Local measurement of hot-electron phase-coherence at metal surfaces

We use scanning tunneling microscopy and spectroscopy to study the energy dependence of hot-electron scattering processes on metal surfaces via the determination of the energy-dependent phase-coherence length. From this an electron lifetime can be determined, which in the case of electrons in the surface state of Ag(111) and in the case of the n=1 image-potential state on Cu(100) shows good agreement with theoretical modeling and other experimental data. The method is based on the quantitative analysis of electron interference patterns. A theoretical analysis shows that the phase-coherence length can be determined in confining nanostructures of a characteristic length scale smaller than the phase-coherence length. PACS 73.20.At; 73.50.Gr; 72.10.Fk; 68.37.Ef     

Electrochemically decorated carbon nanotubes for hydrogen sensing

Low-density networks of single-wall carbon nanotubes have been modified by palladium nanoparticles using an electrochemical method. A major advantage of this approach is that it allows for selective metal deposition on the electrically contacted nanotubes, whereas the remaining substrate surface and the non-contacted tubes remain essentially unaffected. The Pd-decorated networks function as effective hydrogen sensors enabling the detection of hydrogen concentrations as low as 10 ppm at room temperature. The electrochemical metal deposition scheme is promising for the development of sensor arrays suitable for analysing gas mixtures.     

Parallel information-based complexity

We develop the theory of information-based complexity from a parallel point of view. For a model of computation with p processors, each being capable of arithmetic operations and decisions, we analyze the complexity of function approximation, numerical integration, and solution of Fredholm integral equations. We obtain tight bounds on the complexity, considered as a function of three variables simultaneously: the number of processors, the required precision, and (in the case of approximation and integral equations) the number of points, in which the approximate solution is to be determined.     

Suppressing decoherence of quantum algorithms by jump codes

The stabilizing properties of one-error correcting jump codes are explored under the realistic non-ideal condition of finite recovery times. For this purpose the quantum algorithm of the tent-map is decomposed into a universal set of Hamiltonian quantum gates which ensure perfect correction of spontaneous decay processes under ideal circumstances even if they occur during a gate operation. An entanglement gate is presented which is capable of entangling any two logical qubits of different one-error correcting code spaces. With the help of this gate simultaneous spontaneous decay processes affecting physical qubits of different code spaces can be corrected and decoherence can be suppressed significantly.     

Transition-metal-templated synthesis of rotaxanes and catenanes: From small molecules to polymers

This article provides an overview of the fundamental principles of the synthesis of metallocatenanes and metallorotaxanes. It also describes the synthesis and properties of electronic conducting polymers—polypyrrole and polythiophene—built around metallocatenanes and metallorotaxanes. The particular properties of this new class of polymers, including the possibility of transmetallation reactions being performed with them and the observation of electronic coupling between the metal centers and the conducting matrix, are discussed. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3470–3477, 2003     

Hydridoorganostannylene Coordination: Group 4 Metallocene Dichloride Reduction in Reaction with Organodihydridostannate Anions

Organodihydridoelement anions of germanium and tin were reacted with metallocene dichlorides of Group 4 metals Ti, Zr and Hf. The germate anion [Ar*GeH₂]⁻ reacts with hafnocene dichloride under formation of the substitution product [Cp₂Hf(GeH₂Ar*)₂]. Reaction of the organodihydridostannate with metallocene dichlorides affords the reduction products [Cp₂M(SnHAr*)₂] (M=Ti, Zr, Hf). Abstraction of a hydride substituent from the titanium bis(hydridoorganostannylene) complex results in formation of cation [Cp₂M(SnAr*)(SnHAr*)]⁺ exhibiting a short Ti–Sn interaction. (Ar*=2,6-Trip₂C₆H₃, Trip=2,4,6-triisopropylphenyl).     

Application of a universal force field to mixed Fe/Mo-S/Se cubane and heterocubane clusters. 1. Substitution of sulfur by selenium in the series [Fe4X4(YCH3)4]2-; X = S/Se and Y = S/Se

A series of Fe-S and Fe-Se cubane clusters containing all four combinations of the general formula [Fe(4)X(4)(Y-CH(3))(4)](2)(-) (X = S/Se, Y = S/Se) is investigated with FTIR and Raman spectroscopy. The terminally selenolate coordinated clusters (Y = Se) are prepared by a new synthetic route. All four cluster compounds are structurally characterized by X-ray single-crystal structure determination. Infrared and Raman spectra of all compounds are presented and interpreted with normal coordinate analysis. The corresponding force fields are based on that developed for the Fe(4)S(4)-benzyl cluster (Czernuszewicz, R. S.; Macor, K. A.; Johnson, M. K.; Gewirth, A.; Spiro, T. G. J. Am.Chem. Soc. 1987, 109, 7178-7187). An empirical procedure is presented to convert Fe-S into Fe-Se force constants. Only minor changes in force constants are found upon S --> Se exchange, reflecting the similarity of the Fe-S and Fe-Se bonds. The drastic frequency shifts in the metal-ligand region observed upon substitution of sulfur by selenium are, therefore, primarily due to the corresponding mass changes.     

The Determination Of Sub Part-Per-Billion Levels Of Volatile Organic Compounds In Air By Pre-Concentration From Small Sample Volumes

Abstract

Trace level Volatile Organic Compounds (VOCs) in ambient air are normally determined according to EPA Method TO-14. This method describes the analysis in ambient air of 41 VOCs, ranging in boiling point from -29 to 215°C. It covers a concentration range from 0.2 to 20 parts per billion, volume/volume (ppb), and specifies sample enrichment of a 400 mL air sample on glass beads at -160°C. While this sample volume provides sub-ppb levels of VOC detection for target analytes when using a quadrupole mass spectrometer detector in SIM mode or when using GC detectors, the identification of non-target analytes may only be done in full scan mode for higher concentrations. Also with this sample volume a Nafion dryer is needed for water removal thereby lowering the recovery of polar VOCs.

Because of the very high sensitivity of the ion trap MS, relatively small air volumes (60 mL) are adequate to obtain the required or lower detection levels. An integrated air analysis system based on a GC-ion-trap MS has been investigated and is described. This system has a built-in cryogenic trap and necessary valving, internal standard gas sampling loop, and is controlled from the GC-MS workstation. The linearity, precision, and method detection levels obtainable with this system when using small volumes are reported. In addition, examples of the quantitative and qualitative analysis of ambient air samples are shown.     

Effects of Adsorbed Polyaniline on Redox Process on MoO3 Surface

The effects exhibited by adsorbed conducting polyaniline on the redox process on a molybdenum oxide surface were studied. Thermogravimetric results indicate a 4% polyaniline deposition. Cyclic voltammograms of the adsorbed polymer on MoO3 show that polyaniline exerts remarkable effects on the molybdenum blue oxidation-reduction process, with oxidation and reduction potentials of 0.33 and 0.18 V, respectively. This effect strongly enhances the electrode response, and can be used as an important tool in qualitative and/or quantitative determinations of molybdenum in solution as well as in any substrate. Copyright 1999 Academic Press.