Chemical States of Fe atoms Implanted into Solid Oxygen

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We have performed an in-beam Mossbauer spectroscopic experiment using a short-lived ^<57>Mn(T_<1/2>=1.5 min) beam, in order to study the production of the novel chemical species of ^<57>Fe atoms arising from ^<57>Mn implanted into solid oxygen. It is expected that the high electron affinity of oxygen favors the production of high valence states of Fe.^<57>Mn particles were produced as a secondary RI beam following a nuclear projectile-fragmentation reaction of ^<58>Fe beams (E=63 MeV/nucleon) with Be production target, and separated by the in-flight isotope separator using RIPS(Riken Projectile-fragment Separator) in RIKEN Accelerator Research Facility. Subsequently, the ^<57>Mn was implanted into a solid O_2 sample after passing through an Al plate with an appropriate thickness to suppress the beam energy down to about 20 MeV/nucleon. The in-beam Mossbauer spectra were measured at 18, 32 and 40 K during the implantation of ^<57>Mn. The obtained spectrum at 18 K could be analyzed by 4 sets of Lorentzian doublet. From the basis of Mossbauer parameters of the isomer shifts and the quadrupole splittings and the ab-initio molecular orbital calculations, these 4 components were assigned to be Fe(O_2), FeO, and (O_2)Fe(O_2), and a chemical species in a higher oxidation state of Fe atoms. We discussed the reaction products between ^<57>Fe atoms arising from ^<57>Mn and a solid O_2 matrix, compared with the products of laser-ablated Fe atoms with O_2 gas isolated in the Ar matrix at low temperatures.