The effect of an electric field on the overall intensity of the ZEKE spectrum and on the lifetime is discussed for very long living states detected by a pulsed field ionization delayed by several microseconds or more. It is shown that the presence of a de electrical field can shorten the very long lifetimes and that it can also reduce the overall intensity of the very long living states, The decrease in the long lifetimes of the ZEKE states is complementary to the field-induced elongation of the shorter lifetimes. The discussion is based on quantum mechanical considerations and is illustrated by detailed computational studies, for a model problem, using an effective Hamiltonian formalism for an energy range just above the lowest ionization threshold, The model allows for coupling of different Rydberg series built on different excited states of the core where the continuum corresponds to the ground state. Predissociation is not allowed for in the model Hamiltonian. The trends are in accord with the observations of Held et al, as reported in the preceding paper, and the magnitude of the measured lifetimes of the ZEKE states (dozens of microseconds) are reproduced by the computations.
Reprinted in Charles Dickens, Hard Times. A Norton Critical Edition. Ed. Fred Kaplan and Sylvère Monod. Third Edition. New York: Norton, 2001, pp. 465–75.
Review article of S. Heidemann, Das Aleppiner Kalifat (AD 1261): vom Ende des Kalifates in Bagdad über Aleppo zu den Restaurationen in Kairo (Leiden, 1994).
Impact heating of cold molecular clusters moving at high velocities dissipates extreme amounts of energy (often more than several eV per atom) in very short times. Molecular dynamics simulations of larger rare gas clusters show that this excess energy is thermalized in 100 fs or less, depending on cluster size and impact velocity. Dissipation is also extensive for smaller clusters but these shatter before being fully thermalized. A simple analytical hard sphere model that recovers this behavior is discussed. The model attributes the ultrafast relaxation to the random orientation of the interatomic distance before the collision, A perfectly ordered army of atoms is indeed found not to relax. Such an array also allows for a dispersion-free propagation of a shock front. The route to equilibrium is therefore the efficient mixing in phase space caused by the velocity components after the collision having a random part. The implications for the maximum entropy description of cluster impact induced chemistry, for the production of electronically excited and ionic species and for electron emission are discussed.
We present the first application of first-principles molecular dynamics to a chemical process occurring on more than one electronic state. The example is the collisional chemi-ionization of NaI using a novel ab initio technique for the electronic states and a previously described full multiple spawning (FMS) classically motivated quantal method to describe the nuclear dynamics, The results for the dynamics are compared with fully exact quantal propagation, The FMS method which generates quantal amplitudes and inherently conserves normalization is shown to perform remarkably well for this heavy particle problem. The ab initio generated potentials and interstate couplings are compared with empirical potentials for NaI. Particular attention is given to the localized molecular orbital/generalized valence bond (LMO/GVB) method used for the electronic problem and to its interface with the equations of motion for the nuclei. The ability to incorporate atomic input (such as the ionization potential or the electron affinity) into the LMO/GVB method is emphasized. (C) 1996 American Institute of Physics.
A two-step flocculation process is described in which a polycation (polydiallyldimethylammonium chloride) is adsorbed on to neg. charged montmorillonite particles, followed by the addn. of an anionic surfactant [sodium dodecyl sulfate (SDS)]. At a 1:1 molar ratio, optimal flocculation is obtained owing to the formation of an insol. surfactant-polymer complex in the presence of the particles. The pptn. of the insol. complex is also dependent on the SDS and polymer concns., and occurs at a 1:1 SDS:monomer molar ratio. From adsorption isotherms of the polymer-particle and surfactant-polymer systems, the adsorption energy was calcd. and subsequently a flocculation mechanism is suggested. [on SciFinder(R)]
Hydrophobic modification of horseradish peroxidase by fatty acid esters (C8, C12, C16, C18) of N-hydroxysuccinimide was carried out. The degree of modification increases with an increase in the ester:enzyme molar ratio and reaches a maximal value of four modified amino groups when this ratio is 150:1. Covalent attachment of hydrophobic groups to the peroxidase mols. leads to a spontaneous formation of micelle-like colloidal clusters, which have a mean diam. of 65 nm at the maximal degree of modification by C16-ester. The fraction of the enzyme mols. which forms clusters depends on both the length of the attached hydrophobic chain and the degree of modification. The colloidal clusters, which are composed of the modified peroxidase, have about 80 and 50% lower enzymic activities for C12- and C16- modified enzymes. [on SciFinder(R)]
The binding of two model surfactants, SDS and dodecyltrimethylammonium bromide to β-lactoglobulin was studied at room temp. and the thermal stability of the resulting complexes was evaluated by differential scanning calorimetry (DSC) measurements. Binding isotherms indicated both ionic and hydrophobic interactions depending on both the charge of the protein and surfactant at different pHs and on the binding molar ratios of surfactant to the globular protein. Zeta potential measurements indicated charge neutralization of the protein, under suitable conditions, which also lead to aggregation and pptn. of the proteins. Surface tension measurements indicated similarity between the two types of oppositely charged protein-surfactant complexes and a difference between them when protein and surfactants are similarly charged. DSC measurements revealed different behavior in protein conformation in the presence of the two surfactants. The results obtained at room temp. and upon heating are discussed in terms of the nature of the surfactant/protein interactions involved in the complex formation. [on SciFinder(R)]
We propose a novel method to measure the fractal dimension of a submonolayer metal adatom system grown under conditions of limited diffusivity on a surface. The method is based on measuring the specular peak attenuation of He atoms scattered from the surface, as a function of incidence energy. The (Minkowski) fractal dimension thus obtained is that of contours of constant electron density of the adatom system. Simulation results are presented, based on experimental data. A coverage dependent fractal dimension is found from a two-decade wide scaling regime.
