Small carbon clusters (C n , n=2-15) are produced in a molecular beam by pulsed laser vaporization and studied with vacuum ultraviolet (VUV) photoionization mass spectrometry.The required VUV radiation in the 8-12 eV range is provided by the Advanced Light Source (ALS) at the Lawrence Berkeley National Laboratory. Mass spectra at various ionization energies reveal the qualitative relative abundances of the neutral carbon clusters produced. By far the most abundant species is C 3 . Using the tunability of the ALS, ionization threshold spectra are recorded for the clusters up to 15 atoms in size. The ionization thresholds are compared to those measured previously with charge transfer bracketing methods. To interpret the ionization thresholds for different cluster sizes, new ab initio calculations are carried out on the clusters for n=4-10. Geometric structures are optimized at the CCSD(T) level with cc-pVTZ (or cc-pVDZ) basis sets, and focal point extrapolations are applied to both neutral and cation species to 2 determine adiabatic and vertical ionization potentials. The comparison of computed and measured ionization potentials makes it possible to investigate the isomeric structures of the neutral clusters produced in this experiment. The measurements are inconclusive for the n=4-6 species because of unquenched excited electronic states. However, the data provide evidence for the prominence of linear structures for the n = 7, 9, 11, 13 species and the presence of cyclic C 10 .

The protonated acetylene cation, C2H3+, (also known as the vinyl cation) and the proton-bound acetylene dimer cation (C4H5+) are produced by a pulsed supersonic nozzle/pulsed electrical discharge cluster source. The parent ions are also generated with weakly attached argon "tag" atoms, e.g., C2H3+Ar and C4H5+Ar. These ions are mass selected in a specially designed reflectron time-of-flight mass spectrometer and studied with infrared laser photodissociation spectroscopy in the 800-3600 cm-1 region. Vibrational resonances are detected for both ions in the C-H stretching region. C2H3+ has a strong vibrational resonance near 2200 cm-1 assigned to the bridged proton stretch of the nonclassical ion, while C4H5+ has no such free-proton vibration. Instead, C4H5+ has resonances near 1300 cm-1, consistent with a symmetrically shared proton in a di-bridged structure. Although the shared proton structure is not the lowest energy isomer of C4H5+, this species is apparently stabilized under the supersonic beam conditions. Larger clusters containing additional acetylene units are also investigated via the elimination of acetylene. These species have new IR bands indicating that rearrangement reactions have taken place to produce core C4H5+ ions with the methyl cyclopropane cation structure and/or the protonated cyclobutadiene isomer. Ab initio (MP2) calculations provide structures and predicted spectra consistent with all of these experiments.

We report the first observation of the infrared spectrum of the free tert-butyl cation in the gas phase. This ion is produced in a pulsed supersonic expansion equipped with a pulsed electrical discharge. C4H9 + is detected with a time-of-flight mass spectrometer. Its infrared spectrum is recorded using infrared photodissociation spectroscopy and the method of rare gas tagging with argon. For this, the C4H9 +Ar complex is produced, mass selected, and studied with resonance-enhanced IR photodissociation spectroscopy using a tunable optical parametric oscillator laser system. Ab initio computations are employed at the MP2(fc)/6-311+G(2d,2p) level for various isomeric structures of C4H9 + and C4H9 +Ar, and for various configurations of the tert-butyl cation, to elucidate the spectra. We find that argon has a negligible effect on the IR spectra and that the patterns in the spectrum are consistent only with the tert-butyl cation in a C 1 or C s configuration.

The infrared photodissociation spectra (IRPD) in the 700 to 4000 cm(-1) region are reported for H+ (CO2)n clusters (n = 1-4) and their complexes with argon. Weakly bound Ar atoms are attached to each complex upon cluster formation in a pulsed electric discharge/supersonic expansion cluster source. An expanded IRPD spectrum of the H+ (CO2)Ar complex, previously reported in the 2600-3000 cm(-1) range [Dopfer, O.; Olkhov, R.V.; Roth, D.; Maier, J.P. Chem. Phys. Lett. 1998, 296, 585-591] reveals new vibrational resonances. For n = 2 to 4, the vibrational resonances involving the motion of the proton are observed in the 750 to 1500 cm(-1) region of the spectrum, and by comparison to the predictions of theory, the structure of the small clusters are revealed. The monomer species has a nonlinear structure, with the proton binding to the lone pair of an oxygen. In the dimer, this nonlinear configuration is preserved, with the two CO2 units in a trans configuration about the central proton. Upon formation of the trimer, the core CO2 dimer ion undergoes a rearrangement, producing a structure with near C2v symmetry, which is preserved upon successive CO2 solvation. While the higher frequency asymmetric CO2 stretch vibrations are unaffected by the presence of the weakly attached Ar atom, the dynamics of the shared proton motions are substantially altered, largely due to the reduction in symmetry of each complex. For n = 2 to 4, the perturbation due to Ar leads to blue shifts of proton stretching vibrations that involve motion of the proton mostly parallel to the O-H+-O axis of the core ion. Moreover, proton stretching motions perpendicular to this axis exhibit smaller shifts, largely to the red. Ab initio (MP2) calculations of the structures, complexation energies, and harmonic vibrational frequencies are also presented, which support the assignments of the experimental spectra.

The infrared spectra of protonated acetone and the proton bound acetone dimer are obtained revealing vibrational resonances associated with the shared proton motions, which are in agreement with the predictions from ab initio, MP2, harmonic frequency calculations.

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Noble metal carbide cluster cations (MC(n)(+), M = Cu, Au) are produced by laser vaporization in a pulsed molecular beam and detected with time-of-flight mass spectrometry. Copper favors the formation of carbides with an odd number of carbon atoms, while gold shows marked drops in ion intensity after clusters with 3, 6, 9, and 12 carbons. These clusters are mass selected and photodissociated at 355 nm. Copper carbides with an odd number of carbons fragment by eliminating the metal from the cluster; for the small species it is eliminated as Cu(+) and for the larger species it is lost as neutral Cu. Copper carbides with an even number of carbons also lose the metal, but in addition to this they eliminate neutral C(3). This even-odd alternation, with the even clusters having mixed fragments, holds true for clusters as large as CuC(30)(+). No loss of C(2) is observed for even the largest clusters studied, indicating that fullerene formation does not occur. The gold carbide photodissociation data closely parallel that of copper, with even clusters losing primarily C(3) and odd ones losing gold. Comparisons to known carbon cluster ionization potentials give some insight into the structures of carbon photofragments. DFT calculations performed on CuC(3-11)(+) allow comparisons of the energetics of isomers likely present in our experiment, and metal-carbon dissociation energies help explain the even-odd alternation in the fragmentation channels. The simplest picture of these metal-doped carbides consistent with all the data is that the small species have linear chain structures with the metal attached at the end, whereas the larger species have cyclic structures with the metal attached externally to a single carbon.

A fully automated method for the separation of low-concentration uranium from plutonium in environmental swipe samples has been developed. The offline chromatography system features renewable 1 mL Eichrom TEVA and UTEVA column generation from bulk resin slurry. Discrete fractions of the separated actinides are delivered into user defined vials for future analysis. Clean room background levels were achieved outside of a cleanroom environment with this method. Purification of uranium and plutonium from various sample matrixes and at various concentrations was successful. Major and minor isotope ratios for both elements were measured via multiple collector inductively coupled plasma mass spectrometry and were in good agreement with certified reference values. Validation of the separation method was conducted on archived environmental samples and agreed with values previously reported using standard column chemistry.