Ion and Electron Emission from Liquid Metal Sources

In liquid metal ion sources, the emission is located at the apex of a liquid cone (the often so-called Taylor cone), formed by electrostatic forces and surface tension. Reversal of the extraction voltage polarity results in electron emission from the liquid metal surface. For small apex radii, ≤ 1 μm, steady field emission of electrons has been observed, whereas for apex radii ≥ 10 μm, explosive pulsed emission occurs. Since the onset voltage for electron emission has been found to be considerably lower than the critical voltage for the formation of the Taylor cone, it has been concluded that dc (direct current) electron emission from a field stabilized cone is not possible. In situ high-voltage transmission electron microscopy observations seem to confirm this conclusion, although in one case, a field-stabilized liquid micro-cone during electron emission has been observed for a liquid-gallium-indium-tin source. The literature on liquid metal ion and electron sources is reviewed. From in situ transmission electron microscopy observations of the cone formation, relations for cone angle and jet length dependent on ion emission current are derived. Growth and emission of microdroplets at liquid indium ion sources have been imaged. In the case of electron field emission from liquid indium sources, no liquid cone formation has been observed

A Computer Controlled Scanning Transmission Electron Microscope Equipped with an Energy Analyzer for Special Investigations on Electron Diffraction- and Channeling Patterns

A scanning electron microscope was equipped with a double tilting stage, driven by stepping motors, to investigate electron channeling patterns (ECPs) and large angle convergent beam patterns (LACBPs) of single crystals. Transmitted electrons may be energy-selected by a magnetic sector-field energy analyzer. The recording of experimental data and the experimental arrangement are controlled by a microprocessor system, including a picture storage unit of 512 x 512 pixels of 16 bit. Recorded patterns can be stored on 1 Megabyte floppies. A set of useful programs allows one to perform calculations with stored patter ns, e.g., contrast enhancement or -inversion, noise reduction, difference or quotient of two patterns etc. The possibility of background subtraction (e.g., in patterns recorded with characteristic energy loss electrons) allows one to get true K-loss convergent beam patterns. Other recording modes allow one to get two CBPs simultaneously recorded with electrons of different energy losses, to measure angle dependences of energy selected electrons, or to take electron energy loss spectra. A special processor program generates a theoretically calculated CBP or ECP on the TV screen and prints out a list of all band edges up to a chosen limit of Miller indices (hkl). The program requires the coordinates of two known poles and some crystallographic properties of the investigated material. Thus complete indexing of recorded diffraction patterns is easily possible. The system has been applied, e.g., to investigate localization effects of electron Bloch-waves in graphite

Immunological reactivity of a human immunodeficiency virus type I derived peptide representing a consensus sequence of the GP120 major neutralizing region V3

To reduce the opportunities for human immunodeficiency virus type 1 (HIV-1) to evade vaccine induced immunity, the development of subunit vaccines must focus on the characterization of immunogenic epitopes, which are major targets for the immune system. The most dominant site for elicitation of neutralising immune response is located on the external envelope glycoprotein gp120 within the third variable domain (V3). To overcome virus type specificity of antibodies directed to the V3-domain we designed a 36 amino acids long gp120/V3-consensus peptide (V3-C36) based on published biological data and sequence comparisons of various HIV-1 virus isolates. This peptide contains a conserved core sequence which is suggested to form a surface-exposed beta-turn. This peptide also includes T-cell epitopes defined in mice and humans, an ADCC-epitope and two highly conserved cysteine residues which were oxidized to form a cystine derivate, thus allowing correct peptide folding. In ELISA-tests, this peptide reacts with at least 90% of randomly selected sera of European and African patients infected with HIV-1 and is recognized by three different HIV-1/V3 "type-specific" antisera (MN, RF, IIIB-strain). Using this peptide as immunogen in rabbits, antisera could be raised with highly cross-reactive and HIV-1/IIIB strain neutralizing properties. Moreover, HTLV/HIV-1/IIIB specific cytotoxic T-lymphocytes (CTLs) of BALB/c mice infected with a gp120 recombinant vaccinia virus recognized the central 16- and 12-mer peptides of the V3-C36 consensus peptide in cytolytic assays, indicating perfect compatibility of the consensus peptide with the IIIB-primed CTLs. The DNA-sequence encoding the V3-consensus loop region might be an important component in newly designed recombinant subunit vaccines. In addition, due to its broad serological reactivity, the V3-consensus peptide might play an important role in special diagnostic purposes

