Soft X-ray emission spectroscopy

Triguero L, Luo Y, Pettersson LGM, Agren H, Vaterlein P, Weinelt M, Fohlisch A, Hasselstrom J, Kaiis O, Nilsson A (1999) Resonant soft X-ray emission spectroscopy of surface adsorbates Theory, computations and measurements of ethylene and benzene on Cu(llO). Phys Rev B 59 5189... 

Photoelectron spectroscopy provides a useful (because direct) tool for studying the valence-band structure of solids. It is unlike soft x-ray emission spectroscopy where one must contend with transitions (to inner shells) that are constrained by selection rules and where one must take into account the character of the shell to which the transitions occur. In photoelectron spectroscopy, any of the occupied states in the band can be examined by ejecting the band photoelectrons. Thus, the photoelectron spectral shape essentially reflects the structure of the occupied band itself. 

Nordgren, J., Gians, P, and Wassdahl, N. 1991. Progress in ultra-soft X-ray emission spectroscopy. Phys. Scr. 

Boron nitride, cubic BNcub g, ind r i5v to Xlc 6.2 Soft-X-ray emission spectroscopy... 

J. Nordgren Ultra-soft X-ray emission spectroscopy - a progress report, in Proc. 14th Int l Conf. on X-Ray and Inner Shell Processes. J. Physique C 9, Suppl.l2 (1987)... 

Surface resonance Raman spectroscopy Surface Raman spectroscopy specular reflectance speetroscopy Scanning tunneling microscope Sinface-unenhanced Raman spectroscopy Soft X-ray appearance potential spectroscopy Soft X-ray emission spectroscopy Thermodesorption mass spectroscopy Thermal energy atom scattering Third harmonic generation Thin layer cell... 

As a complement to electron spectroscopy molecular ultra soft X-ray emission spectroscopy was developed in our laboratory in 1971. A specially designed instrument for this purpose was constructed and reported at the Asilomar conference. This high resolution 3m grazing incidence spectrometer with a differentially pumped electron excitation arrangement was built in order to observe - if possible - not only individual molecular orbitals but also vibrational fine structures (see ref.54, p.40-45). Such a fine structure was actually observed with the instrument first in and then later on in several... 

IR, Raman, NMR, ESR, UPS, XPS, AES, EELS, SIMS) [1]. However, some industrial carbon materials such as amorphous carbon films and carbon black cannot be easily characterized from the local-structure point of view by these methods, because these materials usually take amorphous and complex structures. Recently, soft X-ray emission and absorption spectroscopy using highly brilliant synchrotron radiation [2] has been utilized to characterize various carbon materials, because information on both the occupied and unoccupied orbitals, which directly reflect the local structure and chemical states, can be provided from the high-resolution soft X-ray measurements. We have applied the soft X-ray spectroscopy to elucidate the local structure and chemical states of various carbon materials [3]. Additionally, we have successfully used the discrete variational (DV)-Xa method [4] for the soft X-ray spectroscopic analysis of the carbon materials, because the DV-Xa method can easily treat complex carbon cluster models, which should be considered for the structural analysis of amorphous carbon materials. 

There are three main detection modes for EAPFS within the appearance potential spectroscopy (APS) technique./31/ First, one may monitor soft-x-ray emission due to the decay of the core hole left by the primary process. This is called SXAPS-EAPFS (Figure le). Second, it is also possible to monitor Auger electrons due to the same core-hole decay, as in AEAPS-EAPFS and AMEFS-EAPFS, cf. Figure If. Third, one may measure the remaining total intensity of... 

Emission of photons Soft X-ray appearance spectroscopy SXAPS... 

A simple cell for liquids is schematically shown in Figure 5.4, developed by Guo et al. [11]. This liquid cell consists of a metal container, an O-ring, and a 100-nm-thin SijN membrane (1x1 mm size). Liquid samples are sealed inside the metal container with O ring and the SijN membrane. The cell is then transferred into the UHV chamber at the beam line to allow access to soft X-rays. The SijN membrane enables photon-in and photon-out so that the absorption signal is measured with fluorescence yield. This also permits X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS) [1], which are not discussed here. 

Our discussion of X-ray emission spectroscopy has so far mainly been focused on the analytical capabilities of systems with moderate resolution. Clearly, X-ray emission spectroscopy is also a field of active research, particularly in the soft X-ray region. Here dipole transitions between well defined inner orbitals and more diffuse (perturbed) valence orbitals are studied yielding valuable information on the latter orbitals. In the measurements it is clearly desirable to increase the resolution as much as possible. Wavelength-dispersive instruments are then mandatory. (Above 3nm two... 

Fig. 4. Schematic representation of the principle of the different core level spectroscopies. Lower part (See caption of fig. 3.) (a) and (b) SXE soft X-ray emission, (a) and (c) AES Auger electron spectroscopy, (d) XPS X-ray photoemission spectroscopy, (e) SXA soft X-ray absorption, (f) EELS electron energy loss spectroscopy. Upper part (See caption of fig. 3.) Half-filled rectangle excited final state with the same electron count as in the initial state, (e), (f). Divided rectangle final state with two electrons less than in the initial state (see also fig. 19b).

H. Tillborg, A. Nilsson, T. Wiell, N. Wassdahl, N. Martensson, J. Nordgren, Electronic-structure of atomic oxygen adsorbed on Ni(lOO) and Cu(lOO) studied by soft-x-ray emission and photoelectron spectroscopies. Phys. Rev. B 47(24), 16464-16470 (1993)... 

In X-ray photoelectron spectroscopy (XPS), a beam of soft X-rays with energy hv s. focused onto the surface of a solid that is held under an ultra-high vacuum, resulting in the ejection of photoelectrons from core levels of the atoms in the solid [20]. Fig. 15 shows an energy level diagram for an atom and illustrates the processes involved in X-ray-induced photoelectron emission from a solid. 

Soft X-ray spectroscopy has been used to investigate the 3d electron distribution in small Ni particles.179 180 In addition to showing modified absorption and emission spectra, the small Ni particles (< 2 nm) have two satellite peaks in the absorption spectrum missing. This is attributed to a loss of long range order due to small particle size. 

Appearance potential spectroscopy involves detection of electronic transitions not of the backscattered electrons as in ELS, but of secondary processes. The latter include increase in soft X-ray (SXAPS) or Auger electron (AEAPS) emission or decrease in elastically scattered primary electrons (DAPS) (382). SXAPS is not as sensitive as AES for surface chemical analysis. However, SXAPS and IS spectra are easier to analyze than AES, since only one core transition is involved. This makes SXAPS and IS quite convenient for detecting heavy elements on catalyst surfaces. 

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