Shaping of intense ion beams into hollow cylindrical form

A specifically tailored plasma lens could shape a high-energy, heavy-ion beam into the form of a hollow cylinder without loss of beam intensity. It has been experimentally confirmed that both a positive as well as a negative radial gradient of the current density in the active plasma lens can be the underlying principle. Calculations were performed that yield the ideal current density distribution for both cases. A numerical simulation of an experiment with an intense ion beam highlights that the shaping of the beam increases the achievable compression in a lead sample.     

Low temperature scanning force microscopy of the Si(111)-(7x7) surface

A low temperature scanning force microscope (SFM) operating in a dynamic mode in ultrahigh vacuum was used to study the Si(111)- (7x7) surface at 7.2 K. Not only the twelve adatoms but also the six rest atoms of the unit cell are clearly resolved for the first time with SFM. In addition, the first measurements of the short range chemical bonding forces above specific atomic sites are presented. The data are in good agreement with first principles computations and indicate that the nearest atoms in the tip and sample relax significantly when the tip is within a few A of the surface.     

Mikrochemie

Ohne Zusammenfassung     

A comparison of intermolecular vibrations and tautomerism in benzimidazole, benzotriazole and their binary water clusters

Resonance-enhanced two-photon ionization spectra of benzotriazole, benzimidazole and their water clusters have been taken in a molecular beam. The tautomerism of the benzotriazole and benzimidazole monomers is studied. In order to obtain an assignment of the vibronic bands of the benzotriazole- and benzimidazole-water clusters, ab initio calculations have been performed, which allow the assignment of the intermolecular vibrations and the determination of the most stable cluster structures. In the case of benzotriazole the cyclic cluster of the 2H-tautomer has been found to be the most stable one, whereas for benzimidazole the most stable cluster could be shown to have a linear arrangement, with the benzimidazole moiety acting as proton donor. Received: 2 March 2000 / Revised version: 31 May 2000 / Published online: 5 October 2000     

Surface-enhanced raman scattering from poly(4-vinyl pyridine)

Surface-enhanced Raman scattering (SERS) has been observed for poly(4-vinyl pyridine) absorbed onto silver island films. Bands near 1219 and 1613 cm^?1, which are weak in normal Raman spectra of PVP, are strong in SERS spectra, and the band near 1020 cm^?1, which is the strongest band in the normal spectra, is relatively weak in SERS. The strongest bands in the SERS spectra all belong to the same symmetry species as α_ZZ, implying that the pyridine moieties are adsorbed through the nitrogen atoms with a vertical conformation. The ring breathing mode of the pyridine rings is observed near 1020 cm^?1, a frequency characteristic of pyridinium ions or coordinated pyridine, providing further evidence for adsorption through the nitrogen atoms. Silver catalyzed photooxidation, which can lead to the appearance of artifacts in SERS spectra, particularly of polymers, can be reduced by overcoating SERS samples with thin films of polymers such as poly(methyl methacrylate) that have low Raman scattering cross sections.