Effect of nuclear motion on Mössbauer relaxation spectra

Abstract

Relaxation effects in M\"ossbauer line shape are studied when the nucleus jumps from one position to another. Relaxation arises from the fact that the nucleus interacts differently with its surroundings at different sites. At the same time, an additional broadening is observed because of nuclear motion. The effect of surroundings on the nucleus is treated classically in a stochastic-theory model in which it is assumed that the surroundings act like a heat bath and drive thermal fluctuations into the system which cause the nucleus to jump from time to time, from one site to another. But as it jumps to a new site it finds itself in a different quantum-mechanical enviornment. This gives rise to relaxation effects. It is found that nuclear motion precludes the occurrence of motional narrowing of lines which is typical of a pure-relaxation spectrum in the fast-relaxation-rate limit. The Singwi-Sj\"olander diffusion broadening and the Blume result for pure-relaxation effects are obtained as two limiting cases of the theory. Application of the model to experiment is discussed.