Influence of rotational diffusion of the Mössbauer spectrum of ultrafine particles in a supercooled liquid

The theory of the Mössbauer line shape of ultrafine particles in a liquid has been developed taking into account both translational and rotational diffusion of the particles. A simple analytical expression has been found for the line shape in the limiting case of fast rotational diffusion. In this limit the line shape appears to be independent of the rotational diffusion constant apart from a constant and a scaling factor. The quadrupole splitting remains visible even in this limiting case in contrast to the case of molecular rotation diffusion spectra. The predictions of the theory are compared with experimental spectra of nanosize iron oxide particles dispersed in supercooled decalin where a rapid decrease of the total area with increasing temperature has been found. The present theory can account for a part of the observed loss of spectral area. It is also demonstrated that the uncertainty in the determination of the total area and diffusion constants from the Mössbauer spectra increases significantly when the rotational diffusion is taken into account.