VEELS

Een tweede doel van het onderzoek was het verbeteren van de selectiviteit van de Diels-Alderreacrie. De vorming van ongewenste bijproducten dient zo veel mogelijk te worden voorkomen. 

In this model of electrostatic interactions, two atoms (i and j) have point charges q and qj. The magnitude of the electrostatic energy (Veel) varies inversely with the distance between the atoms, Ry. The effective dielectric constant is 8. For in vacuo simulations or simulations with explicit water molecules, the denominator equals eRij. In some force fields, a distance-dependent dielectric, where the denominator is eRy Rjj, represents solvent implicitly. 

The degree of surface cleanliness or even ordering can be determined by REELS, especially from the intense VEELS signals. The relative intensity of the surface and bulk plasmon peaks is often more sensitive to surface contamination than AES, especially for elements like Al, which have intense plasmon peaks. Semiconductor surfaces often have surface states due to dangling bonds that are unique to each crystal orientation, which have been used in the case of Si and GaAs to follow in situ the formation of metal contacts and to resolve such issues as Fermi-level pinning and its role in Schottky barrier heights. 

VEELS spectra are limited in practice to tbe relatively narrow energy range of about 30 eV over wbicb plasmons or interband transitions can occur. In contrast to AES, XPS, or even CEELS, where excitations can occur over hundreds of eV, the probability of spectral overlap is much higher for VEELS. It is fortunate that most... 

Figure 4 VEELS and Auger spectra for tiK angles of 0°, 45°, and 60° taken from a tin sample covered by a 0.5-nm oxide layer. The doublet AES peaks are the Sn (410) peaks while the singlet AES peak is the O (510) taken with the same gain. VEELS peaks are oxide related, while the Sn (410) peak is due primarily to the metallic tin beneath the oxide, illustrating the superior depth resolution of VEELS.

REELS will continue to be an important surface analytical tool having special features, such as very high surface sensitivity over lateral distances of the order of a few pm and a lateral resolution that is uniquely immune from back scattered electron effects that degrade the lateral resolution of SAM, SEM and EDS. Its universal availability on all types of electron-excited Auger spectrometers is appealing. However in its high-intensity VEELS-form spectral overlap problems prevent widespread application of REELS. 

A. J. Bevolo, M. L. Albers, H. R. Shanks, and J. Shinar. J. Appl. Phys. 62, 1240, 1987. VEELS in fixed-spot mode to depth profile hydrogen in amorphous silicon films to determine hydrogen mobility at elevated temperatures. 

P. Braun. Surf. Sci. 126,714,1983. VEELS study of bulk and surface plasmon energies across Al—Mg alloy phase diagram. 

VEELS VWence Electron Energy-Loss Spectroscopy... 

Lemke, J. (1998). Multiplying meanings Visual and verbal semiotics in scientific text. In J. R. Martin R. Veel (Eds.), Reading science Critical andfunctionalperspectives on discourse of science (pp. 87-113). London Routledge. 

Figure 6.2 VEEL spectra when a mixture of CO and 02 was coadsorbed at a Cu(l 10)-Cs surface (ctCs — 3.5 x 1014 cm-2) at 80 K and the adlayer warmed to 298 K. Note the formation of surface carbonate (cf. Figure 6.9). (Reproduced from Ref. 6).

Het in sediment identifieeren van ebemisebe stoffen die een potentieel risico vormen voor bet eeosysteem of voor de menselijke gezondbeid, is dan ook bet onderwerp van veel studies. 

Room-temperature Raman wavenumbers for ethyne (C2H2 and C2D2) onNi/Si02have beenrecorded by Krasseref al. (72), but without illustration of the spectra. The higher wavenumber listed bands at 2988, 2910, 1204, 1044, 864, and 806 cm"1 show some promising coincidences, particularly the 1204-cm"1 feature from C—C, with the bands in the VEEL spectrum of ethyne on Ni(lll) (8). 

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