H. Schaber

Mass spectra of Si, Ge, and Sn clusters

Clusters of group IV B elements have been produced and observed in a mass spectrometer. Irregularities in the ion intensities identify clusters with particularly high or low stability. For example, Si+x,Ge+x, and Sn+x show an enhanced stability for x=6, 10. However, Sn+14 is unstable, whereas Ge+14 is particularly stable.

High‐resolution time‐of‐flight mass spectrometers: Part I. Effects of field distortions in the vicinity of wire meshes

Field distortions in the vicinity of wire meshes can change the direction of passing ions. Small deflection angles of ions passing through wire meshes in the reflector can reduce the mass resolution of time‐of‐flight mass spectrometers. We calculate these effects.

Evidence for a size‐dependent melting of sodium clusters

Geometric shell structure in the mass spectra of sodium clusters was found to disappear as the clusters were heated. The exact temperature at which the shells disappeared was dependent on the size of the clusters. These observations are interpreted as evidence for a size‐dependent melting. Clusters containing 1000 atoms appear to melt at 288 K, clusters containing 10 000 atoms at 303 K. Both values lie well below the bulk melting temperature of 371 K.

Metal coated fullerene molecules and clusters

Coevaporation of C60 and an alkali metal in a gas aggregation cell yields a distribution of clusters with composition (C60)nMx with 0≤x<150, n=1,2,3, and M=Li, Na, and K. For singly ionized clusters the mass peaks are strong for odd values of x but only after reaching the composition C60M7+. It is suggested that the onset of even–odd alternation marks the end of electron transfer between metal and C60 and the beginning of metal–metal bonding.

High resolution time‐of‐flight mass spectrometers. Part III. Reflector design

The reflector has a central influence on the properties of a time‐of‐flight mass spectrometer. The mass resolution can be greatly improved using a reflector with homogeneous fields. A reflector with homogeneous fields can also have focusing properties and thus enhance sensitivity of the instrument. In this case it is important to be aware of design limitations. We present a mathematical method of determining these limitations and also give a guideline for mechanical design.

High‐resolution time of‐flight mass spectrometers. Part II. Cross beam ion optics

Injecting neutral species at right angles to the direction of acceleration and using quadrupole ion optics allows for highest resolution and good sensitivity in time‐of‐flight mass spectrometers. We have designed such ion optics using a finite element field calculation. Imaging and timing errors have been compensated. With an acceleration voltage of 6.5 kV it accepts neutral species having energies up to 500 eV.

Matrix isolated alkali halide monomers and clusters

The far infrared absorption spectra of sodium and cesium halide monomers and clusters isolated in solid argon are reported. For low concentrations of alkali halide vapor in argon, four absorption lines were usually seen. The strongest and highest frequency line could be assigned to the monomer. The three remaining lines were due to planar rhombic dimers. For higher concentrations of alkali halide vapor, new absorption lines appeared which seem to be due to trimer and tetramer formation. The equilibrium configurations of alkali halide molecules containing from two to eight atoms were calculated by minimizing the total energy. Both rigid and polarizable ion models were used to describe the two particle interaction. The second derivative of the total energy with respect to the atomic displacements was then calculated and the resulting dynamical matrix diagonalized to give the infrared absorption spectrum.

Matrix isolated copper and silver halide clusters

Infrared absorption spectra are reported for matrix isolated (CuCl)n, (CuBr)n, (AgCl)n, (AgBr)n and for mixed clusters with composition CunClmBrn–m. Silver halide clusters with n=1 to 3 are already present in the vapor in equilibrium with the melt. The monomer and dimer forms of the copper halides could be produced only by using a two‐stage oven to partially dissociate the predominant trimer. The stable configurations and corresponding infrared active vibrational frequencies are calculated and compared with experiment. By means of this comparison we come to the following tentative conclusions concerning the structure of these clusters: The silver dimers appear to be rhombic with an X–M–X angle differing considerably from 90°. Both the Cu and the Ag halide trimers are deformed six‐rings. The Cu halide tetramer appears to be an eight‐ring, not a cube. Larger clusters, (MX)6,7,8, formed within the matrix at high halide to argon concentrations also appear to have planar structures.

Matrix isolated II-VI molecules: Sulfides of Mg, Ca, Sr, Zn and Cd

Abstract Infrared absorption spectra are reported for the molecules MS, MS 2 and M 2 S, where M is Zn, Cd, Mg, Ca or Sr. All of these molecules are unstable in a high temperature vapor. Vapor in equilibrium with solid CaS and SrS contains predominately metal and sulfur atoms. In this investigation the atoms were combined in a low temperature argon matrix in order to form stable molecules. The vapor above ZnS, CdS and MgS contains metal atoms and S 2 molecules. A two stage oven was used to further dissociate the S 2 molecules into atomic sulfur. The triatomic alkaline earth sulfide molecules appear to have the configurations M-S-M and S-M-S. These structures and the vibrational frequencies are fairly well explained using an ionic interatomic potential derived from the diatomic molecule. ZnS 2 and CdS 2 , on the other hand, appear to have the structure M-S-S.

Thermally induced structural transition in (C60)n clusters

Abstract Both neutral and charged (C 60 ) n clusters were heated to temperatures at which they evaporated molecules. Particularly stable clusters resisted evaporation and manifested themselves as enhanced peaks in the mass spectra. At 490 K, the set of enhanced mass peaks (e.g., n =13, 19, 39, 46, 49, 55, 116, 131, 147) indicated the presence of icosahedral structures. A different set of intense peaks (e.g., n =38, 48, 58, 68, 71, 75, 77, 84, 98) dominated the mass spectra at temperatures near 585 K. The new set correlates rather well with that expected for close-packed and decahedral structures. The two sets of enhanced peaks were found to be independent of the cluster charge at all temperatures investigated.