The Effects of Temperature and Line Overlap in the Analysis of Multiple Line Moessbauer Spectra.

The primary information of a Mossbauer transmission spectrum consists of the positions and intensities of the spectral lines, and the temperature dependencies of these parameters provide information concerning the relative abundance, surrounding electronic structure, and dynamical nature of the various absorption site species within a sample. Parameter values can be obtained directly if all lines are well separated and the sample size allows the use of a thin absorber approximation in which the spectrum can be treated as a superposition of Lorentzian lines. Complexities aries in thick samples with saturation effects, in samples where overlap interferes with measurement of line areas, and in samples where heterogeneous lattice structure causes differences in the zero phonon fraction associated with absorption sites. These complexities are considered here and are illustrated with experimental examples. The interference effect of overlapping lines can be separated out from any saturation effects. An analytical expression for that part of the total spectral area is derived here for an arbitrary number of overlapping lines and then analyzed extensively for the simplest case, that of a symmetric doublet. In such a system the interference term is found to be a function of the line separation to line width ratio, and a phase angle defined in terms of this ratio leads to an analogy with optical interference. The spectra of porphyrin cytochrome c-ferrous ion complexes were found to approximate an overlapping symmetric doublet but a greater than predicted difference between total integrated line intensity and the thin absorber approximation suggests a significant Gaussian component to the lineshape. An approximation to the dynamic structure of these complexes has been obtained using a Debye model. A more complex spectrum arising from absorption site heterogeneity is that exhibited by a multivalent zwitterion, triferrocenyl monoferricenyl boron. The temperature dependence of the spectral parameters of this compound and its component ligands, ferrocene and the ferricenium cation, have been used to establish valence structure, demonstrate intramolecular phase transitions, and find an approximation to dynamical structure using a single mode Einstein model.