electric-field induced capillary interaction of charged particles at a polar interface

We study the electric-field induced capillary interaction of charged particles at a polar interface. The algebraic tail of the electrostatic pressure of each charge results in a deformation of the interface u approximately r(-4), where r is the lateral distance. The capillary interaction of nearby particles is repulsive and varies as rho(-6) with their distance rho. As a consequence, electric-field induced capillary forces cannot be at the origin of the secondary minimum observed recently for charged poly(methyl methacrylate) particles at an oil-water interface.     

Photoelectron imaging of helium droplets

The photoionization and photoelectron spectroscopy of He nanodroplets (10(4) atoms) has been studied by photoelectron imaging with photon energies from 22.5-24.5 eV. Total electron yield measurements reveal broad features, whose onset is approximately 1.5 eV below the ionization potential of atomic He. The photoelectron spectra are dominated by very low energy electrons, with less than 0.6 meV. These results are attributed to the formation and autoionization of highly vibrationally excited He(*)(n) Rydberg states within the cluster, followed by strong final state interactions between the photoelectron and the droplet.     

Fluorination of the epsilon-Keggin Al13 polycation

Fluorination of the Al13 polycation in epsilon-Keggin (Kepsilon-J) conformation, leads to partial substitution of di-mu2-OH bridges by fluorine and enhances the formation of Al30 polycation.     

Confined crystallization of PVDF and a PVDF‐TFE copolymer in nanolayered films

Layer‐multiplying coextrusion was used in conjunction with isothermal recrystallization to study the confined crystallization of polyvinylidene fluoride (PVDF) and polyvinylidene fluoride‐tetrafluoroethylene (PVDF‐TFE) using polycarbonate (PC) and polysulfone (PSF) as confining materials. Three layered systems were produced (PC/PVDF, PSF/PVDF, and PC/PVDF‐TFE) with layer thicknesses ranging from 525 to 28 nm. The crystal morphology was affected by both layer thickness and recrystallization temperature. Specifically, increased recrystallization temperature and decreased layer thickness facilitated the formation of high aspect ratio in‐plane crystals in both PVDF based polymers. On the other side of the spectrum, thicker layers and lower recrystallization temperatures produced on‐edge PVDF crystals and isotropic PVDF‐TFE crystals. The morphology was correlated with oxygen permeability, which decreased by almost two orders of magnitude compared with the bulk. A variety of crystal structures were obtained and explained with nucleation and diffusion theory. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Morphology-to-properties correlations in anodic porous InP layers

In this paper, we investigate the properties of porous structures anodically grown onto n-InP (100) in HCl. In situ electrochemical characterizations show the pore morphology strongly influences the properties of the InP surfaces. Both dc- and ac-electrochemical measurements reveal an enhancement of the capacitive current and a modification of the electronic distribution at the interface. Photocurrent spectra performed during the pore growth are also strongly modified. For low anodic charges, an increase of the photocurrent with a redshift of the absorption edge is measured. These evolutions can be respectively ascribed (i) to a reflection decrease due to a surface roughening and (ii) to the creation of surface states within the band gap. For higher anodic charges, the photocurrent drops with a narrowing of the spectrum. Using a model based on the “dead” layer, the porous layer is considered as an absorbent film that progressively attenuates the photocurrent of the bulk semiconductor.     

Hydrogen electrosorption in nanocrystalline Ti-based alloys

The electrochemical behavior in alkaline solution (1 M NaOH) of nanocrystalline Ti:Ru:Fe:O (2:1:1:2) prepared by high-energy ball milling was studied over its whole electroactivity domain, with a particular emphasis on the hydrogen evolution reaction (her). Comparison has also been made with nanocrystalline Ti:Ru:Fe (2:1:1) and a mixture of Ti:TiO:Ru:Fe₂O₃ (3/2:1/2:1:1/2). It was shown by cyclic voltammetry, open circuit potential decay and chronopotentiometry measurements that hydrogen absorption in the electrode material occurs during hydrogen discharge. The electrochemical behavior of nanocrystalline Ti:Ru:Fe:O (2:1:1:2) closely follows that of Ti:Ru:Fe (2:1:1), but differs radically from that of Ti:TiO:Ru:Fe₂O₃ (3/2:1/2:1:1/2). This is due to the fact that the former two compounds contain a significant fraction of B2 phase (59 and 97 wt.%, respectively), while the latter does not. In steady state conditions, the ratio H/B2 phase in nanocrystalline Ti:Ru:Fe:O (2:1:1:2) is 0.15, about 1.6 times less than that for the O-free nanocrystalline compound. The coefficient of diffusion of hydrogen in nanocrystalline Ti:Ru:Fe:O (2:1:1:2) is 2.6×10⁻¹³ cm² s⁻¹, more than three times less than that in nanocrystalline Ti:Ru:Fe (2:1:1). The difference between the hydrogen absorption characteristics of both nanocrystalline compounds are tracked down to the fact that their B2 phases have different stoichiometries.     

A statistical learning strategy for closed-loop control of fluid flows

This work discusses a closed-loop control strategy for complex systems utilizing scarce and streaming data. A discrete embedding space is first built using hash functions applied to the sensor measurements from which a Markov process model is derived, approximating the complex system’s dynamics. A control strategy is then learned using reinforcement learning once rewards relevant with respect to the control objective are identified. This method is designed for experimental configurations, requiring no computations nor prior knowledge of the system, and enjoys intrinsic robustness. It is illustrated on two systems: the control of the transitions of a Lorenz’63 dynamical system, and the control of the drag of a cylinder flow. The method is shown to perform well.