Characterization of rodlike aggregates generated from a cationic surfactant and a polymerizable counterion

The polymerization of elongated micellar structures offers a novel approach to the production of high-aspect-ratio, water-soluble amphiphilic nanorods. A cationic surfactant with a vinyl-containing counterion, cetyltrimethylammonium 4-vinylbenzoate, has been synthesized and polymerized to produce high-aspect-ratio nanoparticles which are insensitive to changes in solution conditions. Aggregates are polymerized over a range of initiator concentrations allowing for control of the product length. Small-angle neutron scattering and light scattering are used to characterize the dimensions of the polymerized aggregates, showing them to have a fixed radius of 2 nm and contour lengths varying from 96 to 340 nm. Proton NMR verifies the chemical structure and provides insight into the mobility of the aggregate components. Finally, gel permeation chromatography of the polymer extracted from the aggregates indicates that the polymerization reaction controls the aggregate dimensions.     

Formation of novel rare-gas-containing molecules by molecular photodissociation in clusters

Recent work by R?s?nen and coworkers showed that photolysis of hydrides in rare-gas matrices results in part in formation of novel, rare-gas-containing molecules. Thus, photolysis of HCl in Xe and of H2O in Xe result respectively in formation of HXeCl and HXeOH in the Xe matrices. Ab initio calculations show that the compounds HRgY so formed are stable in isolation, and that by the strength and nature of the bonding these are molecules, very different from the corresponding weakly bound clusters Rg...HY. This paper presents a study of the formation mechanism of HRgY following the photolysis of HY in clusters Rgn(HY). Calculations are described for HXeCl, as a representative example. Potential energy surfaces that govern the formation of HXeCl in the photolysis of HCl in xenon clusters are obtained, and the dynamics on these surfaces is analyzed, partly with insight from trajectories of molecular dynamics simulations. The potential surfaces are obtained by a new variant of the DIM (diatomics in molecules) and DIIS (diatomics in ionic systems) models. Non-adiabatic couplings are also obtained. The main results are: (1) Properties of HXeCl predicted by the DIM-DIIS model are in reasonable accord with results of ab initio calculations. (2) The potential along the isomerization path HXeCl-->Xe...HCl predicted by DIM is in semiquantitative accord with the ab initio results. (3) Surface-hopping molecular dynamics simulations of the process in clusters, with "on the fly" calculations of the DIM-DIIS potentials and non-adiabatic couplings are computationally feasible. (4) Formation of HXeCl, following photolysis of HCl in Xe54(HCl), requires cage-exit of the H atom as a precondition. The H atom and the Cl can then attack the same Xe atom on opposite sides, leading to charge transfer and production of the ionic HXeCl. (5) Non-adiabatic processes play an important role, both in the reagent configurations, and at the charge-transfer stage. The results open the way to predictions of the formation of new HRgY species.     

Robust Algorithms for the Stable Set Problem

The stable set problem is to find in a simple graph a maximum subset of pairwise non-adjacent vertices. The problem is known to be NP-hard in general and can be solved in polynomial time on some special classes, like cographs or claw-free graphs. Usually, efficient algorithms assume membership of a given graph in a special class. Robust algorithms apply to any graph G and either solve the problem for G or find in it special forbidden configurations. In the present paper we describe several efficient robust algorithms, extending some known results.     

Optimal focusing by spatio-temporal inverse filter. I. Basic principles

A focusing technique based on the inversion of the propagation operator relating an array of transducers to a set of control points inside a medium was proposed in previous work [Tanter et al., J. Acoust. Soc. Am. 108, 223-234 (2000)] and is extended here to the time domain. As the inversion of the propagation operator is achieved both in space and time, this technique allows calculation of the set of temporal signals to be emitted by each element of the array in order to optimally focus on a chosen control point. This broadband inversion process takes advantage of the singular-value decomposition of the propagation operator in the Fourier domain. The physical meaning of this decomposition is explained in a homogeneous medium. In particular, a definition of the number of degrees of freedom necessary to define the acoustic field generated by an array of limited aperture in a focal plane of limited extent is given. This number corresponds to the number of independent signals that can be created in the focal area both in space and time. In this paper, this broadband inverse-focusing technique is compared in homogeneous media with the classical focusing achieved by simple geometrical considerations but also with time-reversal focusing. It is shown that, even in a simple medium, slight differences appear between these three focusing strategies. In the companion paper [Aubry et al., J. Acoust. Soc. Am. 110, 48-58 (2001)] the three focusing techniques are compared in heterogeneous, absorbing, or complex media where classical focusing is strongly degraded. The strong improvement achieved by the spatio-temporal inverse-filter technique emphasizes the great potential of multiple-channel systems having the ability to apply completely different signal waveforms on each transducer of the array. The application of this focusing technique could be of great interest in various ultrasonic fields such as medical imaging, nondestructive testing, and underwater acoustics.     

Stringent tests of shell model calculations in fp shell nuclei (46, 48)Ti and (50,52)Cr from measurements of g factors and B(E2) values

Measurements of magnetic moments and lifetimes of 2(+)(1) and 4(+)(1) states of (46,48)Ti and (50,52)Cr were performed with high accuracy via projectile Coulomb excitation and the technique of transient magnetic fields. The high quality of the data allows for the first time to establish stringent constraints on large scale shell model calculations. Whereas the global behavior of the data is well explained by full fp shell model calculations, distinct deviations in the g factors and B(E2) values of (46,48)Ti from theoretical predictions can be attributed to excitations of the 40Ca core. This suggestion is supported by recent Monte Carlo calculations which provide evidence that 48Ca is a better inert core.     

Low temperature scanning force microscopy of the Si(111)-(7x7) surface

A low temperature scanning force microscope (SFM) operating in a dynamic mode in ultrahigh vacuum was used to study the Si(111)- (7x7) surface at 7.2 K. Not only the twelve adatoms but also the six rest atoms of the unit cell are clearly resolved for the first time with SFM. In addition, the first measurements of the short range chemical bonding forces above specific atomic sites are presented. The data are in good agreement with first principles computations and indicate that the nearest atoms in the tip and sample relax significantly when the tip is within a few A of the surface.     

Obtaining the dividends-penalty identities by interpretation

The dividends-penalty identity is a relation between three functions: the discounted penalty function without dividends, the discounted penalty function if a barrier dividend strategy is applied, and the expected discounted dividends until ruin. The classical model of risk theory is modified in that the deterministic premiums are replaced by a compound Poisson process with exponential jumps. In this model, the dividends-penalty identity is new and can be derived by interpretation. Then the dividends-penalty identity in the classical model is obtained as a limit.     

A study of the electron quenching of excimers in a KrF* laser excited by a coaxial electron beam

Measurements of the output energy, the optical pulse length and the build-up time of the laser pulse, obtained with a coaxially e-beam pumped KrF^* laser, were performed varying the total gas fill pressure, the F₂ content and the e-beam current from 1–5 bar, 0.1–0.8% and 13.3–26.6 kA, respectively. The maximum specific extraction energy amounts to 64 J/l. The large range of measurements, especially at low F₂ concentrations, reveals the necessity to extend the kinetics of the F₂ chain in the usual computer model. With the introduction of electron quenching of KrF^* and ArF^* by dissociative attachment the predictions are also for low F₂ concentration in agreement with experiments.     

Aspects of hydrogen CARS thermometry

The temperature of hydrogen in a graphite tube furnace has been determined using broadband coherent anti-Stokes Raman scattering (CARS) spectroscopy. Significant laser-induced population changes were observed within the vibrational state manifold, whereas the rotational envelope in both fundamental and hot band remained unchanged.