Mossbauer transition

The Bragg scattering of X-rays by a periodic lattice in contrast to a Mossbauer transition is a collective event which is short in time as compared to the typical lattice vibration frequencies. Therefore, the mean-square displacement (x ) in the Debye-Waller factor is obtained from the average over the ensemble, whereas (r4) in the Lamb-Mossbauer factor describes a time average. The results are equivalent. 

T-violation experiment The effect of time-reversal (7) violation on the transition of photons through a system of magnetized foils that have a Mossbauer transition with comparable M and E2 strength (such as in Ru or Au)... 

Another study of the temperature dependence of the 6.2 keV Mossbauer resonance of Ta has been carried out by Salomon et al. [197] for sources of WAV metal and W/Ta metal in the temperature range from 15 to 457 K. In more recent investigations, Salomon et al. [198] have extended such studies of the temperature behavior of the 6.2 keV Mossbauer transition of Ta in tantalum metal to temperatures up to 2,300 K which has been the highest temperature range for any Mossbauer study so far. 

The short wavelength (2 = 0.267 A) of the 46.5 keV Mossbauer transition of limits the lowest accessible angle to sin(rf)//l = 0.12 The measurements for glycerol were performed in the direction b corresponding to the first liquid structure peak at 1.3 A and those for polycrystalline myoglobin from 0.12 to 0.44 A. ... 

Apart from the determination of nuclear parameters, the Mossbauer transition in Os, especially the 36.2 and 69.6 keV transitions, are suited for chemical applications. As shown below, the 36.2 keV level, in spite of its large half-width, can be well used for the measurement of isomer shifts, whereas the 69.2 keV state is favorable for the characterization of electric quadrupole or magnetic dipole interactions. Both Mossbauer levels are populated equally well by the parent isotope lr, and simultaneous measurement is possible by appropriate geometrical arrangement. 

Table 7.8 Summary of results obtained for the four Os Mossbauer transitions studied. The absorber thickness d refers to the amount of the resonant isotope per unit area. The estimates of the effective absorber thickness t are based on Debye-Waller factors / for an assumed Debye temperature of 0 = 400 K. For comparison with the full experimental line widths at half maximum, Texp, we give the minimum observable width = 2 S/t as calculated from lifetime data.

Fig. 7.55 Isomer shift 5 = 5 — 5sod of the 69.6 keV Mossbauer transition versus the isomer shift of the 36.2 keV line in s (from [264])...

There are two iridium isotopes, ir and Ir, suitable for Mossbauer spectroscopy. Each of them possesses two nuclear transitions with which nuclear resonance absorption has been observed. Figure 7.58 (from [266]) shows the (simplified) nuclear decay schemes for both iridium Mossbauer isotopes the Mossbauer transitions are marked therein with bold arrows. The relevant nuclear data known to date for the four Mossbauer transitions are collected in Table 7.1 at the end of the book. 

It is a matter of historical interest that Mossbauer spectroscopy has its deepest root in the 129.4 keV transition line of lr, for which R.L. Mossbauer established recoilless nuclear resonance absorption for the first time while he was working on his thesis under Prof. Maier-Leibnitz at Heidelberg [267]. But this nuclear transition is, by far, not the easiest one among the four iridium Mossbauer transitions to use for solid-state applications the 129 keV excited state is rather short-lived (fi/2 = 90 ps) and consequently the line width is very broad. The 73 keV transition line of lr with the lowest transition energy and the narrowest natural line width (0.60 mm s ) fulfills best the practical requirements and therefore is, of all four iridium transitions, most often (in about 90% of all reports published on Ir Mossbauer spectroscopy) used in studying electronic stractures, bond properties, and magnetism. 

Fig. 7.58 Simplified decay scheme leading to the population of the four nuclear Mossbauer transitions of lr and Ir (from [266])...

A Japanese group [297] measured the hyperfine field at Ir nuclei in the alloys Feo.yPto.aJrx (0.03 < x < 0.2) using the Ir (73 keV) Mossbauer transition, in order to understand the mechanism associated with the decrease of the magnetic moment (and negative hyperfine field) in these alloys. The isomer shift was found to increase with increasing x, which could be rationalized by the contraction of the lattice volume. 

Fig. 7.70 Simplified decay scheme of The two Mossbauer transitions have energies of 98.86 keV and 129.74 keV (from [1])...

The recoilless nuclear resonance absorption of y-radiation (Mossbauer effect) has been verified for more than 40 elements, but only some 15 of them are suitable for practical applications [33, 34]. The limiting factors are the lifetime and the energy of the nuclear excited state involved in the Mossbauer transition. The lifetime determines the spectral line width, which should not exceed the hyperfine interaction energies to be observed. The transition energy of the y-quanta determines the recoil energy and thus the resonance effect [34]. 57Fe is by far the most suited and thus the most widely studied Mossbauer-active nuclide, and 57Fe Mossbauer spectroscopy has become a standard technique for the characterisation of SCO compounds of iron. 

FIGURE 2. Quadrupole splitting and Mossbauer transitions for (a) 127I and (b) l29I... 

Mossbauer Bulk 1- 1000 ppm Site locations, structure, bonding, chemical environment Isotopes with Mossbauer transitions... 

The nuclear ground and excited levels involved in the Mossbauer transition are shifted or split because of the electrostatic interactions between the nuclear charge and the surrounding electric charge (Fig. 2). The first interaction, sometimes called the electric monopole interaction, shifts only the nuclear levels and is related to the perturbation resulting from the electrons inside the nuclear volume. This shift is... 

Synchrotrons produce photons with energies in the range of nuclear Mossbauer transitions and can, in principle, be used to excite these transitions. However, synchrotron radiation can be monochromatized to only about 1 meV with new monochromators. Because the accessible nuclear levels are extremely narrow (between 10 and 10 eV), it is only about 10 of the incident photons that can excite the nuclear levels (excitation cross-section could be as much as 10 Fq). This is far weaker than radiation that is non-resonantly scattered by the electronic processes in the solid arising from the scattering of the entire 1 meV width of the incident radiation. 

The transmission throngh an absorber of thickness teff as a ftmction of the relative velocity V between source and absorber is given by the evalnation of the transmission integral discussed, for example, in Ref 2. In case of Fe Mdssbaner spectroscopy for almost all samples which are not enriched in the Mossbauer isotope Fe (the natural abundance of Fe is 2%), the evaluation of the transmission integral leads to Lorentzian line shape with a minimum linewidth of 0.19mms , which is twice the natural linewidth of the Fe Mossbauer transition. The absorption cross section as a ftmction of source velocity is then given as (with... 

---