Optical techniques EELS

Because fluorescence detection is an optical technique, it is also subject to Beer s law. For dilute solutions, where eel < 0.01,... 

Although some optical techniques, such as soft X-ray absorption and optical reflectance measurements, provide comparative information about solids with higher energy resolution, EELS enjoys several unique advantages over optical spectroscopies. First of all, unlike optical reflectance measurements which are sensitive to the surface condition of the sample, the transmitted EELS represents the bulk properties of the material. Secondly, EELS spectra can be measured with q along specific controllable directions and thus, can be used to study the dispersion of plasmons, excitons, and other excitations [8.1-8.5]. Such experiments offer both dynamics as well as symmetry information about the electronic excitations in solids. In addition, the capability to probe the electronic structure at finite momentum-transfer also allows one to investigate the excited monopole or quadrupole transitions, which cannot be directly observed by conventional optical techniques limited by the dipole selection rule. 

The major disadvantage of EELS, especially compared to optical techniques, is the relatively poor instrumental resolution, which usually varies between 3 and 10 meV (25-80 cm ). The spectral resolution hinders assignment of vibrations due to individual modes, although peak assignments can be made to within 10 cm The high sensitivity of EELS coupled with the advantages discussed above has encouraged rapid development and use of this technique, despite resolution limitations, such that it has now been used to study hundreds of adsorptions systems. 

The first two advantages listed above allow an optical method like transmission or reflection IR spectroscopy to be used for studies which would be impossible for a widely used competitive technique, electron energy loss spectroscopy (EELS). EELS must... 

DETERMINATION OF THE OXIDATION STATES OF THE 4th PERIOD TRANSITION METALS IN MINERALS. The study of the oxidation states of polyvalent cations is an important component of the characterization of minerals as they not only have vital crystal-chemical implications, but are also useful monitors of the ambient oxygen fugacities [21, 22]. The techniques conventionally employed for the determination of the oxidation states, such as Mdssbauer, optical and x-ray absorption, etc., have limited applicability because of the difficulty in obtaining a sufficiently pure amount of the fine grained or inhomogenous mineral sample. EELS avoids... 

In this paper, the optical and Mossbauer spectroscopic techniques for measuring Fe and Fe and determining their site occupancies will be examined. For each of these methods, an overview of practice and theory will be presented. Although there have been other reviews of optical spectroscopy (Rossman 1984) and Mossbauer spectroscopy (Kalinchenko et al. 1973 Ericsson et al. 1977 Heller-Kallai and Rozenson 1981 Pollack and Stevens 1986 Dyar 1987 Redhammer 1998 Murad 1998 Rancourt 1998), emphasis here is on historical and international inclusiveness, and discussion is restricted to micas rather than to the broader topic of clay minerals. Because they predate the other methods, optical studies will be discussed here first, followed by a discussion of the Mossbauer studies that constitute the majority of spectroscopic studies of mica. XPS and EELS spectroscopies are discussed in the Appendix, as they represent emerging technologies... 

The source of information is dependent on electron current, primary beam energy and electron energy loss. The plasmon losses are typically of the order of 10 eV and can provide information on the permitivity of the material under study. EELS is thus complementary to optical reflectivity techniques. Interband transitions can be compared with theoretical models of the electronic structure of the material. 

The foremost spectroscopic techniques that yield information on the chemical state of adsoibates are optical (IRS, Raman, SFG, SHG), electron (EELS, XPS, UPS) and ion (SIMS) spectroscopies. Quantitative structural techniques such as LEED and XPD have also contributed to solving the question of the chemical state of the primary and secondary intermediate species. In this context, sections 1.4.3, 1.4.5-1.4.8 should also be consulted. 

Chemical reactions at the gas-surface interface can be followed by monitoring gas-phase products with, for example, a mass spectrometer, or by directly analyzing the surface with a spectroscopic technique such as Auger electron spectroscopy (AES), photoelectron spectroscopy (PES), or electron energy loss spectroscopy (EELS), all of which involve energy analysis of electrons, or by secondary ionization mass spectrometry (SIMS), which examines the masses of ions ejected by ion bombardment. Another widely used surface probe is low-energy electron diffraction (LEED), which can provide structural information via electron diffraction patterns. At the gas-liquid interface, optical reflection elHpsometry and optical spectroscopies are employed, such as Eourier transform infrared (ET IK) and laser Raman spectroscopies. 

Every effort is made here to achieve the highest possible absolute power of detection. Microdistribution analysis represents the primary field of application for microprobe techniques based on beams of laser photons, electrons, or ions, including electron microprobe analysis (EPMA), electron energy-loss spectrometry (EELS), particle-induced X-ray spectrometry (PIXE), secondary ion mass spectrometry (SIMS), and laser vaporization (laser ablation). These are exploited in conjunction with optical atomic emission spectrometry and mass spectrometry, as well as various forms of laser spectrometry that are still under development, such as laser atomic ab.sorption spectrometry (LAAS), resonance ionization spectrometry (RIS). resonance ionization mass spectrometry (RIMS), laser-enhanced ionization (LEI) spectrometry, and laser-induced fluorescence (LIF) spectrometry [36]-[44],... 

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