Non-perturbative approach to high-index-contrast variations in electromagnetic systems

We present a method that formally calculates exact frequency shifts of an electromagnetic field for arbitrary changes in the refractive index. The possible refractive index changes include both anisotropic changes and boundary shifts. Degenerate eigenmode frequencies pose no problems in the presented method. The approach relies on operator algebra to derive an equation for the frequency shifts, which eventually turn out in a simple and physically sound form. Numerically the equations are well-behaved, easy implementable, and can be solved very fast. Like in perturbation theory a reference system is first considered, which then subsequently is used to solve another related, but different system. For our method precision is only limited by the reference system basis functions and the error induced in frequency is of second order for first-order basis set error. As an example we apply our method to the problem of variations in the air-hole diameter in a photonic crystal fiber.     

Slow-light enhanced optical detection in liquid-infiltrated photonic crystals

Slow-light enhanced optical detection in liquid-infiltrated photonic crystals is theoretically studied. Using a scattering-matrix approach and the Wigner–Smith delay time concept, we show that optical absorbance benefits both from slow-light phenomena as well as a high filling factor of the energy residing in the liquid. Utilizing strongly dispersive photonic crystal structures, we numerically demonstrate how liquid-infiltrated photonic crystals facilitate enhanced light–matter interactions, by potentially up to an order of magnitude. The proposed concept provides strong opportunities for improving existing miniaturized absorbance cells for optical detection in lab-on-a-chip systems.     

The response of microstructure to processing in a series of poly(siloxaneimide) copolymers

Poly(siloxaneimide) (PSI) segmented copolymers exhibit organized microdomains if the blocks are sufficiently incompatible. As with neat diblock and triblock copolymers, the processing route employed to prepare films of PSI materials is expected to influence the dimensions and/or morphology of the resultant microstructure. In this work, small-angle neutron scattering (SANS) is utilized to characterize the disordered microstructure found in films of a series of PSI copolymers which are subjected to solvent casting and various thermal treatments. Microstructural dimensions such as the periodicity and correlation length are deduced from the Teubner-Strey (TS) model for disordered microemulsions. The scattering intensity of each copolymer up to q = 5.0 nm^?1, where q is the scattering vector, is found to scale as q^?2.8+?0.1. Results indicate that processing the materials as cast films or as melt-pressed films allowed to cool slowly has a small, but discernible, effect on microstructural characteristics. SANS profiles of films quenched from elevated temperatures reveal a clear transition in microdomain periodicity, which correlates well with the glass transition temperature of the imide microphase in these and other materials of similar chemical structure. © 1993 John Wiley & Sons, Inc.     

Scavenging of acetylperoxyl radicals and quenching of triplet diacetyl by beta-carotene: mechanisms and kinetics

Beta-carotene scavenges triplet diacetyl generated by laser flash photolysis with a second-order rate constant of 9.1+/-0.9 x 10(9) M(-1) s(-1) in deaerated benzene at 20 degrees C. In the presence of oxygen diacetyl dissociates to generate acetylperoxyl radicals. It is demonstrated that diacetyl does not dissociate to any appreciable extent in the absence of oxygen. The acetylperoxyl radical is scavenged by beta-carotene with second-order rate constant 9.2+/-0.6 x 10(8) M(-1) s(-1) in aerated benzene at 20 degrees C to give an adduct between the acetylperoxyl radical and beta-carotene, whereas no evidence of oxidation of beta-carotene by the strongly oxidizing acetylperoxyl radical to give the beta-carotene radical cation is found. This adduct decays with first-order rate constant 1.35+/-0.16 x 10(3) s(-1) to give (presumably) a beta-carotene epoxide and the acetyloxyl radical.     

Specific determination of 20 primary aromatic amines in aqueous food simulants by liquid chromatography-electrospray ionization-tandem mass spectrometry

A multi-analyte method without any pre-treatment steps using reversed-phase liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS) was developed and applied for the determination of 20 primary aromatic amines (PAA) associated with polyurethane (PUR) products or azo-colours. The method was validated in-house for water and 3% acetic acid food simulants using spiked migrates from plastic laminates. Detection limits ranged from 0.27 to 3 microg amine/L food simulants, and RSD values of within-laboratory reproducibility at the 2 microg PAA/L level ranged from 3.9 to 19%. PAA migration from plastic laminates and black nylon cooking utensils were determined with the method, and high levels of 4,4'-methylenedianiline and aniline were found in migrates from about half of the tested cooking utensils. The method fulfils present legislative demands in the EU for screening and verification of PAA migration from food contact materials.     

Analysing fluorobenzoate tracers in groundwater samples using liquid chromatography-tandem mass spectrometry. A tool for leaching studies and hydrology

A sensitive LC-MS-MS method for the direct determination and quantification of 15 fluorobenzoic acids (FBAs) was developed. FBAs are used as conservative tracers for hydrological modelling of water flow and in studies of pesticides and other xenobiotic compounds. The use of FBAs is discussed in relation to other tracers (bromide, chloride, uranine). The method covers mono-substituted fluorobenzoic acid, difluorobenzoic acid, trifluorobenzoic acid, and tetrafluorobenzoic acid. The general detection limit in ground water was 1 microg/l using electrospray ionisation and 20 microg/l using atmospheric pressure chemical ionisation. Analysis time was less than 10 min, small sample volumes were needed and no clean-up was required.     

The physicochemical surface characteristics of Brevibacterium linens

The physicochemical surface characteristics and interfacial behavior of two strains of Brevibacterium linens (BL-MGE and BL-9174), that may enhance cheese flavor, were assessed. Cell surface hydrophobicity was determined by measuring the contact angle of a variety of polar and non-polar wetting agents on lawns of the bacterial cells. The contact angles obtained were used to calculate the cell surface free energy components γ^LW, γ^AB, γ⁺ and γ⁻. The Lifshitz van der Waals component and the Lewis acid-base component were approximately 35 mJ m⁻² and 22 mJ m⁻², respectively, for both strains. Under conditions of physiological pH and low ionic strength, neither strain exhibited affinity for an octyl ligand in hydrophobic interaction chromatography. This occurred despite a favorable free energy of interfacial interaction which was attributable almost entirely to favorable acid-base interactions between cells and octyl-sepharose. The nature of surface functional groups was evaluated using X-ray photoelectron spectroscopy. Excluding H, the mean percentage of atomic fraction for C, O, N and P for BL-MGE was 57.7, 37.8, 3.9 and 0.6%, respectively and 61.0, 31.8, 7.0 and 0.2%, respectively for BL-9174.     

Fourth-order diagrammatic MB-RSPT calculations of the interaction energy between two helium atoms

The efficiency of the MB-RSPT in the calculations of the correlation contributions to the interaction energies was investigated, using He₂ as a model Van der Waals system. The attention has been focused on the convergency of the perturbation expansion in the calculations of the interaction energy and on the analysis of the fourth-order terms of MB-RSPT. The rôle of the renormalization term in the correct long-range behaviour of the interaction potential has been emphasized.