Udo Werner

A system for correlated fragment detection in dissociation experiments

Abstract We describe a detector system for the study of the dissociation dynamics of small molecules, clusters or molecular ions. The system is based on a two-dimensional multiparticle detector which allows the position-sensitive and time-correlated detection of all fragments produced in one particular dissociation process. As a test example we discuss the double ionization and ensueing fragmentation of H 2 by fast H + and He + ions, in which case our detector allows an absolute measurement of the dissociation energy of the two correlated H + -fragments. The results are in excellent agreement with the prediction of a Franck-Condon calculation.

Coster-Kronig and fluorescence yields of Au L subshells derived from photoionisation measurements

Photoionisation studies provide a direct method for an accurate determination of vacancy decay parameters. The authors report on such measurements performed for the Au L shell. Thin Au foils were irradiated by photons with energies ranging from 11.5 to 15.2 keV. The mass absorption coefficient as well as the induced L X-radiation were measured. From the absorption coefficient the ionisation cross sections of the individual L subshells were derived. Comparison of the cross sections and the induced X-ray fluorescence gives Coster-Kronig factors, fluorescence yields and branching ratios for different X-ray lines. For the Coster-Kronig factors and fluorescence yields significant deviations from previously adopted values were found.

Fluorescence, Coster-Kronig and Auger yields of the 62Sm L subshells measured with the synchrotron photoionization method

Using monochromatized synchrotron radiation a sample of thin samarium and titanium foils was excited, and the induced Sm L X-rays and Ti K X-rays were detected by a Si(Li) detector. When the primary photon energy is tuned across the Sm L2 and L1 edges, the intensities of the Sm L X-ray lines normalized to the Ti K alpha intensity exhibit abrupt changes. From these intensity changes the Coster-Kronig factors were determined: f23=0.14+or-0.03, f13=0.18+or-0.03 and f12=0.19+or-0.03. The value of f13 is substantially smaller than predicted by theory and extrapolated from heavier elements. Furthermore, from the total intensities of all lines originating from the same Sm L subshell the ratios of the fluorescence yields omega 1/ omega 3=0.48+or-0.07 and omega 2/3=1.05+or-0.05 were obtained.

Fabrication of nanopores in 1 nm thick carbon nanomembranes with slow highly charged ions

We describe the use of slow highly charged ions as a simple tool for the fabrication of nanopores with well-defined diameters typically between 10 and 20 nm in freestanding, 1 nm thick carbon nanomembranes (CNMs). When CNMs are exposed to a flux of highly charged ions, for example Xe40+, each individual ion creates a circular nanopore, the size of which depends on the kinetic and potential energy of the impinging ion. The controlled fabrication of nanopores with a uniform size opens a path for the application of CNM based filters in nanobiotechnology.

ION-IMPACT-INDUCED FRAGMENTATION OF WATER-MOLECULES

Abstract The multiple ionization and fragmentation of H 2 O by fast H + , He + , O 6+ and O 7+ ions was studied utilizing a position-and time-sensitive multi-particle detector which allows the coincident measurement of the momenta of correlated fragments. Thereby, the fragmentation energy and angular correlations for each individual event can be derived. Of special interest are the processes H 2 O → H + + H + + O q + , where a simple Coulomb-explosion model is insufficient to explain the measured energy and angular spectra. Better agreement was achieved with ab initio MCSCF calculations which take into account several molecular states of the fragmenting highly charged H 2 O ( q +2)+ ion.

