Tandem Mass Spectrometry of Trimethylsilyl-Terminated Poly(Dimethylsiloxane) Ammonium Adducts Generated by Electrospray Ionization

Ammonium adducts of trimethylsilyl-terminated poly(dimethylsiloxane) (CH₃-PDMS) produced by electrospray ionization were submitted to collision induced dissociation and revealed a particular MS/MS behavior: the same three main product ions at m/z 221, 295, and 369 were always generated in very similar relative abundances regardless of the size of the precursor ion. Combining accurate mass measurements and ab initio calculation allowed very stable cyclic geometries to be obtained for these ionic species. Dissociation mechanisms were proposed to account for the three targeted ions to be readily generated in a two-step or a three-step reaction from any CH₃-PDMS ammonium adducts. A second set of three product ions was also observed with low abundance at m/z 207, 281, and 355, which were shown in MS³ experiments to be formed in secondary reactions. An alternative dissociation process was shown to consist of a concerted elimination of ammonia and methane and the need for a methyl of an end-group to be involved in the released methane molecule would account for this reaction to mainly proceed from the smallest precursor ions.     

Neutron spin-echo investigation of slow spin dynamics in kagomé-bilayer frustrated magnets as evidence for phonon assisted relaxation in SrCr9xGa12-9xO19

A neutron spin-echo investigation of the low temperature spin dynamics in two well-characterized kagomé bilayer compounds SrCr9xGa12-9xO19 (x=0.95, SCGO) and Ba2Sn2ZnCr7xGa10-7xO22 (x=0.97, BSZCGO) reveals two novel features. One is the slowing down of the relaxation rate without critical behavior at Tg, where a macroscopic spin-glass-like freezing occurs. The second is, in SCGO at 4 K (approximately Tg)相似文献   

Tracking the effects of rigidity percolation down to the liquid state: relaxational dynamics of binary chalcogen melts

The stochastic dynamics of binary liquids with formula AxB1-x, x=0-0.4 is investigated by neutron spin-echo spectroscopy. These compositions comprise samples of varying chemical connectivity, ranging from twofold-coordinated liquid Se to higher average coordinated As2S3. The parameters giving the temperature dependence of the relaxation patterns show a quasilinear dependence on the average coordination number. The results thus extend the validity of the rigidity concept into the normal liquid state and emphasize the role played by the fine details of atomic bonding on the dynamics at 10 ps-1 ns scales.     

A Conservative Space-time Mesh Refinement Method for the 1-D Wave Equation. Part II: Analysis

Summary. We introduced in [2] a new method for space-time refinement for the 1-D wave equation. This method is based on the conservation of a discrete energy through two different discretization grids which guarantees the stability of the scheme. In this second part, we analyse the accuracy of this scheme in a detailed way by means of a plane wave analysis and numerical experiments that permit us to point out spurious numerical phenomena and explain how to control them. Mathematics Subject Classification (2000):65N12     

Observation of microscopic CO dynamics on Cu(001) using 3He spin-echo spectroscopy

We present momentum resolved measurements of quasielastic helium atom scattering made using a new 3He spin-echo spectrometer. Our data for the dynamics of CO on Cu(001) indicates an activated jump mechanism which we analyze in detail using molecular dynamics simulations. A nearly isotropic potential energy surface is found with an average barrier height of approximately 125 meV, yielding comparable hopping rates along both the <110> and <100> directions. The measurements provide the first rigorous experimental test of state-of-the-art first-principles calculations previously made on this system.     

A refined He-LiF(001) potential from selective adsorption resonances measured with high-resolution helium spin-echo spectroscopy

The authors have developed a new experimental approach for measuring gas-surface selective adsorption resonances with much higher energy resolution and over a wider range of kinematic conditions than has previously been possible. The technique involves using a 3He spin-echo spectrometer as a Fourier transform helium atom scattering apparatus. The authors applied the technique to the He-LiF(001) system. They developed a new empirical potential for the He-LiF(001) system by analyzing and refining the best existing potentials in the light of the new data set. Following an initial free-particle model analysis, the authors used exact close coupling scattering calculations to compare the existing potentials with the new experimental data set. Systematic differences are observed between the two. The existing potentials are modified by simple transformations to give a refined potential that is consistent with and fully reproduces the experimental data. Their technique represents a new approach for developing very high precision empirical potentials in order to test first principles theory.     

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