The Mössbauer effect in surface studies Fe57 on Ag
The Mossbauer effect for atoms located at crystal surfaces is discussed. We show that the Einstein model is valid for describing qualitatively the mean square displacement of lattice atoms. The mean square displacement (Mossbauer recoilless fraction) and mean square velocity (MOssbauer second-order Doppler shift) of atoms in one and three-dimensional crystals with free surfaces are treated in a many-body approach The simple models indicate that at high temperatures surface atom mean square displacements are on the order of twice bulk atom mean square displacements (or equivalently that surface atom Debye temperatures are about 1//2 times the corresponding bulk values) and that surface effects penetrate only a few atomic spacings into a crystal. A possible temperature dependence of the Mossbauer isomer shift due to anharmonic binding forces at surfaces is pointed out. The experimental difficulties faced in a surface Mossbauer study, performed under ultrahigh vacuum conditions and the apparatus used in an attempt to overcome them are described. M~ssbauer spectra of Fe57 on LiF, in Ag, and £n Ag have been obtained in the temperature range 100-400oK. The LiF experiment is assumed to be a:measurement of Fe on W which had contaminated the LiF. This experiment yielded a broad line which we interpret as a quadrupole split pair of lines due to a field gradient normal to the surface of about -4 x 1016 volts/cm2 e The Fe in Ag experiment gave a single narrow line shifted at room temperature (.056 + .002) cm/sec with respect to iron in iron, a temperature gradient of line position -(7.5 ± 0.5) x 10-5 cm/(sec.deg), and an effective Debye temperature obtained from the temperature dependence of the recoilless fraction of (253 + 12)oK. The Fe on Ag measurements yielded an asymmetric line interpreted as a quadrupole split line due to a field gradient of -3 x 1016 volts/cm2• A very small isomer shift with respect to iron in iron was observed along with a temperature dependence of the line position of -(5.1 + 0.6) x 10-5cm/(sec.deg). An effective Debye temperature of (380 + 30)oK was found for Fe on Ag.
Mossbauer effect, crystal surface studies, Einstein model, Mossbauer recoilless fraction, Mossbauer second-order Doppler shift, Fe57 on Ag, many-body
University of Illinois - Urbana-Champaign
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