Studies on processing, particle formation, and immunogenicity of the HIV-1 gag gene product: a possible component of a HIV vaccine

Antigens in a particulate conformation were shown to be highly immunogenic in mammals. For this reason, the particle forming capacity of derivatives of the HIV-1 group specific core antigen p55 gag was assayed and compared dependent on various expression systems: recombinant bacteria, vaccinia- and baculoviruses were established encoding the entire core protein p55 either in its authentic sequence or lacking the myristylation consensus signal. Moreover, p55 gag was expressed in combination with the protease (p55-PR) or with the entire polymerase (p55-pol), respectively. Budding of 100-160 nm p55 core particles, resembling immature HIV-virions, was observed in the eucaryotic expression systems only. In comparison to the vaccinia virus driven expression of p55 in mammalian cells, considerably higher yields of particulate core antigen were obtained by infection of Spodoptera frugiperda (Sf9) insect cells with the recombinant Autographa californica nuclear polyhedrosis (AcMNPV) baculovirus. Mutation of the NH2-terminal myristylation signal sequence prevented budding of the immature core particles. Expression of the HIV p55-PR gene construct by recombinant baculovirus resulted in complete processing of the p55 gag precursor molecule in this system. The introduction of an artificial frameshift near the natural frameshift site resulted in constitutive expression of the viral protease and complete processing of p55, both in Escherichia coli and in vaccinia virus infected cells. Interestingly, significant processing of p55 resembling that of HIV infected H9 cells could also be achieved in the vaccinia system by fusing the entire pol gene to the gag gene. Moreover, processing was not found to be dependent on amino-terminal myristylation of the gag procursor molecule, which is in contrast to observations with type C and type D retrovirus. However, complete processing of p55 into p24, p17, p9 and p6 abolished particle formation. Purified immature HIV-virus like particles were highly immunogenic in rabbits, leading to a strong humoral immune response after immunization. Empty immature p55 gag particles represent a noninfectious and attractive candidate for a basic vaccine component

Electronic structure of barium-doped C₆₀

We have investigated the electronic structure of Ba-doped C60 films with Ba concentrations of up to x≈12 (BaxC60) by applying valence-band photoemission and x-ray-absorption spectroscopy. A crystal orbital (CO) formalism based on a semiempirical Hamiltonian of the intermediate-neglect-of-differential-overlap type has been employed to derive solid-state results for the Ba-doped C60 fullerides. Using x-ray diffraction, we show three distinct phases for the bulk BaxC60 system with Ba concentrations of up to x=6. In all cases, the experimental observations strongly indicate that fulleride formation leads to the occupation of hybrid bands on both sides of the Fermi level. The theoretical data indicate that the alkaline-earth atoms are essentially monovalent and hybridize strongly with the π-type functions of the C60 network. The Ba atoms in the BaxC60 fullerides deviate from the limit of complete charge transfer as a consequence of the competition between covalent Ba-C60 bonding and ionic contributions. Furthermore, it is shown that the calculated density-of-state profiles reproduce the photoemission data in the extreme outer valence-band region

Lifetimes of ultralong-range Rydberg molecules in vibrational ground and excited state

