Coherent interactions in two-beam excite-probe absorption measurements: Phase gratings in flowing samples

As a rule, phase gratings yield no power transfer in two-beam excite-probe experiments. We show that an exception to this rule is that of ultrashort-pulse excite-probe experiments involving slow phase effects, such as thermal gratings, occurring in a moving sample medium under high-repetition-rate excitation. A symmetric coherent artifact results, and depending on the direction of the sample motion, its amplitude can be positive, negative, or zero. This artifact occurs only in multiple-pulse experiments, involving accumulated gratings, while, on the other hand, the antisymmetric artifact (described by Palfrey and Heinz) results only from single-pulse effects. We discuss these two complementary effects and present both theory and experiments.This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Chemical Sciences Division     

直接键连原子间的NMR偶合常数的自然杂化轨道研究V.~(13)C-~(13)C和~(13)C-~(31)P核自旋偶合常数~1J_(CC)和~1J_(CP)

在前文工作的基础上，结合ＭＮＤＯ／ＥＨＭＯ分子轨道方法和自然杂化轨道方法，具体计算了ＣＣ键和ＣＰ键的核自旋偶合常数．计算结果表明，１ＪＣＣ和１ＪＣＰ主要由成键原子的轨道杂化作用和键极性这两种结构因素所决定．为从简单价键理论角度解释和计算１ＪＣＣ和１ＪＣＰ值提供了简便直观的方法．     

Early stages in the oxidation of magnesium,aluminium and magnesium/aluminium alloys: I. Exoelectron emission and long wavelength photoemission

The initial stages in the oxidation of magnesium, aluminium and magnesium/aluminium alloys have been studied using a photoelectron spectrometer (XPS and exoelectron energy spectra) and the diode technique (surface potentials and volumetric adsorption of oxygen). This paper describes and characterises the exoelectron emission and the visible wavelength photoemission which occurs during oxidation. At low oxygen exposures (< 10 L), exoelectrons with a 4 eV energy spread are emitted in the dark from magnesium and magnesium/aluminium alloy. Aluminium shows no such emission. Another, dark pressure-dependent emission of exoelectrons occurs only on magnesium/aluminium alloy and aluminium at high oxygen exposures. The onset of this emission lies 3 eV higher and it again has a 4 eV spread. When the surface is illuminated, it is possible to distinguish photo/exoelectron emission, which occurs during oxygen uptake, and true photoemission which occurs in vacuo. Both these types of emission, which were only recorded on magnesium, show a resolved triplet. Photoelectric work functions as low as 0.3 eV were observed. A patchy surface evidently develops during oxidation. This process, which may correspond to the nucleation of oxide islands, is slower than the chemisorption of of oxygen. The nature of the low work function patches, which we associate with the electron emissions, is therefore very dependent on the oxygen ambient pressure. A tentative energy scheme is given for such “exopatches” which are neither metal nor oxide and contain a curious energy level that must be situated above the vacuum level of the patch, as well as above the metal Fermi level. The energy with which exoelectrons are emitted derives from the heat of adsorption.     

Franck-Condon analysis of predissociations in the lower valence states of the NH radical

The origins of the predissociations observed by Smith, Brzozowski, and Erman for the A³Πand c¹Π states of NH have been investigated via a Franck-Condon analysis. It is shown that the measured variation of the predissociation probability is is proportional to the Franck-Condon densities between the bound rotational levels of the A and c states and the vibrational continuum of a ⁵Σ^- state arising from the ground state asympote. Similar calculations for the X³Σ^- ground state vibrational continuum, the only other state with a lower dissociation limit, showed that state to be ineffective in producing the observed predissociation.     

Security of quantum bit string commitment depends on the information measure

Unconditionally secure nonrelativistic bit commitment is known to be impossible in both the classical and the quantum world. However, when committing to a string of n bits at once, how far can we stretch the quantum limits? In this Letter, we introduce a framework of quantum schemes where Alice commits a string of n bits to Bob, in such a way that she can only cheat on a bits and Bob can learn at most b bits of information before the reveal phase. Our results are twofold: we show by an explicit construction that in the traditional approach, where the reveal and guess probabilities form the security criteria, no good schemes can exist: a + b is at least n. If, however, we use a more liberal criterion of security, the accessible information, we construct schemes where a = 4log2(n) + O(1) and b = 4, which is impossible classically. Our findings significantly extend known no-go results for quantum bit commitment.     