A detailed treatment is provided of the various free-energy terms that contribute to the transfer of a polyalanine ct-helix from the aqueous phase into lipid bilayers. In agreement with previous work, the hydrophobic effect is found to provide the major driving force for helix insertion, However, an opposing effect of comparable magnitude is also identified and is attributed to the large free-energy penalty associated with the desolvation of peptide hydrogen bonds on transfer to the low dielectric environment of the bilayer. Lipid perturbation effects as well as the entropy loss associated with helix immobilization in the bilayer are also evaluated. Two configurations of a membrane-bound 25mer polyalanine helix were found to be lower in free energy than the isolated helix in the aqueous phase, The first corresponds to the case of vertical insertion, in which a helix terminus protrudes from each side of the bilayer. The second minimum is for the case of horizontal insertion, for which the helix is adsorbed upon the surface of the bilayer. The calculated free-energy minima are found to be in good agreement with recent measurements of related systems, Large free-energy barriers resulting from desolvation of unsatisfied hydrogen-bonding groups al the helix termini are obtained for both insertion processes. The barriers for insertion are significantly reduced if the helix termini are assumed to be `'capped'' through the formation of hydrogen bonds with polar sidechains, For uncapped helices, our results support recently proposed models in which helices are inserted by first adsorbing on the membrane surface and then having one terminus `'swing around'' so as to penetrate the bilayer,
Reprinted in G.R. Hawting, editor. Muslims, Mongols and Crusaders: An Anthology of Articles Published by the Bulletin of the School of Oriental and African Studies. London and New York: RoutledgeCurzon, 2005.
Persian translation: “Ghāzān, islām va sunnat - i mughūl az dīd - i salāṭ in- i mamlūk,” in Hujūm -i mughūl bi īrān va payāmad - hā - yi ān . Tehran: Dānishgāh - i Shahīd - i Bihishtī, 1379. 1287-1315.
This paper investigates the tendency of high technology firms in Israel to choose metropolitan locations. A series of hypotheses are presented that link this spatial behavior with the firms' life cycle characteristics, its' network context and technological characteristics. These hypotheses are then structured in a causal framework and the choice of metropolitan location is modeled as a discrete choice problem. Firm behavior is taken as utility-maximizing rather than profit-maximizing. Empirical results, based on a survey of over 160 Israel high technology firms are presented.These suggest that the choice of metropolitan location is often used to substitute for disadvantages that the firm experiences. For example, metropolitan location for new firms can mitigate the negative effects associated with a precarious market position; for firms with weak network structures, metropolitan location can substitute for this drawback. The public policy implications of these findings with respect to prospects for network-based regional development in Israel, are also discussed.
Felsenstein D. 1996. “High Technology Location and Metropolitan growth.” In Gradus Y and Lipshitz G (eds), The Mosaic of Israeli Geography, Pp. 217-226. Beer Sheva: Ben Gurion University Press of the Negev.
Time evolution when many states are strongly coupled is approximated by a reduced description where nearly degenerate states are taken to be equally populated, on the average. This grouping, valid at longer times, can significantly reduce the dimension of the problem, thereby making tractable computations which include all nearly isoenergetic states. The validity of the approach is examined by comparison with exact results, The application illustrated is to high molecular Rydberg states where a large basis size is required for convergence because, at times of experimental interest for ZEKE spectroscopy, many zero-order states have been accessed.
