Mössbauer Studies onFe57Atoms in Rare-Gas Matrices between 1.45 and 20.5 K

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Abstract
The Mössbauer absorption spectra of Fe57 have been measured in the rare-gas matrices argon, krypton, and xenon with iron atomic concentrations from 0.3 to 3% and matrix temperatures between 1.45 and 20.5 K. All of the spectra show an absorption line with an isomer shift of δ=−0.75±0.03 mm/sec with respect to an iron foil at 300 K. This isomer shift is independent of rare-gas matrix, iron concentration, and matrix temperature. This line is ascribed to an isolated Fe57 atom (monomer) with an atomic configuration of 3d64s2. The measured isomer shift gives a new calibration point in the isomer-shift-versus-electron-density plot for Fe57. The observed 1T temperature dependence of the monomer linewidth shows that the direct phonon process is dominant in the spin-lattice relaxation mechanism. Spin-lattice relaxation times of the order of 2.5 × 10−10 sec are obtained by assuming a hyperfine field of 1.1 × 106 Oe at the Fe57 nucleus due to an iron atom with unquenched orbital momentum. From the temperature dependence of the Mössbauer f factor, the Mössbauer temperatures ΘM in the Debye model are calculated and compared with the values expected from specific-heat measurements. The Mössbauer spectra show, in addition to the monomer absorption line, a pair of narrow lines (Γ=0.22−0.3 mm/sec), which is intensified with increasing iron concentrations. These lines can be interpreted as the result of the quadrupole splitting of the I=32 excited state of Fe57 in the axial field produced by an iron nearest neighbor (dimer). The measured quadrupole splitting is ΔEQ=4.05±0.04 mm/sec, and the isomer shift for the dimer is δ=−0.14±0.02 mm/sec, corresponding to an effective atomic configuration of 3d64sx with x=1.47±0.04. For Fe57 in krypton and xenon, the measured dimer/monomer ratio is that which one would expect from probability considerations, but in argon it is a factor of approximately 3 higher than expected.