Combinatorial approach to the study of particle size effects in electrocatalysis: synthesis of supported gold nanoparticles

A high-throughput method for physical vapor deposition has been applied to the synthesis of libraries of supported gold particles on amorphous substoichiometric TiO(x)() and carbon supports. The TiO(x)() substrate stoichiometry can be varied or kept constant across a supporting sample, and subsequent deposition of particle sizes on supports are controlled through the nucleation and growth process. TEM measurements indicate nucleation and growth of Au particles takes place, with the smallest particles initially observed at 1.4 nm with a maximum density of 5.5 x 10(12) cm(-2) on titania, and 2.6 nm with concomitantly lower density on carbon. The 1.4-nm particles on titania exhibit a binding energy shift in the Au(4f) core level of 0.3 eV from bulk gold, and the shift is approximately 0.1 eV by the time particles grow to a mean size of 2.5 nm. These shifts are associated with final state effects, and the supported gold particles are metallic and appear to be relatively stable in air. When combined with appropriate substrates and screening techniques, this method provides a highly controllable method for the high-throughput synthesis of model supported catalyst.     

Physical vapor deposition method for the high-throughput synthesis of solid-state material libraries

A method that combines co-evaporation of pure elements from multiple finite-size sources on temperature-controlled substrates with independently controlled source shutters has been used for the synthesis of solid-state material combinatorial libraries. The source shutters are positioned to achieve a controlled gradient of the deposited elements across the substrate and are fixed during the course of deposition. Choice of the shutter position and the rate of deposition for each source allow the direct synthesis of continuous and controlled materials of varying composition. There are significant advantages of the method over alternatives which rely on sequential deposition and subsequent heat treatment to produce thin film materials. The parameters governing the creation of gradients have been identified and defined. Simulations and experimental data have been compared in the case of a single source. Results are presented for the synthesis of a ternary alloy library to demonstrate the methodology.     

Hinge peptide combinatorial libraries for inhilbitors of botulinum neurotoxins and saxitoxin: Deconvolution strategy

Combinatorial library screening offers a rapid process for identifying potential therapies to toxins. Hinge peptide libraries, which rely on conformational diversity rather than traditional molecular diversity, reduce the need for huge numbers of syntheses and screening steps and greatly expedite the discovery process of active molecules. Hinge peptide libraries having the structures: Acetyl-X₁–X₂–hinge–X₃–X₄–NH₂ (capped) and X₁–hinge–X₂–X₃ (uncapped), where X₁ through X₄ are near-equimolar mixtures of twelve L-amino acids and hinge = 4-aminobutyric acid, were screened for inhibitory activity in bioassays for botulinum neurotoxins A and B (BoNT/A, BoNT/B) and saxitoxin. The zinc protease activity of the reduced light chains of BoNT/A and /B was assayed by measuring the cleavage of synthetic substrates. Saxitoxin activity was measured by the restoration of the viability of neuroblastoma cells treated with ouabain and veratridine. Deconvolution of libraries was accomplished by fixing one position at a time beginning with the C-terminus. Primary library subsets in which position 4 was fixed showed moderate levels of inhibition for BoNT/A. Secondary library subsets showed stronger inhibition in the bioassays. In each of the bioassays, inhibitory potency was stronger when the second position to be fixed was on the opposite side of the hinge, rather than on the same side with respect to the C-terminus, suggesting that the hinge facilitates the interaction of side chains. Inhibitors for all three of the toxins studied were discovered within library subsets, although not necessarily in primary subsets. These studies demonstrate that (1) the best strategy for deconvoluting hinge peptide libraries is by fixing residues alternately on each side of the hinge moiety, and (2) it is essential to investigate secondary subsets even when primary subsets are inactive. The present findings support the concept that the increased flexibility imposed by the inclusion of a central hinge residue in small peptides increases the opportunity for side chain interactions, providing a distinct advantage for hinge peptide libraries over conventional peptide libraries. Hinge peptide libraries are a rich source of novel ligands for modulation of biomechanisms. The library subsets uncovered in this study may possess peptides that will lead to effective therapies to neurotoxin poisoning.     

Emission Spectra Excited in Benzene

Recently, an emission from molecular benzene has been observed in our electron beam excitation source when benzene vas used as a parent molecule for the C₂ molecule¹ the primary subject of our study. Due to the discussion in this journal ^2,3 concerning emission excited in a benzene system as observed by Robinson and Rosamund⁴, it was felt that our results should be communicated to dispel any ambiguity that may remain for this emission system.