Showing papers by "Lutz Schweikhard published in 1994"

Collision induced dissociation of stored gold cluster ions

Abstract: The stability of gold cluster ions Au + (2≦n≦23) has been investigated via collision induced dissociation in a Penning trap. Threshold energies and dissociation channels have been determined. The cluster stability exhibits a pronounced odd — even alternation: Clusters with an odd number of atoms,n, are more stable than the even-numbered ones. Enhanced stabilities are found for Au 3 + , Au 9 + , and Au 19 + in accordance with the Clemenger-Nilsson and the deformed jellium model of delocalized valence electrons. Excited odd cluster ions withn≦15 predominantly decay by evaporation of dimers; all others decay by monomer evaporation. From the dissociation channels estimates of the binding energies are deduced.

First Fourier-transform ion cyclotron resonance signals of very highly charged atomic ions

Abstract: We report on the first application of Fourier-transform ion cyclotron resonance mass spectrometry to the very highly charged ions produced in the high-energy electron beam ion trap (SuperEBIT) at the Lawrence Livermore National Laboratory. Initial results are described, and spectra are presented from ions as highly charged as Cs53+.

Au n+‐induced decomposition of N2O

Abstract: Reactions between small gold cluster ions, Au, and N2O were studied in a Penning trap mass spectrometer. Gold clusters were produced by laser vaporization and injected into a Penning trap. After reaction times of 50–7000ms the products were detected by time-of-flight mass spectrometry. For the major reaction channel, Au + N2OAu1,2N + NO+, rates of (0.9±0.1)×10−12 cm3 s−1 and (2.4±0.4)×10−12 cm3 s−1 were determined which are about a factor 500 below the collision rate. The corresponding activation energies for N2O decomposition were estimated to lie below 0.6 eV and 0.3 eV. Additional products with small branching ratios were detected, viz. the ions Au1O+, Au1N2O+, Au2N+, Au2NO+, Au2N2O+, Au3O+, Au3N2O+, and Au4N2O+. This indicates a preference for nitrogen-oxygen bond rupture in the case of Au and Au, and for nitrogen-nitrogen bond rupture in the case of Au.

Fragmentation of gold clusters stored in a penning trap

Abstract: The collision-induced dissociation of positively charged gold clusters (2 to 23 atoms) stored in a Penning trap has been studied. After collisions with rare gases, excited clusters predominantly decay by emission of one or two atoms. The loss of two atoms occurs most likely through the emission of a dimer rather than a sequential evaporation. The minimum kinetic energies of clusters required to induce dissociation exhibit a pronounced odd-even effect. Clusters with an even number of delocalized electrons are more stable than the odd ones.

Excitation and detection geometries for Fourier‐transform mass spectometry

Abstract: A short overview of current excitation and detection techniques for Fourier-transform mass spectrometry and related ion cyclotron resonance experiments is given, and some ideas are discussed which may lead to new developments.

Internal ion impact ionization for Fourier‐transform ion cyclotron resonance

Abstract: A general scheme for ionization and fragmentation of ions in Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICRMS) is introduced. The technique is based upon excitation of high-frequency (i.e. lowmass) primary ions (e.g. N, Al+, H2O+·) which may be generated by standard ionization methods (e.g. electron ionization or laser desorption) inside the trap. Generation of the primary ions is followed by excitation of their cyclotron motion to high translational energy to serve as projectile ions. The projectiles serve two functions: to ionize neutral gas atoms and/or molecules (secondary ions) within the trap internal ion impact ionization (IIII), and to collisionally activate the secondary ions to induce fragmentation. Ionization by IIII is analogous to charge-exchange ionization, in which a primary ion reacts with neutral analyte to produce ionization. When the projectile ions' cyclotron orbits are not excited (even for very long delay periods between ionization and detection), lower ion abundances are observed for the secondary ions derived from the neutral analyte(s), verifying that high energy ion impact must be responsible for the relatively high ion abundances observed when projectile cyclotron motion is excited: i.e., the mechanism of IIII is likely to be high-energy charge exchange. Several advantages of the new technique include: accessibility of very high impact energy (keV) in FT-ICRMS in both the laboratory and the center-of-mass frames of reference; ease in controlling the average energy transferred from projectiles to sample neutrals by moderation of the projectile ion's cyclotron radius; and the high ion-molecule reaction rates attainable in FT-ICR due to the long path length for trapped primary projectile ions at low pressure. Theory of ion formation and the problems associated with this technique (e.g. formation of secondary ions at different pre-excitation ICR orbits) are discussed.