Adi Scheidemann

Preparative Separation of Mixtures by Mass Spectrometry

A specially designed mass spectrometer which allows for preparative separation of mixtures is described. This mass spectrometer allows for large ion currents, on the order of nanoamperes, to be produced by electrospray and transmitted into a high vacuum. Accumulation of nanomole quantities of collected and recovered material in several hours is demonstrated. The use of high-velocity ions reduces space charge effects at high ion currents. Separation of mass occurs simultaneously for all ions, providing a 100% duty cycle. The use of a linear dispersion magnet avoids compression at higher m/z ratios. A deceleration lens slows the ions to allow for soft landing at low kinetic energy. The ions are neutralized by ion pairing on an oxidized metal surface. Retractable landing plates allow for easy removal of the separated components.

Micromachined Faraday cup array using deep reactive ion etching

A micromachined Faraday cup array (MFCA) for position sensitive ion detection has been developed using a deep reactive ion etching (DRIE) process. Linear, closely spaced arrays of 64, 128, and 256 cups have been produced with pitches of 250 /spl mu/m and 150 /spl mu/m. Low leakage MOS capacitors formed into DRIE trenches form effective ion collection traps with stable and electrostatically isolated capacitances. These closely spaced arrays of Faraday cups enable a new generation of compact mass spectrometers with true multi-channel detection capability. Since all of the incident ion flux is continuously intercepted by the array, no ion flux is lost as in scanning systems, and the overall sensitivity of the mass spectrometer is drastically improved by a factor approximately equal to the number of cups in the array. The MFCA is thus an ideal component for miniaturized mass spectrometers, ion beam profiling, and chemical analyzers which must work with very small sample volumes or high throughputs.

Capture of lithium by 4He clusters: Surface adsorption, Penning ionization, and formation of HeLi+

We investigate the capture of lithium atoms by a beam of large cold helium clusters and subsequent ion production by electron impact. Li atoms are efficiently picked up by the He droplets, although with a cross section significantly lower than that for other atoms and molecules. The mass spectrum reveals the presence of Li atom and dimer ions, as well as the weakly bound complex HeLi+, confirming that capture by He-cluster beams can be used for efficient soft ionization of fragile species. The electron-energy dependence of the Li+, Li2+, and HeLi+ yield shows that they are formed primarily by Penning ionization in a collision with a metastable He atom in the droplet. This leads to the conclusion that lithium metal atoms are not submerged in the helium clusters but locate on the surface, corroborating theoretical predictions for bulk helium surfaces and spectroscopic measurements.

Scattering of neutral metal clusters: Long‐range interactions and response properties

The absolute integral cross sections for low‐energy collisions of neutral sodium clusters Nan (n=2–40) with atoms and molecules (Ar, N2, O2, and halogens) have been measured. The cross sections are found to be exceptionally large (up to thousands of square angstroms), showing the dominant role of long‐range intermolecular interactions. Elastic scattering proceeding under the influence of the van der Waals force, and a reaction channel involving electron transfer can successfully describe the measurements. The strength of the van der Waals potential is defined by such cluster response properties as the electric polarizability and the frequency of the giant dipole resonance. The reactive electron‐jump channel, in turn, is described by the ‘‘harpooning’’ mechanism which is sensitive to the cluster ionization potential. Employing parameters taken from spectroscopic studies of alkali clusters, we obtain good agreement with the observed cross sections. This provides a direct connection between beam scattering e...

Optical spectra of sodium microclusters

Photoabsorption spectra have been measured for free neutral sodium clusters containing fromN=3 to 40 atoms. In the size range ofN≈3 to 5, a transition occurs from molecule-like absorption to collective excitations of the valence electrons. ForN≈6 to 12, the data are well described by an ellipsoidal shell model. In open-shell clusters, the multiple surface plasma resonances expected for spheroidal or ellipsoidal shapes are observed. The experimental resonance positions provide a sensitive measurement of the cluster distortions. ForN≳13, the per atom strength of these collective resonances is reduced; this may be due to peak fragmentation caused by interaction between the surface plasmon and nearby single-electron resonances. In three distinct wavelength regions, one of which corresponds to the position of the Na atom “D-lines”, additional absorption is seen in the spectra of all investigated clusters.

Faraday cup detector array with electronic multiplexing for multichannel mass spectrometry

A Faraday cup detector array (FCDA) and electronic multiplexing circuit have been developed for position sensitive ion beam detection. The entire FCDA always remains open to intercept the incident ion beam flux, and each cup is periodically and sequentially discharged through the electronic multiplexer. This produces true multichannel ion beam detection since none of the incident ion beam flux is lost, as is the case for scanning position sensitive detectors, and higher sensitivity detection is thus obtained. The FCDA consists of a one-dimensional or two-dimensional array of individual cups which are electrostatically isolated from each other by means of an intervening ground conductor, with resulting fill factors F of 58% to 85%. Each cup acts as a charge collector and integrator which is quickly discharged during the readout to create a time-multiplexed output signal that gives the position distribution of the ion beam. When N cups are sequentially scanned and read out, the ion collection efficiency is ...

Growing ultracold sodium clusters by using helium nanodroplets

Abstract The aggressive doping of helium droplets (∼10 5 atoms) with sodium vapor results in the growth of sodium clusters having up to at least 13 atoms, as determined by a Penning ionization technique. Signatures of electronic shell effects are observed in Na k + cluster ions, including an odd–even intensity oscillation and an enhanced Na 9 + peak, which is a magic number effect. The size distributions are consistent with cluster ion fragmentation rather than simply sequential pickup statistics. The dependence of ion yield on mean ionizing electron energy suggests that neutral alkali clusters are located preferentially on the droplet surfaces.

Microfabricated components for miniaturized chemical analysis systems

The roles of microfabricated components for miniaturized chemical analysis systems are reviewed and the fundamental advantages of these components are illustrated in typical analysis and separation systems, including mass spectrometry, electroanalytical chemistry, capillary electrophoresis, and chromatography. In each instance the scaling laws that affect the resolution of the miniaturized instrument are supported by key enabling micromachined components. Micromachined components are also enabling elements for system integration and for coupling multiple techniques together in parallel or cascade.

Measurement of pick-up cross-sections of 4He clusters: Polar versus non-polar molecules

Abstract Pick-up cross-sections of free large 4He clusters formed in free jet expansions have been measured to determine the influence of the molecular properties of target molecules in the capture process. The pick-up cross-section depends strongly on the dipole moment of the molecule. For non-polar molecules, earlier measurements of pick-up cross-sections ranging from 2000 to 4300 A2 were ponfirmed; for polar systems, however, cross-sections between 6400 and 8100 A2 were observed. In both cases a linear cross-section versus mass dependence is observed.

Electron-cluster interactions

Beam depletion spectroscopy has been used to measure absolute total inelastic electron-sodium cluster collision cross sections in the energy range fromE≈0.1 toE≈6 eV. The investigation focused on the closed shell clusters Na8, Na20, and Na40. The measured cross sections show an increase for the lowest collision energies where electron attachment is the primary scattering channel. The electron attachment cross section can be understood in terms of Langevin scattering, connecting this measurement with the polarizability of the cluster. For energies above the dissociation energy the measured electron-cluster cross section is energy independent, thus defining an electron-cluster interaction range. This interaction range increases with the cluster size.