Soft X-Ray Emission and Absorption

In the case of compounds and alloys the use of soft X-ray spectroscopy has the advantage that it yields separate spectra associated with the core levels on the different elements (provided these are not too close in energy). Indeed, the more complicated the system, the more this technique has to offer in comparison with optical spectroscopy. 

X-ray photoemission (or ESCA) yields rather similar information to that given by X-ray emission—this technique is currently in a period of rapid development (see, e.g.. Ref. 91). 

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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. 

McGuinness, C., Stagarescu, C.B., Ryan, P.J., Downes, J.E., Fu, D., Smith, K.E., and Egdell, R.G. Influence of shallow core-level hybridization on the electronic structure of post-transition-metal oxides studied using soft X-ray emission and absorption. Pl s. Rev. B 2003, 68,165104. 

In either optical absorption or soft X-ray emission and absorption the results are generally interpreted in terms of a simple band picture in which the spectrum is assumed to be given by an integral of a transition probability over occupied (i) and unoccupied (y) states of the same k vector. For the case of optical absorption this may be written as... 

The cubic modification (3C-SiC) is the only SiC with zinc blende structure. The band structure of 3C-SiC has been calculated theoretically by Kobayashi [1], and estimated from optical absorption and reflection [6], luminescence [7], soft X-ray emission [8] and X-ray photoelectron [9] spectra. 

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).

The emission spectmm of Co, as recorded with an ideal detector with energy-independent efficiency and constant resolution (line width), is shown in Fig. 3.6b. In addition to the expected three y-lines of Fe at 14.4, 122, and 136 keV, there is also a strong X-ray line at 6.4 keV. This is due to an after-effect of K-capture, arising from electron-hole recombination in the K-shell of the atom. The spontaneous transition of an L-electron filling up the hole in the K-shell yields Fe-X X-radiation. However, in a practical Mossbauer experiment, this and other soft X-rays rarely reach the y-detector because of the strong mass absorption in the Mossbauer sample. On the other hand, the sample itself may also emit substantial X-ray fluorescence (XRF) radiation, resulting from photo absorption of y-rays (not shown here). Another X-ray line is expected to appear in the y-spectrum due to XRF of the carrier material of the source. For rhodium metal, which is commonly used as the source matrix for Co, the corresponding line is found at 22 keV. 

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. 

DIET has also been applied to the study of rare earth oxides, YBaCuO oxides as well as Si02 with significant contribution to the understanding of their structure. Loubriel, Knotek, Stulen, Koel and Parks studied the ESD and PSD of oxide films of Ce and Er and compared the results with those obtained by Soft X-ray absorption, SXA [135]. From the ESD experiments they concluded that the 5p level in the rare earth is involved in the electronic transitions for ion desorption from the surface. Schmidt-May, Sent, Voss, Kunz, Flodstrom, Nyholm and Stockbauer later studied the resonant electron and ion emission and the desorption mechanism from oxides of Sm, Eu and Yb [136]. They observed that the enhancement in photoelectron at the 4d edge is also found in the ion desorption yield. They concluded that the use of PSD as a proof of the specific desorption site is strongly limited because the initial excitation does not occur in the surface complex from which the detected ion is being desorbed. 

If we are speaking about observations of thermal emission of young cooling NSs then most of their properties depend on their age, and their detectability in X-rays obviously strongly depends on the distance (not only because of the flux dilution, but also because of the strong interstellar absorption of soft X-rays). In that sense it is useful to plot an age-distance diagram (ADD) for these objects. 

Most NEXAFS beamlines measnre soft x-ray absorbance by monitoring the energetic decay of the excited state, which occurs nonradiatively as Auger electron emission and radiatively as photon flnorescence. Auger electron emission provides the most common quantification of soft x-ray absorption into bonnd states. Anger emission occurs when an outer shell electron from the same atom decays into a core hole formed by soft x-ray excitation. The decay energy, which is the difference between onter shell and core orbital energies, is transferred to a second onter shell... 

KeV) on samples with concentrations above 10 (atomic ratio), standard X-ray transmission techniques are used. The incident and transmitted fluxes are typically measured by photo-ionization chambers. In the soft X-ray range (i.e., with hv < 1000 eV), absorption spectra may be efficiently measured by recording core-hole decay products. If we describe the inner-shell photo-ionization process as a two-step process, then in the first step the photon excites a core-hole electron pair, and in the second step the recombination process of the core-hole takes place. There are many channels suitable for core-hole recombination. These channels may produce the emission of photons, electrons, or ions, all of which are collected by special detectors. The recombination channel that is normally used to record bulk XAS spectra of dilute systems is the direct radiative core-hole decay that produces X-ray fluorescence lines. When fluorescence lines have high photon energies, this technique probes the bulk. In Figure 4 a beam line with an apparatus to record absorption spectra in the fluorescence mode is schematically represented. 

Koningsberger DC, Prins R (eds) (1988) X-ray absorption Principles, applications, techniques of EXAFS, SEXAFS, and XANES. John Wiley Sons, New York Lindle DW, Cowan PL, Jach T, LaVilla RE, Deslattes RD, Perera RCC (1991) Polarized X-ray emission studies of methyl chloride and chlorofluoromethanes. Phys Rev A 43 2353-2362 Loo BW, Sauerwald IM, Hitchcock AP, Rothman SS (2001) A new sample preparation method for biological soft X-ray microscopy nitrogen based contrast and radiation tolerance properties of glycol methacrylate-embedded and sectioned tissue. J Microsc 204 69-86 Matsui F, Yeom HW, Matsuda I, Ohta T (2000) Adsorption and reaction of acetylene and ethylene on the Si(001)2xl surface. Phys Rev 62 5036-5044... 

Grush MM, Home CR, Perera RCC, Ederer DL, Cramer SP, Caims EJ, Callcott TA (2000) Correlating electronic stmcture with cycling performance of substituted LiMn204 electrode materials a study using the techniques of soft x-ray absorption and emission. Chem Mater 12 659-664... 

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. 

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