Frequency Spectra of Quantum Beats in Nuclear Forward Scattering of 57Fe: The Mössbauer Spectroscopy with Superior Energy Resolution

Abstract

Frequency spectra of quantum beats (QB) in nuclear forward scattering (NFS) are analysed and compared to Mossbauer spectra. Lineshape, number of lines, sensitivity to minor sites, and other specific properties of the frequency spectra are discussed. The most characteristic case of combined magnetic and quadrupole interactions is considered in detail for 57Fe. Pure magnetic Zeeman splitting corresponds to a eight-line spectrum of QB, six of which show the same energy separation as the six lines in Mossbauer spectra. Two other lines (called 2′ and 3′) are the lower-energy satellites of the lines 2 and 3. As the quadrupole interaction EQ appears, the satellites remain unsplit in the quantum beat frequency spectra, as well as the first (zero-frequency) and the 6th (largest frequency) lines. Each of the lines 3 and 5 generates a doublet split by 2EQ, and the lines 2 and 4 generate triplets. In QB frequency spectra (QBFS) of thin absorbers of GdFeO3 we demonstrate the enhanced spectral resolution compared to Mossbauer spectra. Small particle size in an antiferromagnet (Fe2O3) was found to affect the QBFS via enhancement of the intensity around zero-frequencies. An asymmetric hyperfine field distribution mixes up into the hybridization with dynamical beats, which enlarges the frequencies of the low-lying QBFS lines and makes their shifts relatively large compared to the shift of the highest-frequency line.