Outer-Shell Internal Conversion of the 14.4-keV Transition inFe57: Chemical Effects and Line-Shape Differences

Internal conversion of the 14.4-keV transition in the outer shells of ${\mathrm{Fe}}^{57}$ following the decay of ${\mathrm{Co}}^{57}$ has been examined at 0.05% momentum resolution. The intensity of the conversion in the outermost shell, $N_1$ in the free atom, relative to that of the $M_1$ shell shows a dependence on the chemical environment of the decaying ${\mathrm{Co}}^{57}$ atom. ${\mathrm{Co}}^{57}$ ions from an electromagnetic isotope separator were deposited at <25 eV on the surface ("oxide state") or were allowed at 500 eV to penetrate a natural graphite crystal lattice ("metallic state") where oxidation could not occur. The intensity ratio $\frac{N_1}{M_1}$ ("oxide") is 0.024±0.002 and $\frac{N_1}{M_1}$ ("metal") is 0.034 ±0.003. The $N_1$-shell line shapes show less low-energy tail than the $M_1$ lines (which are only ∼0.7% lower in energy), an effect probably due to less outer-shell electron shakeoff in the case of the $N_1$ lines. This result throws doubt on the accuracy of the analyses of previous experiments in which the outermost-shell conversion lines were not well enough resolved to observe this effect and in which the assumption of similar line shapes were used.