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THE APPLICATION OF SELECTIVE EXCITATION DOUBLE MÖSSBAUER TECHNIQUES TO THE STUDY OF RELAXATION IN FERRICHROME A
B. Balko, E. Mielczarek, R. Berger
To cite this version:
B. Balko, E. Mielczarek, R. Berger. THE APPLICATION OF SELECTIVE EXCITATION DOUBLE
MÖSSBAUER TECHNIQUES TO THE STUDY OF RELAXATION IN FERRICHROME A. Journal
de Physique Colloques, 1979, 40 (C2), pp.C2-17-C2-19. �10.1051/jphyscol:1979204�. �jpa-00218462�
JOURNAL DE PHYSIQUE Collogue C2, supplement au n° 3, Tome 40, mars 1979, page C2-17
THE APPLICATION OF SELECTIVE EXCITATION DOUBLE MOSSBAUER TECHNIQUES TO THE STUDY OF RELAXATION IN FERRICHROME A
B. Balko, E.V. Mielczarek and R.L. Berger
Laboratory of Technical Development, National Heart, Lung, and Blood Institute, national Institutes of Health, Bethesda, MD 20014 U.S.A.
Résumé.- Une technique d'excitation sélective par double effet Mossbauer a été utilisée pour étudier la relaxation électronique dans le polypeptide ferrichrome A. Les calculs de la forme des raies SEDM à partir du formalisme du superopérateur ont été faits et comparés avec les spectres expérimentaux à 300 K et à 5,4 K. Les résultats ne confirment pas le modèle théorique du mécanisme de relaxation dans ce matériau proposé par d'autres investigateurs, à partir d'informations obtenues par des expériences de transmission.
Abstract.- 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 ma- terial proposed by other investigators based on information obtained from transmission experiments.
The study by the Mossbauer effect of time de- pendent 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 stu- dies are performed in transmission geometry. The advantages of using selective excitation double Moss- bauer (SEDM) techniques for these investigations have been discussed / ] / and the technique has alrea- dy been used to study a variety of phenomena /2,3/.
However, no investigation of the line shape with re- laxation occuring in the scatterer in a well charac- terized 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 S7Fe environ- ments. This is especially timely because recently several theoretical calculations of the SEDM line- shape have appeared in the literature /4,5,6/. These calculations were done for arbitrary electronic re- laxation rates and general interaction Hamiltonians but were restricted to thin scatterers. We have ex- tended 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 mo- del for our investigation. In the transmission geo- metry 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 57Fe 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 ina- dequate for a paramagnet like ferrichrome A. However, it does explain some general features of the relaxa- tion lineshape obtained in the transmission geometry.
To reveal more detailed information about the rela- xation mechanism a more specific and descriminating technique needs to be employed.
Our experimental arrangement is shown in figu- re 1. The constant velocity drive (CVD) was set to excite the ferrichrome A scatterer a particular energy. We used a 90 mCi 57Co source mounted in a Pd matrix and an enriched polycrystalline ferrichro- me A (70 mg of 57Fe) scatterer. The scattered
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979204
c2- 18 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.
( D R I V E I
1
"CODRIVE 2
r l
Sample g;taining
--
Single Line Absorber
Fig. 1 : Schematic representation of the experimen- tal SEDM configuration. CVD stands for constant ve- locity drive, and CAD for constant acceleration dri- ve.
The results of our experiments on ferrichrome A at 300 K are shown in the composite figure 2. The top spectrum (Fig. 2a) was obtained in the transmis- sion geometry using an absorber with f3 = 22 and shows a complete collapse of the hyperfine pattern due to fast electronic relaxation. The arrows indi- cate the positions of the excitations (determined by the CVD setting) in the SEDM experiments for figu- res 2b and 2c. The resulting SEDM lineshapes are shown in figure 2b for excitation at position 1 and in figure 2c for excitation at position 2. The dashed lines in the figures represent the expected spectra calculated for a static iron environment in the scatterer. This previous result showed that the SEDM line in the absence of relaxation appears only at the excitation energy (Balko and Hoy) / 1 /. Compa- ring this with the broader experimental SEDM line
(Fig. 2b) clearly indicates relaxation in the scat- terer. Figure 2c shows even more clearly that rela- xation is occuring in the scatter. Notice the large deviation of the data from the dashed line.
The solid lines in figure 2b and 2c represent our calculated results based on the SEDM lineshape theory of Afanseev and Gorobchenko /4/ which we have modified for thick scatterers. For this calculation we used the effective field
-
spin flip model of Wickman which assumes relaxation in the,Kramers dou- blets (SZ = 2 5/2,+
3/2,+
1/2, where S is the electron spin). This mechanism reverses effective magnetic field at the nuclear site.In calculating the theoretical fit to these
experimental data we used only one free parameter, the elastic Rayleigh contribution to the peak at the excitation energy.
