Spectrometer loss spectroscopy

Firstly, the energy losses of the incident electrons which produce the inner shell excitations may be detected as peaks in electron energy loss spectroscopy (EELS). The elecrons transmitted by the specimen are dispersed in a magnetic field spectrometer and the peaks, due to K, L and other shell excitations giving energy losses in the range of 0-2000eV, may be detected and measured. 

Figure 5.38. Illustrating the incident electron beam convergence angle (a), the scattering angle (6) and the spectrometer acceptance angle (/l) in electron energy loss spectroscopy. (After Joy, 1981.)...

Local chemical composition from areas less than 1 nm in diameter can be measured by energy dispersive X-ray spectroscopy (EDS) or electron energy loss spectroscopy (EELS). Such spectroscopic information may be presented in 2D maps showing the spatial element distribution in the specimen (13). Furthermore, information about the local density of unoccupied electron states of a specific element can be extracted from EELS data and used to estimate the oxidation state and the local coordination geometry of the excited atoms (14). In some favorable cases, electronic structure information with a resolution of about 1 eV from individual atomic columns has been attained (15,16). Recent developments of monochromators and spectrometers have brought the resolution down to 0.1 eV (17,18), and this capability may offer new opportunities to determine relationships between electronic structure information, the atomic arrangements and the catalytic activities of solids. 

The electron-energy-loss spectroscopy (EELS) was performed in transmission with a primary beam energy of 170 keV in a purpose-built UHV spectrometer described in detail elsewhere [5]. For the valence level excitations and elastic scattering (electron diffraction) data the momentum resolution of the instrument was set to 0.04 A 1 with an energy resolution of 90-140 meV. The core level excitations were performed with a momentum and energy resolution of 0.2 A"1 and 90-140 meV, respectively. All EELS experiments were conducted at room temperature. 

Ibach H and Mills D L 1982 Electron Energy Loss Spectroscopy and Surface Vibrations (New York Academic) Ibach H 1991 Electron Energy Loss Spectrometers the Technology of High Performance (Berlin Springer)... 

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. 

Another issue in trap-loss detection is the competition between homonuclear and heteronuclear trap loss in a given experiment, making assignments quite difficult. For example, when searching for KRb trap loss just below the %i/2 + 5pi/2 asymptote, stronger trap loss is simultaneously observed from just below the Rb2 5 i/2 + 5pi/2 asymptote. However, for detection of KRb+ ions by REMPI from theX and/or a states of KRb, there is no competition from ions due to Rb2 PA (the Rb+ and RbJ ions are easily distinguishable by their differing TOFs). Thus we believe that REMPI TOF mass spectrometer detection of X- and a-state heteronuclear molecules is normally preferable to trap-loss spectroscopy. 

Another technique in an early stage of development - spin polarised electron energy loss spectroscopy (SPEELS) - is expected to have particularly strong impact on rare earth surface science. In electron spectrometers with good temperature control of the sample the technique should unfold a wealth of interesting magnetic phases at surfaces and in ultra thin films on substrates. 

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