LEIS spectroscopy

In this article, the study of alloy electrodes by a combination of EC with LEISS is discussed. LEIS spectroscopy is unparalleled in its ability to interrogate only the outermost layer this is because of the repulsive nature of ion-atom interactions that serves to mask interactions between sub-surface species with the probe ions. LEISS is an established surface physics technique but its adoption in surface electrochemical investigations has not been pervasive. For the work reviewed here, an instrument that incorporates EC with LEISS, LEED, XPS and TPD was employed. 

If a collimated atom beam expands into vacuum, the velocity component of the atoms in the beam direction is much greater than the thermal velocity component perpendicular to the beams axis, in accord with the low divergence of the beam. Consequently, the Doppler width of atom transitions, as seen by an intersecting laser field perpendicular to the axis of the atom beam, is reduced significantly. Isotopes, which have been excited selectively by the tunable laser, can then be detected by LIES, by ionization techniques, such as RIS and FILS, or by RIMS after photoionization. LEI spectroscopy cannot be applied since collisional ionization does not occur in the atom beam. 

Laser-enhanced ionization (LEI) spectroscopy is based on the difference in collisional ionization rates of atoms in their ground and excited states. If... 

In the previous section we described how optical resonance could be detected by direct observation of electrical phenomena. From this point of view we described a special type of opto-galvanic spectroscopy. When electrical detection of optical resonance in connection with measurements on electrical discharges or flames is performed the term Laser-Enhanced Ionization (LEI) spectroscopy is frequently used. In Fig.9.9 opto-galvanic spectroscopy (LEI) on a gas discharge is illustrated. The laser beam is directed into the discharge and when it is tuned to an optical transition the discharge current is changed, since the probability of collisional ionization is different for... 

It has generally been assumed that the outermost surfaces of bulk bismuth-molybdate catalysts are just an extension of one of its bulk crystal planes. According to this model, the mechanism has been proposed that molecular O2 dissociatively chemisorbs on the surface bismuth sites, where it becomes incorporated into the bulk lattice and the propylene chemisorbs and reacts on the surface Mo sites where it is oxidised by oxygen supplied from the bismuth-molybdate bulk lattice. The exchange of gas-phase molecular O2 with lattice O in bismuth-molybdate catalysts has been confirmed with Raman studies using isotopically labeled 02. Recent LEIS spectroscopy analyses of the outermost surface of bismuth-molybdate, however, revealed that its surface is enriched in Mo sites and that instead of Bi, the... 

HEED = high energy electron diffraction IILE = ion-induced light emission INS = ion-neutralization spectroscopy IRS = infrared spectroscopy ISS = ion-scattering spectroscopy LEED = low energy electron diffraction LEIS = low energy ion scattering ... 

In low energy ion scattering (LEIS, also called ion scattering spectroscopy, ISS) a beam of noble gas ions with energy of a few keV scatters elastically from a solid sur-... 

IRS Internal reflectance spectroscopy LEI Laser-enhanced ionisation... 

It is usually found that the conformation of the three chelate rings in [Co(en)3]3+ is such that each C-C vector lies approximately parallel (lei) with respect to the molecular C3 axis. This has been examined with a number of experimental and theoretical methods including NMR and IR,616 vibrational circular dichroism617 spectroscopy as well as molecular mechanics calculations.618... 

In situ characterization. Catalysts should preferably be investigated under the conditions under which they are active in the reaction. Various reasons exist why this may not be possible, however. For example, lattice vibrations often impede the use of EXAFS, XRD and Mossbauer spectroscopy at reaction temperatures the mean free path of electrons and ions dictates that XPS, SIMS and LEIS are carried out in vacuum, etc. Nevertheless, one should strive to choose the conditions as close as possible to those of the catalytic reaction. This means that the catalyst is kept under reaction gases or inert atmosphere at low temperature to be studied by EXAFS and Mossbauer spectroscopy or that it is transferred to the vacuum spectrometers under conditions preserving the chemical state of the surface. 

Covering monometallic (Pd, Sn) and multimetallic (Pd-Sn, Pd-Ag) systems, several examples are presented in this chapter to illustrate the possibility offered by this chemistry to control the particle size distribution and the bimetallic interaction at a molecular level. This work is supported by a multitechnique characterization approachusing SnM6ssbauerspectroscopy,X-rayphotoelectron spectroscopy (XPS), low-energy ion spectroscopy (LEIS), and transmission electron microscopy (TEM). Catalytic performances in hydrogenation of different unsaturated hydrocarbons (phenylacetylene, butadiene) are finally discussed in order to establish structure-reactivity relationships. 

H. Lei, K. Ugurbil and W. Chen, Measurement of unidirectional Pi to ATP flux in human visual cortex at 7 T by using in vivo P magnetic resonance spectroscopy. Proc. Natl. Acad. Sci. USA, 2003,100,14409-14414. 

ISS ion-scattering spectroscopy LEIS low-energy ion scattering SEM scanning electron microscopy SIMS Secondary ion mass spectrometry TEM transmission electron microscopy TP temperature-programmed XANES X-ray absorption near edge spectroscopy. 

Techniques based on the interaction of ions with solids, such as secondary ion mass spectrometry (SIMS) and low-energy ion scattering (LEIS) have undoubtedly been accepted in catalyst characterization, but are by no means as widely applied as for example X-ray photoelectron spectroscopy (XPS) or X-ray diffraction (XRD). Nevertheless, SIMS, with its unsurpassed sensitivity for many elements, may yield unique information on whether or not elements on a surface are in contact with each other. LEIS is a surface technique with true outer layer sensitivity, and is highly useful for determining to what extent a support is covered by the catalytic material. Rutherford backscattering (RBS) is less suitable for studying catalysts, but is indispensable for determining concentrations in model systems, where the catalytically active material is present in monolayer (ML)-like quantities on the surface of a flat model support. 

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