Simultaneous dual-element analyses of refractory metals in naturally occurring matrices using resonance ionization of sputtered atoms

The combination of secondary neutral mass spectrometry (SNMS) and resonance ionization spectroscopy (RIS) has been shown to be a powerful tool for the detection of low levels of elemental impurities in solids. Drawbacks of the technique have been the laser-repetition-rate-limited, low duty cycle of the analysis and the fact that RIS schemes are limited to determinations of a single element. These problems have been addressed as part of an ongoing program to explore the usefulness of RIS/SNMS instruments for the analysis of naturally occurring samples. Efficient two-color, two-photon (1+1) resonance ionization schemes were identified for Mo and for four platinum-group elements (Ru, Os, Ir, and Re). Careful selection of the ionization schemes allowed Mo or Ru to be measured simultaneously with Re, Os, or Ir, using two tunable dye lasers and an XeCl excimer laser. Resonance frequencies could be switched easily under computer control, so that all five elements can be rapidly analyzed. In situ measurements of these elements in metal grains from five meteorites were conducted. From the analyses, estimates of the precision and the detection limit of the instrument were made. The trade-off between lower detection limits and rapid multielement RIS analyses is discussed

Tunneling study of cavity grade Nb: possible magnetic scattering at the surface

Tunneling spectroscopy was performed on Nb pieces prepared by the same processes used to etch and clean superconducting radio frequency (SRF) cavities. Air exposed, electropolished Nb exhibited a surface superconducting gap delta=1.55 meV, characteristic of clean, bulk Nb. However the tunneling density of states (DOS) was broadened significantly. The Nb pieces treated with the same mild baking used to improve the Q-slope in SRF cavities, reveal a sharper DOS. Good fits to the DOS were obtained using Shiba theory, suggesting that magnetic scattering of quasiparticles is the origin of the gapless surface superconductivity and a heretofore unrecognized contributor to the Q-slope problem of Nb SRF cavities.Comment: 3 pages, 3 figure

Multiphoton Ionization Followed by Time-of-Flight Mass Spectroscopy of Sputtered Neutrals

Multiphoton ionization (MPI) by pulsed, tunable lasers provides a sensitive means for detection of neutral atoms, resulting from the high probability achievable in both the ionization and subsequent detection steps. Substantial selectivity is achieved by excitation between energy levels of the atom of interest. This resonant MPI technique can access all atomic states including ground and metastable levels. The high efficiency of MPI technique permits detailed sputtering data to be obtained with minimal target damage. The goal is to obtain velocity and angular distributions for each energy level of every sputtered species. In practice, two types of experimental configurations have been employed. In one method, the photoionized atoms are allowed to strike a spatially resolved detector near the target, with extraction fields that preserve the angular distribution information. Velocity information is obtained by time of flight (TOF). This method is most suitable for majority species in the sputtered flux. In the case of minority species (either very dilute surface constituents or highly excited states produced), additional noise reduction is necessary. A suitable configuration involves extraction of the photoions into a sector-field TOF mass spectrometer. In standard, isochronous operation, energy and angular spreads at the point of ionization are compensated in flight to produce sharp TOF mass spectra. Noise sources (photons, metastable and scattered atoms) escaping through transparent grids are strongly suppressed. Angular distributions can be mapped pointwise by varying the relation between the point of ion beam impact and the photoionization volume. Velocity data can be obtained from the TOF spectra or by Doppler scanning on any resonant step of the laser excitation. Recent data are discussed

Supernova Reverse Shocks and SiC Growth

We present new mechanisms by which the isotopic compositions of X-type grains of presolar SiC are altered by reverse shocks in Type II supernovae. We address three epochs of reverse shocks: pressure wave from the H envelope near t = 10$^6$s; reverse shock from the presupernova wind near 10$^8-10^9$s; reverse shock from the ISM near 10$^{10}$s. Using 1-D hydrodynamics we show that the first creates a dense shell of Si and C atoms near 10$^6$s in which the SiC surely condenses. The second reverse shock causes precondensed grains to move rapidly forward through decelerated gas of different isotopic composition, during which implantation, sputtering and further condensation occur simultaneously. The third reverse shock causes only further ion implantation and sputtering, which may affect trace element isotopic compositions. Using a 25M$_{\odot}$ supernova model we propose solutions to the following unsolved questions: where does SiC condense?; why does SiC condense in preference to graphite?; why is condensed SiC $^{28}$Si-rich?; why is O richness no obstacle to SiC condensation?; how many atoms of each isotope are impacted by a grain that condenses at time t$_0$ at radial coordinate r$_0$? These many considerations are put forward as a road map for interpreting SiC X grains found in meteorites and their meaning for supernova physics.Comment: 28 pages, 14 figures, animation for Figure 3 and machine-readable Table 3 can be found at http://antares.steelangel.com/~edeneau/supernova/DHC_2003, Submitted to Ap

Surface analysis of rubbers and plastics using secondary neutral mass spectrometry.

Rubber and plastics are complex mixtures of long-chain polymers, smaller organic additives used as plasticizers, mildicides, fungicides, colorants, etc., and inorganic additives such as carbon and silica. Surface analysis of such materials is at once difficult and important. The difficulty lies in the need to identify specific carbon molecules on a very similar organic surface. The importance arises from the dramatic effect that the distribution and concentration of additives have on the physical properties of polymeric materials. Recently we have had some success in applying laser desorption post-ionization mass spectrometry in measuring the surface concentration and distribution of both additives and the polymer molecules themselves. The key has been to use the photoionization properties of the analyte of interest to augment mass spectrometric information