Multi-fragmentation of C60 after collisions with Arz+ ions

We studied the fragmentation of C60 molecules in collisions with slow Ar+ ions (v ≈ 0.04–0.55 au) and with Arz+ ions (z = 2, 3) at somewhat higher velocities (v ≈ 0.3–0.95 au). The measured mass spectra show C60q+ ions (q ≤ 5), smaller C60−2mq+ fullerenes and C60−2mq+ + C n+ fission products. Special attention is devoted to the C60 multi-fragmentation process, i.e. the breakup of the C60 cage into several small Cn+ fragments (n = 1–29). Up to seven correlated small Cn+ fragments (n = 1–5) have been observed. The multi-fragmentation pattern depends on the projectile charge. Furthermore, it varies strongly with the projectile velocity for slow ions, whereas it becomes nearly constant for faster ions. This is in accordance with recent non-adiabatic quantum molecular dynamic calculations.

Multiple ionization and fragmentation of H2O in collisions with fast highly charged Xe ions

The multiple ionization and fragmentation of H2O in collision with 5.9 MeV u-1 Xe17+, Xe18+ and Xe43+ ions was studied using a position- and time-sensitive multiparticle detector which allows the coincident measurement of the momenta of correlated fragment ions. If all fragments from a particular fragmentation are detected, a kinematically complete study of the molecular break-up process is possible. For these Coulomb explosion processes the kinetic energy as well as angular correlations are determined.

Protein resistant oligo(ethylene glycol) terminated self-assembled monolayers of thiols on gold by vapor deposition in vacuum

Protein resistant oligo(ethylene glycol) (OEG) terminated self-assembled monolayers SAMs) of thiols on gold are commonly used for suppression of nonspecific protein adsorption in biology and biotechnology. The standard preparation for these SAMs is the solution method (SM) that involves immersion of the gold surface in an OEG solution. Here the authors present the preparation of 11-(mercaptoundecyl)-triethylene glycol [HS(CH2)11(OCH2CH2)3OH] SAMs on gold surface by vapor deposition (VD) in vacuum. They compare the properties of SAMs prepared by VD and SM using x-ray photoelectron spectroscopy (XPS), polarization modulation infrared reflection absorption spectroscopy, and surface plasmon resonance measurements. VD and SM SAMs exhibit similar packing density and show a similar resistance to the nonspecific adsorption of various proteins bovine serum albumin, trypsin, and myoglobin) under physiological conditions. A very high sensitivity of the OEG SAMs to x-ray radiation is found, which allows tuning their protein resistance. These results show a new path to in situ engineering, analysis, and patterning of protein resistant OEG SAMs by high vacuum and ultrahigh vacuum techniques.

How molecules and clusters explode

Abstract The multiple ionization and fragmentation of small molecules, e.g. N2 and H2O, by fast H+, He+, Arq+ and Oq+ ions was studied utilizing a position- and time-sensitive multi-particle detector which allows the coincident measurement of the momenta of correlated fragments. Thereby, a kinematically complete image of the molecular break-up can be derived for each individual event. Of special interest are the “Coulomb explosion” processes like N2 → Np+ + Nq+ or H2O → H+ + H+ + Oq+. In these cases the pure Coulomb explosion model — though successful in case of H2 — turns out to insufficient to explain the measured energy and angular spectra. In case of H2O better agreement is achieved with ab initio MCSCF calculations of the intermediate H2O(q+2)+ parent ion.

Multiple ionisation and fragmentation of molecules

The multiple ionisation and dissociation of molecules, e.g. H2, N2, H2O, CH4 and C60, by fast ions was studied using a position- and time-sensitive multi-particle detector. The data obtained allow a clear separation of various reaction channels. Of special interest are the ‘Coulomb explosion’ processes like N2 → Nq+ +Nn+ or H2O → H+ + H+ +On+. For these reactions the coincident measurement of the momenta of correlated fragment ion yields a kinematically complete image of the molecular break-up process, and the fragmentation energy as well as angular correlations can be derived for each individual event. In the case of H2 as target molecule the CE model describes the fragmentation energy rather well, whereas in the case of more complex molecules, e.g. N2, CO and CH4, the simple CE model is insufficient to explain the measured energy and angular spectra. Better agreement was achieved with ab initio MCSCF calculations which take into account several molecular states of the fragmenting highly charged molecular ion.