Fig. 2 : This figure shows our results with ferri- chrome A at 300 K. (lmm/s = 9.09 channels). In fi- gure 2a we show a transmission spectrum taken with a 6 = 22 absorber and in figures 2b and 2c the SEDM results with an enriched 8 = 350 scatterer. Excita- tions at positions 1 and 2 indicated by arrows in figure 2a lead to the SEDM results shown in figures 2b and 2c respectively. The dashed line represents the calculated spectrum assuming no relaxation. The solid line is a calculated SEDM spectrum with te =
1.2 x 10-~s.
The relaxation time t = 1.2 x I O - ~ S . used in these SEDM calculations was determined by finding the best fit to the transmission spectra (using the spin flip model). In general our calculated SEDM line shapes
(a
-
solid lines) give fairly good fits to the expe- rimental results. Deviations from the data occur in the wings of the line in figure 2b and the absence of a sharp peak at the resonance energy (Channel 61) in our calculated spectrum of figure 2c.The results obtained at 5.4K are shown in fi- gure 3. The hyperfine splittingfrom the SZ = 2 5/2 Kramers doublet leads to a partially resolved spec- trum as can be seen from the transmission results in
figure 3a.
Fig. 3 : This figure shows our results with ferri- chrome A at 5.4 K. (I mm/s = 9.09 channels). In figure 3a we show the transmission spectrum taken with a f3 = 22 absorber and in figures 2b and 2c the SEDM results with the enriched 6 = 350 scatterer.
Excitation positions 1 and 2 indicated by arrows in figure 3a lead to the SEDM results shown in figures 3b and 3c respectively. The dashed line represents the calculated spectrum assuming no relaxation. The solid line is a calculation of the SEDM line shape assuming spin flip type relaxation with te = 1.9 x I O - ~ S .
In the SEDM experiments we excited the main central peak (position 1) and obtained a good fit. This is shown in figure 3b. However, exciting the outside peak at position 2 (thereby inducing the nuclear transition
I
I, m > =I-
1-
1 > f I$, >) did not give2' 2
results predicted by the spin flip theory of Wick- man. An electronic spin flip occuring during the nu- clear lifetime should give a line at the energy cor- responding to position 2 in the SEDM spectrum. The calculated SEDM lineshape using the superoperator formalism and assuming this spin flip relaxation mechanism shows a peak at this position for a rela- xation time of te = 1.9 x IO-'s. (This value of te was determined from a fit to the transmission spec- tra). The experimental SEDM results, however, clear-
ly show a broad line at the central resonance posi- tion (Channel 61) of the absorption cross section instead of the expected peak. Notice that even in the results shown in figure 2b and 2c there are de- viations from the SEDM spin flip model calculation.
This indicates that the relaxation mechanism in ferrichrome A is basically different from the simple spin flip model. A more general model which includes other relaxation mechanisms such as transitions amng the Kramer doublets or nonadiabatic terms may have to be invoked in order to explain the appearance of the broad central line in figure 3c, and the devia- tions from the predicted result in figure 2c.
Using SEDM techniques we have obtained new in- formation about relaxation processes in ferrichrome A. Further work along these lines is necessary to understand fully the electron spin dynamics in this system. Perhaps the most important conclusion we can draw from these results is that time dependent hyper- fine interactions in general should be investigated using SEDM techniques. Even in such a well studied and apparently simple system such as ferrichrome A this was found advantageous.
Acknowledgements.- We are grateful to Professor J.B. Neilands of the University of California, Ber- keley and Dr. J.F. Geibel of the University of Cali- fornia, San Diego for the preparation of the 5 7 ~ e
enriched ferrichrome A sample. One of us (B.B.) would like to thank Professor G.R. Hoy of Boston University for many helpful discussions and Dr.3.
Obrien of Arcon Corp. and Ms. Karen Anderson of Duke University for assistance with the computations.
References
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10
(1974)36, also Gssbauer Effect Methodology, edited hy I.J. G r u v e m n (Plenum, New York) 1974, Vo1.9.
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(1976) 2729./ 3 / Balko, B. and Hoy, G.R., J. Physique Colloq.
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(1976) C6-89.
/4/ Afanasev, A.M. and Gorobchenko, V.D., Sov. Phys.
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(1975) 1114.151 Hartmann-Boutron, F., J. Physique
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(1976) 537./ 6 / Banarjee, S., Ph.D. Dissertation, State Univ. of New York at Stony Brook (1977).
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