Since their first experimental observation, ultralong-range Rydberg molecules consisting of a highly excited Rydberg atom and a ground state atom have attracted the interest in the field of ultracold chemistry. Especially the intriguing properties like size, polarizability and type of binding they inherit from the Rydberg atom are of interest. An open question in the field is the reduced lifetime of the molecules compared to the corresponding atomic Rydberg states. In this letter we present an experimental study on the lifetimes of the ^3\Sigma (5s-35s) molecule in its vibrational ground state and in an excited state. We show that the lifetimes depends on the density of ground state atoms and that this can be described in the frame of a classical scattering between the molecules and ground state atoms. We also find that the excited molecular state has an even more reduced lifetime compared to the ground state which can be attributed to an inward penetration of the bound atomic pair due to imperfect quantum reflection that takes place in the special shape of the molecular potential

Electronic Structure of the C₆₀ Fragment in Alkali- and Alkaline-earth-doped Fullerides

The electronic structure of the C60 fragment in alkali- and alkaline-earth-doped fullerides is studied theoretically. With increasing metal-to-C60 charge transfer (CT) the n electronic properties of the soccerball are changed. In the undoped solid and for not too high a concentration of doping atoms the hexagon-hexagon (6-6) bonds show sizeable double bond character while the hexagon-pentagon (6-5) bonds are essentially of single bond type. In systems with a high concentration of doping atoms this relative ordering is changed. Now the 6-5 bonds have partial double bond character and the 6-6 bonds are essentially single bonds. The high ability of the C60 unit to accomodate excess electrons prevents any sizeable weakening of the overall n bonding in systems with up to 12 excess electrons on the soccerball. A crystal orbital (CO) formalism on the basis of an INDO (intermediate neglect of differential overlap) Hamiltonian has been employed to derive solid state results for potassium- and barium-doped C60 fullerides. For both types of doping atoms an incomplete metal-to-C60 CT is predicted. In the potassium-doped fullerides the magnitude of the CT depends on the interstitial site of the dopant. The solid state data have been supplemented by INDO and ab initio calculations on molecular C60, C6-60 and C12-60. The calculated bondlength alternation in the neutral molecule is changed in C12-60 where the length of the 6-6 bonds exceeds the length of the 6-5 bonds. The geometries of the three molecular species have been optimized with a 3-21 G* basis. The theoretically derived modification of the C60 (π) electronic structure as a function of the electron count is explained microscopically in the framework of two quantum statistics accessible for π electronic ensembles. In the π ensemble of the C60 fragment so-called hard core bosonic properties are maximized where the Pauli antisymmetry principle has the character of a hidden variable only. Here the electronic degrees of freedom are attenuated only by the Pauli exclusion principle. This behaviour leads to the changes in the π electronic structure mentioned above

Sustained Water Oxidation by Direct Electrosynthesis of Ultrathin Organic Protection Films on Silicon

Artificial photosynthesis allows exceeding the efficiency and stability limits of natural photosynthesis. Based on the use of semiconducting absorbers, high efficiency in water photolysis has been achieved in various photoelectrode configurations. However, integrated systems are limited in their stability, and more stable half-cell electrodes use protection films prepared by laborious methods. Herein, the facile low-temperature preparation of ultrathin organic protection coatings is demonstrated. The formation is based on the catalytic properties of water oxidation catalysts toward alcohol-polymerization reactions, which results in the formation of hitherto unknown protection layers on silicon. The interfacial layers are generated via iodine-mediated electro-reductive polymerization of ethanol, concomitantly forming during electrophoretic transport of RuO_2 onto silicon supports. Reaction chemistry analyses show that the RuO_2-induced catalysis introduces E2-elimination reactions which result in a carbon sp^3 –sp^2 transformation of the film. For the two modes of photoelectrochemical operation, the photovoltaic and the photoelectrocatalytic mode, 20 and 15 mA cm^(−2) photocurrent densities, respectively, are obtained with unaltered output for 8 and 24 h. The interfacial layer enables Si photovoltages of 500 mV, demonstrating extraordinary electronic interface quality. Since only hydrogen termination of the surface is a prerequisite for growth of the organic protection layer, the method is applicable to a wide range of semiconductors