A Mössbauer study of aftereffects of Auger events in cobalt chelates with varying degree of conjugation

Abstract

The electron-capture decay of a cobalt-57 atom triggers an Auger event resulting in the loss of several electrons from the molecule in which it is incorporated. The 14.4 keV Mössbauer emission conveys information regarding the chemical forms in which the daughter iron-57 is stabilized within 10−7 sec following electron capture. The electronic relaxation occurs very rapidly and several tens of eV electronic excitation energy is deposited on the molecule during charge neutralization. Emission Mössbauer spectra of cobalt-57 labeled complexes of the strong field ligands phenanthroline and ethylenediamine, namely [57Co(III)(phen)3](ClO4)3⋅2H2O, [57Co(III)(phen)2(en)]Cl3⋅H2O, [57Co(III)(phen)(en)2] (NO3)3, and [57Co(III)(en)3]Cl3 were studied. The chelate molecules possess varying degree of conjugation. The presence of strong field ligands in the chelates made it easier to identify the high spin ionic iron species Fe2+ and Fe3+, which can form only as a result of fragmentation. Molecules which have no conjugation, e.g., [57Co(III)(en)3]3+, or which possess small degree of conjugation like [57Co(III)(phen) (en)2]3+, fragment in practically 100% of the Auger events. On the other hand, the molecules of [57Co(III)(phen)2(en)]3+, which possess relatively higher degree of conjugation, escape fragmentation in about 20% of the events; while the highly conjugated molecules of [57Co(III) (phen)3]3+ seem to escape fragmentation in almost 100% of the events. One can perhaps conclude that the probability of fragmentation of a molecule as a consequence of the Auger event diminishes with higher degree of conjugation in the molecule. It is speculated that the aftereffects of the Auger event result in collective excitation of the π-electrons in a highly conjugated system. The de-excitation of the ’’plasmon’’ occurs rapidly, presumably through emission of a single electron or an energetic photon. The mixed ligand chelates, [57Co(III)(phen)2(en)]3+ and [57Co(III)(phen) (en)2]3+ show an interesting phenomenon. The cationic iron species formed as a result of fragmentation recombines with electron(s) to form an electronically excited cation, presumably 57Fe+2 which interacts with neighboring cobalt chelate molecules. The phenanthroline ligands are preferentially transferred onto the electronically excited cation forming [57Fe(III)(phen)3]3+ quite efficiently. The transfer of ligands occur in less than 10−7 sec., as is evidenced by the manifestation of the tris-phenanthroline chelate in the emission Mössbauer spectra.