Mossbauer spectroscopy velocity

Fig. 3.19 Schematic illustration of the measurement geometry for Mossbauer spectrometers. In transmission geometry, the absorber (sample) is between the nuclear source of 14.4 keV y-rays (normally Co/Rh) and the detector. The peaks are negative features and the absorber should be thin with respect to absorption of the y-rays to minimize nonlinear effects. In emission (backscatter) Mossbauer spectroscopy, the radiation source and detector are on the same side of the sample. The peaks are positive features, corresponding to recoilless emission of 14.4 keV y-rays and conversion X-rays and electrons. For both measurement geometries Mossbauer spectra are counts per channel as a function of the Doppler velocity (normally in units of mm s relative to the mid-point of the spectrum of a-Fe in the case of Fe Mossbauer spectroscopy). MIMOS II operates in backscattering geometry circle), but the internal reference channel works in transmission mode...

The parameters that Mossbauer spectroscopy is sensitive to depend on the immediate vicinity of the absorbing nucleus. However, for heavier elements, the electrons close to the nucleus move at high velocities, close to the speed of light. In this situation, relativistic effects become significant and should be treated at some level of detail. 

Mossbauer spectroscopy The Mossbauer effect is resonance absorption of 7 radiation of a precisely defined energy, by specific nuclei. It is the basis of a form of spectroscopy used for studying coordinated metal ions. The principal application in bioinorganic chemistry is Fe. The source for the 7 rays is Co, and the frequency is shifted by the Doppler effect, moving it at defined velocities (in mm/s) relative to the sample. The parameters derived from the Mossbauer spectrum (isomer shift, quadrupole splitting, and the hyperfine coupling) provide information about the oxidation, spin and coordination state of the iron. 

Further identification of the particles is made with 57Fe Mossbauer spectroscopy. Mossbauer spectra were recorded with a conventional constant acceleration spectrometer with 57Co in Rh matrix as a y-ray source. Velocity calibration was made using a 5-pm a-Fe foil at 293 K. Figure 1.6.10 shows the Mossbauer spectra of the sample recorded at 293 K and 4.2 K. Spectra were fitted with theoretical... 

Most often the transmission mode is found to be the most convenient in Mossbauer spectroscopy, i.e., the y radiation passes from the source through the absorber, and the attenuation of the primary beam is measured at the various Doppler velocities. However, there are a number of cases where a "scattering geometry may be advantageous (SO). The basis for this geometry lies in those processes that take place after resonant absorption of y radiation by the Mossbauer isotope. Specifically, after excitation the Mossbauer isotope may reemit the y ray, or it may decay by emission of internal conversion electrons and X rays [with the probability of internal conversion equal to a/(l + a)]. 

Fig. 41. Oxygen-propylene interaction with a Sn-Mo catalyst as observed using Mossbauer spectroscopy, (a) Original (b) after adsorption of oxygen and propylene. Zero velocity is with respect to Sn02. Figure according to Firsova et at. (245).

Mossbauer spectroscopy provides phase identification, determination of oxidation states, and incidentally structure information and particle size. A little used application is to follow in real time the kinetics of phase transitions (carburization, reduction) in catalysts by monitoring the intensities of a few selected peaks in a single velocity experiment. Examples of applications on catalysts have recently been reviewed [43]. 

One of the more difficult experimental aspects of Mossbauer spectroscopy is the accurate determination of the absolute velocity of the drive. The calibration is comparatively easy for constant-velocity instruments, but most spectrometers now use constant-acceleration drives. The least expensive method, and therefore that commonly used, is to utilise the spectrum of a compound which has been calibrated as a reference. Unfortunately, suitable international standards and criteria for calibration have yet to be decided. As a result, major discrepancies sometimes appear in the results from different laboratories. The problem is accentuated by having figures quoted with respect to several different standards, necessitating conversion of data before comparison can be made. However, calibration of data from an arbitrary standard spectrum will at least give self-consistency within each laboratory. 

The transmission through an absorber of thickness /eff as a function of the relative velocity V between source and absorber is given by the evaluation of the transmission integral discussed, for example, in Ref. 2. In case of Fe Mossbauer spectroscopy for almost all samples which are not enriched in the Mossbauer isotope Fe (the natural abundance... 

Mossbauer spectroscopy, which involves the resonant absorption of a y-ray photon by a nucleus. The resonant condition is achieved via the Doppler effect, by sweeping the velocity of a sample relative to the source. The chemical environment of the nucleus causes characteristic fl equency shifts. 

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