Metal particles resonance line

Another contribution to variations of intrinsic activity is the different number of defects and amount of disorder in the metallic Cu phase. This disorder can manifest itself in the form of lattice strain detectable, for example, by line profile analysis of X-ray diffraction (XRD) peaks [73], 63Cu nuclear magnetic resonance lines [74], or as an increased disorder parameter (Debye-Waller factor) derived from extended X-ray absorption fine structure spectroscopy [75], Strained copper has been shown theoretically [76] and experimentally [77] to have different adsorptive properties compared to unstrained surfaces. Strain (i.e. local variation in the lattice parameter) is known to shift the center of the d-band and alter the interactions of metal surface and absorbate [78]. The origin of strain and defects in Cu/ZnO is probably related to the crystallization of kinetically trapped nonideal Cu in close interfacial contact to the oxide during catalyst activation at mild conditions. A correlation of the concentration of planar defects in the Cu particles with the catalytic activity in methanol synthesis was observed in a series of industrial Cu/Zn0/Al203 catalysts by Kasatkin et al. [57]. Planar defects like stacking faults and twin boundaries can also be observed by HRTEM and are marked with arrows in Figure 5.3.8C [58],... 

For molecules adsorbed on colloidal metals, the tumbling motion of the metal particles can be rapid enough to average out the chemical shift anisotropy so that the lines are narrow enough for a simple liquids NMR experiment. Using this method, the resonance has been observed in colloidal solutions of Pd and Pt particles (see Sections IV.C and IV.D). 

Originally, the goal of this technique was to determine the rates of exchange of a nucleus between two environments in solution, when the lifetimes are comparable to the spin lattice relaxation times and long with respect to the inverse of the frequency difference (43). However, it has also been used for indirect detection of a broad resonance in one of the two environments by monitoring a narrow resonance in the other environment. The exchanging unit was the "CO molecule, which occurred either as a solute in the solvent or as an adsorbate on a colloidal metal particle. The broad line (the adsorbed state) could not be detected directly with the liquid high-resolution equipment used (see Section IV.D). 

Fig, 65. Poinl-by-point Rh NMR spectra for a sample of Rh/TiO , and another of Rh ill PVP film at two temperatures. The size distributions peak in the 2- or 3-nm region, but they are not particularly narrow. The Rh/TiO, has more large particles than docs the Rh/ PVP. I he long, dashed lines indicate the bulk metal Rh resonance position (the shift scale on top assumes that this is 0.36%)-, the positions of the short lines divide the integrals into halves. The remarkable result is that the spectrum broadens in both directions and not only to low field, as for platinum. The broadening is more important for the sample with fewer large particles. 

Los and coworkers at the FOM institute also used a sputtering source to study ionization cross sections of alkali metal atom collisions with O2 at relative velocities below 13 km s . These authors used magnetic fields in the charged particle detection part of their experiment, allowing them to differentiate between negative ions and e . The K + O2 measurements of Moutinho et al. are shown in Fig. 18. They are compared with the K(4 P -> 4 5) resonance line excitation cross sections of Lacmann and Herschbach after scaling them to the estimated cross section of Kempter et A dotted line indicates the measurements of Kempter et al. in-... 

NaY zeolite loaded with sodium or rubidium metals vapor phase deposition has been investigated by ESR, Xe, Na and Rb NMR. Exposure of the zeolite to a high Na concentration leads to a single ESR line which is attributed to Na metallic particles inside the zeolite cavities. Xe NMR spectrum of NaY zeolite loaded with Na shows three lines at 88, 94 and 134 ppm which are interpreted in terms of domains of nonuniformely distributed metal particles. By annealing at 670 K the spectrum collapses to a single line at 120 ppm, characteristic of a narrow particle size distribution. Na and Rb NMR spectra in the temperature range 260 K-300 K were obtained for Rb loaded NaY zeolite. The observed resonances can be explained by the presence of Na/Rb alloy phase in the zeolite cavities. 

To measure an atomic absorption signal, the analyte must be converted from dissolved ions in aqueous solution to reduced gas-phase free atoms. The overall process is outlined in Figure 6.16. As described earlier, the sample solution, containing the analyte as dissolved ions, is aspirated through the nebulizer. The solution is converted into a line mist or aerosol, with the analyte still dissolved as ions. When the aerosol droplets enter the flame, the solvent (water, in this case) is evaporated. We say that the sample is desolvated. The sample is now in the form of tiny solid particles. The heat of the flame can melt (liquefy) the particles and then vaporize the particles. Finally, the heat from the flame (and the combustion chemistry in the flame) must break the bonds between the analyte metal and its anion, and produce free M° atoms. This entire process must occur very rapidly, before the analyte is carried out of the observation zone of the flame. After free atoms are formed, several things can happen. The free atoms can absorb the incident radiation this is the process we want. The free atoms can be rapidly oxidized in the hostile chemical environment of the hot flame, making them unable to absorb the resonance lines from the lamp. They can be excited (thermally or by collision) or ionized, making them unable to absorb the resonance lines from the lamp. The analyst must control the flame conditions, flow rates, and chemistry to maximize production of free atoms and minimize oxide formation, ionization, and other unwanted reactions. While complete... 

Surface-enhanced Raman spectroscopy (SERS) " involves obtaining Raman spectra in the usual way on samples that are adsorbed on the surface of colloidal metal particles (usually silver, gold, or copper) or on roughened surfaces of pieces of these metals. For reasons that are finally becoming understood, at least semiquantitatively, the Raman lines of the adsorbed molecule are often enhanced by a factor of 10 to lO. When surface enhancement is combined with the resonance enhancement technique discussed in the previous section, the net increase in signal intensity is roughly the product of the intensity produced by each of the techniques. Consequently, detection limits in the range of 10 to 10 " M have been observed. 

Low pressure mercury lamps emit mainly the resonance line at 234 nm. The corresponding energy would promote valence electrons from either of the three photocatalysts to their conduction band. On reaching the surface, these electrons reduce Ag ions to metallic silver which is deposited as a dark layer on the catalyst particles. The increase in mass after irradiation indicates the mass of silver deposited. H2O2 restores the colour of ZnO to white by converting Ag to Ag faq.). 

MeV a-particles and used the Au/Ir source after annealing without any further chemical or physical treatment. Commercially available sources are produced via Pt(p, n) Au. The most popular source matrix into which Au is diffused is platinum metal although it has the disadvantage of being a resonant matrix - natural platinum contains 33.6% of Pt. Using copper and iridium foils as host matrices for the Au parent nuclide, Buym et al. [327] observed natural line widths and reasonable resonance absorption of a few percent at 4.2 K. 

Line-width broadening may also be caused by other fast relaxation mechanisms in addition to a small particle size. For example, it is well known that, for spherical particles, radiation losses become more pronounced with increasing radius. In some metals, these relaxation mechanisms are so strong that a well-defined plasmon resonance is not observed, as in Fe, Pd, and Pt. Nanosized particles are interesting because the optical resonance can be designed in. For example, in a nanoshell consisting of a dielectric core surrounded by a metallic outer layer, the relative dimensions of these components can be varied. This, in turn, varies the optical resonance, possibly over several-hundred nanometers in wavelength. 

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