Ionization edges, detection

Electron-impact energy-loss spectroscopy (EELS) differs from other electron spectroscopies in that it is possible to observe transitions to states below the first ionization edge electronic transitions to excited states of the neutral, vibrational and even rotational transitions can be observed. This is a consequence of the detected electrons not originating in the sample. Conversely, there is a problem when electron impact induces an ionizing transition. For each such event there are two outgoing electrons. To precisely account for the energy deposited in the target, the two electrons must be measured in coincidence. 

The direct proof that H is present in certain centers in Ge came from the substitution of D for H, resulting in an isotopic energy shift in the optical transition lines. The main technique for unraveling the nature of these defects, which are so few in number, is high-resolution photothermal ionization spectroscopy, where IR photons from an FTIR spectrometer excite carriers from the ls-like ground state to bound excited states. Phonons are used to complete the transitions from the excited states to the nearest band edge. The transitions are then detected as a photocurrent. 

Although originally mainly the more pronounced near-edge features were observed and compared or interpreted (usually empirically), it is evident from a recent review (18) that today considerably greater emphasis is given to the study of the extended fine structure. This stands in relation to the improved experimental methods for detection of weak modulations and to the currently more advanced theoretical description of the EXAFS part of the spectrum. Complete understanding of the Kossel structure at the threshold part of an element s inner-shell spectrum, which contains among others valence orbital, ionization, and chemical shift information, is relatively slow due to the... 

Measurements using extended x-ray absorption fine-structure (EXAFS) spectroscopy were made on the insertion-device beam line of the Materials Research Collaborative Access Team (MRCAT) at the Advanced Photon Source, Argonne National Laboratory. Measurements were made in transmission mode with ionization chambers optimized for the maximum current with linear response ( 10 photons detected/sec). A cryogenically cooled double-crystal Si (111) monochromator with resolution (AE) better than 2.5 eV at 11.564 keV (Pt L3 edge) was used in conjunction with a Rh-coated mirror to minimize the presence of harmonics [9]. The integration time per data point was 1-3 sec, and three scans were obtained for each processing condition. 

The primary X-ray beam is first monochromatized using a double-crystal monochromator, usually made of Si or Ge single crystals. The intensities Zq and Ii of the X-ray beam before and behind the sample are determined using ionization chambers. To detect instabilities in the energy scales of subsequent measurements, a reference sample that exhibits an edge in the scanned region can be measured simultaneously using a third ionization chamber. 

Electrons ejected after the core ionization can be measured either selectively by their energy as Auger electrons or unselected as the so-called total electron yield. Due to the small free path that electrons have in condensed matter, these electrons stem from a thin layer of the surface of the sample. Under these conditions, XAS becomes a surface-sensitive probe [41] known as SEXAFS (Surface EXAFS) and NEXAFS (Near Edge X-ray Absorption Fine Structure with the same meaning as XANES,but applied exclusively to near-edge spectra detected using surface-sensitive measurements). These methods have become very important... 

---