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ANALYTICAL CURRENTS STM and TOF combo picks out atoms R. Micheletto and colleagues at Kyoto University and Unisoku Co., Ltd. (both in Japan) have found a powerful combination for analyzing surfaces at the nanometric scale. The researchers integrated scanning tunneling microscopy (STM) and time-offlight (TOF) MS to detect—with help from a laser beam—individual atoms from nanoscale surfaces at room temperature. They say this laser-assisted STM-TOF (LSTOF) system shows that detection and desorption of single atomic species are possible in nanolocalized regions, which may be helpful in nanotechnology applications and the fabrication of semiconductor devices. They applied the method in their own work involving dynamic observations of reactions on silicon surfaces. The LSTOF performs conventional STM imaging. But in the TOF analysis mode, the atoms under the STM tip are locally ionized and desorbed by an electric pulse, which is induced directly by the tip itself and a quasi-simultaneous laser pulse applied to the same region. The additional pulse enhances the extraction and ionization induced by the STM tip. An electric field then guides the extracted ions to a TOF chamber for analysis, thus allowing for simultaneous STM structural analysis and atomic spectroscopy in the nanometric region of interest. The instrument’s resolution is mainly limited by the STM probe. The ionization field must be weak, say the researchers,
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Diagram of the laser-assisted STM-TOF instrument. (Adapted with permission. Copyright 2002 American Institute of Physics.)
to induce ionization only on the surface immediately under the tip without altering or damaging the probe. Therefore, fewer ions are desorbed, resulting in lower efficiency and resolving power. (Rev. Sci. Instrum. 2002, 73, 3227–3231)
MS follows heterogeneous ozone reactions Heterogeneous reactions of ozone with unsaturated gas-phase organic compounds may be significant in the loss of ozone from the troposphere. To better understand these procedures, Tomas Baer, Roger Miller, and colleagues at the University of North Carolina–Chapel Hill monitored the heterogeneous reaction of ozone with oleic acid, a representative species, by coupling an aerosol flow tube to a dual-laser single-particle mass spectrometer. Schematic of the aerosol flow tube where the reaction between ozone and From the resulting data, the researchers developed oleic acid particles took place. a new reaction model, which suggests a possible as a function of particle size confirmed that the reactive uprole for particle morphology. take coefficient, , depended on particle size and ranged The researchers followed the reaction by measuring the from (7.3 ± 1.5) 10–3 for particles ~680 nm in radius to concentrations of condensed-phase species. They chose a (0.99 ± 0.09) 10–3 for particles ~2.45 µm in radius. dual-laser mass spectrometer because the separate vaporizaThe resulting reaction model accounted for the simultation and ionization lasers produced less fragmentation and neous reaction and diffusion of ozone and oleic acid. Solumade it possible to gently probe the identities of the contions obtained from this model suggested that oleic acid densed-phase species. diffused more slowly than expected within a particle, perhaps Because many heterogeneous reactions occur near the because of particle morphology, and that decoupling reaction surfaces of particles, the researchers suspected that the upand diffusion processes may not provide an accurate picture take kinetics might be limited by the reactants’ rates of difof reactions. (J. Phys. Chem. A. 2002, 106, 8085–8095) fusion to the surface. Measurements of the uptake fraction 562 A
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