Mössbauer spectra of the quadrupole hyperfine structure in the presence of the rotation of the principal axis of the electric field gradient tensor

It is found that the rotation of the principal axis of the electric field gradient tensor about a certain axis radically changes the shape of the Mössbauer spectra of the quadrupole hyperfine structure. This change may be a manifestation of a qualitatively different physical origin of rotational dynamics in liquids. In particular, the absorption spectra of $^{57}$Fe nuclei may contain the triplet, quartet, and octet of lines instead of the standard quadrupole doublet. Simple analytical expressions derived for a number of limiting cases describe the spectra in terms of the effective quadrupole coupling constant and the rotational splitting constant for the main lines. The observed features of the formation of quadrupole hyperfine structure spectra must be taken into account in analysis of experimental Mössbauer spectra in liquids.