Influence of Local Phonon Damping on Mössbauer Spectra

Abstract

The contribution of localized-mode phonon emission and absorption to the Mössbauer emission process is evaluated in the presence of damping. Net-zero-phonon processes, in which the number of localized phonons (of a given type) emitted is the same as the number absorbed, are shown to contribute to the intensity in the vicinity of the unshifted gamma-ray energy. This contribution is shown to be distributed over a minimum width $\Gamma +2\mathrm{}\gamma$ where $\Gamma$ is the gamma-ray natural width, and $\gamma$, the width of the localized phonon, is larger than $\Gamma$. The complete Mössbauer spectrum including all multiphonon contributions can be obtained in our approximation by using the continuous spectrum of nonlocalized phonons plus a Lorentz-broadened peak associated with each localized phonon. Optical absorption at impurities in solids may have linewidths $\Gamma >$ the phonon widths $\gamma$, thus permitting local phonon enhancement of the zero-phonon transitions in impurity absorption or emission.