THE APPLICATION OF SELECTIVE EXCITATION DOUBLE MÖSSBAUER TECHNIQUES TO THE STUDY OF RELAXATION IN FERRICHROME A

The selective excitation double Mossbauer (SEDM) technique has been applied to the study of electronic relaxation in the polypeptide ferrichrome A. Calculations of the SEDM lineshape using the superoperator formalism were performed and compared with the experimental spectra at 300 K and 5.4 K. These results do not agree with the theoretical model of the relaxation mechanism in this material proposed by other investigators based on information obtained from transmission experiments. The study by the Mossbauer effect of time dependent hyperfine interactions such as those induced by electronic relaxation has become increasingly important to the understanding of the properties of biological and physical systems. Most of these studies are performed in transmission geometry. The advantages of using selective excitation double Mossbauer (SEDM) techniques for these investigations have been discussed / ] / and the technique has already been used to study a variety of phenomena /2,3/. However, no investigation of the line shape with relaxation occuring in the scatterer in a well characterized system, has been performed. Such a study is required for the complete understanding of SEDM spectra and is a prerequisite for the application of the SEDM technique to complex dynamic Fe environments. This is especially timely because recently several theoretical calculations of the SEDM lineshape have appeared in the literature /4,5,6/. These calculations were done for arbitrary electronic relaxation rates and general interaction Hamiltonians but were restricted to thin scatterers. We have extended this calculation to more realistic scatterers of arbitrary thickness 8, ($ = n0 f), so that our •experimental results could be compared with the theory. The thickness correction for scatterers without relaxation has previously been discussed (Balko, Hoy) /l/. Details of the calculation for SEDM lineshapes with relaxation in the scatterer On leave from George Mason University, Fairfax, VA 27030, U.S.A. will be presented in a future publication. In the present paper we will concentrate on the application of this theory to the study of electronic relaxation in ferrichrome A. Ferrichrome A is an iron containing polypeptide which has been studied in detail by transmission techniques 111 and thus serves as an appropriate model for our investigation. In the transmission geometry between IK and 300K it exhibits a Mossbauer relaxation spectrum characteristic of a paramagnetic material. At low temperatures the spectrum shows a split six line pattern. As the temperature increases the lines broaden and collapse to form a single line at 300K. This behavior as a function of temperature was inferred by Wickman et al. to occur because of electronic relaxation. They assumed that the Fe nucleus is subjected to a time dependent randomly varying effective hyperfine field. We show in this paper that such an adiabatic spin flip model is inadequate for a paramagnet like ferrichrome A. However, it does explain some general features of the relaxation lineshape obtained in the transmission geometry. To reveal more detailed information about the relaxation mechanism a more specific and descriminating technique needs to be employed. Our experimental arrangement is shown in figure 1. The constant velocity drive (CVD) was set to excite the ferrichrome A scatterer a particular energy. We used a 90 mCi Co source mounted in a Pd matrix and an enriched polycrystalline ferrichrome A (70 mg of Fe) scatterer. The scattered Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979204 c218 JOURNAL DE PHYSIQUE radiation was analyzed by a single line absorber (with 8 = 14) mounted on a constant acceleration drive (CAD) in the transmission geometry. ( DRIVE I 1 "CO DRIVE 2 r l