51 research outputs found
Electronic ground states of Fe and Co as determined by x-ray absorption and x-ray magnetic circular dichroism spectroscopy
The electronic ground state of the Co diatomic molecular cation
has been assigned experimentally by x-ray absorption and x-ray magnetic
circular dichroism spectroscopy in a cryogenic ion trap. Three candidates,
, , and , for the electronic ground state of Fe
have been identified. These states carry sizable orbital angular momenta that
disagree with theoretical predictions from multireference configuration
interaction and density functional theory. Our results show that the ground
states of neutral and cationic diatomic molecules of transition elements
cannot generally be assumed to be connected by a one-electron process
Direct Observation of High-Spin States in Manganese Dimer and Trimer Cations by X-ray Magnetic Circular Dichroism Spectroscopy in an Ion Trap
The electronic structure and magnetic moments of free Mn and Mn
are characterized by x-ray absorption and x-ray magnetic circular
dichroism spectroscopy in a cryogenic ion trap that is coupled to a synchrotron
radiation beamline. Our results show directly that localized magnetic moments
of 5 are created by states at each ionic core,
which are coupled in parallel to form molecular high-spin states via indirect
exchange that is mediated in both cases by a delocalized valence electron in a
singly-occupied derived orbital with an unpaired spin. This leads to total
magnetic moments of 11 for Mn and 16 for Mn, with
no contribution of orbital angular momentum
Coordination-driven magnetic-to-nonmagnetic transition in manganese doped silicon clusters
The interaction of a single manganese impurity with silicon is analyzed in a
combined experimental and theoretical study of the electronic, magnetic, and
structural properties of manganese-doped silicon clusters. The structural
transition from exohedral to endohedral doping coincides with a quenching of
high-spin states. For all geometric structures investigated, we find a similar
dependence of the magnetic moment on the manganese coordination number and
nearest neighbor distance. This observation can be generalized to manganese
point defects in bulk silicon, whose magnetic moments fall within the observed
magnetic-to-nonmagnetic transition, and which therefore react very sensitively
to changes in the local geometry. The results indicate that high spin states in
manganese-doped silicon could be stabilized by an appropriate lattice
expansion
Spin and orbital magnetic moments of size-selected iron, cobalt, and nickel clusters and their link to the bulk phase diagrams
Spin and orbital magnetic moments of cationic iron, cobalt, and nickel
clusters have been determined from x-ray magnetic circular dichroism
spectroscopy. In the size regime of atoms, these clusters show
strong ferromagnetism with maximized spin magnetic moments of 1~ per
empty state because of completely filled majority spin bands. The
only exception is where an unusually low average spin
magnetic moment of ~ per unoccupied state is
detected; an effect, which is neither observed for nor
.\@ This distinct behavior can be linked to the existence
and accessibility of antiferromagnetic, paramagnetic, or nonmagnetic phases in
the respective bulk phase diagrams of iron, cobalt, and nickel. Compared to the
experimental data, available density functional theory calculations generally
seem to underestimate the spin magnetic moments significantly. In all clusters
investigated, the orbital magnetic moment is quenched to \,\% of the
atomic value by the reduced symmetry of the crystal field. The magnetic
anisotropy energy is well below 65 eV per atom
Chemical shifts and cluster structure
The 2p core-level electron binding energies of size-selected silicon cluster
ions have been determined from soft x-ray photoionization efficiency curves.
Local chemical shifts and global charging energy contributions to the 2p
binding energy can be separated, because core-level and valence-band electron
binding energies exhibit the same inverse radius dependence. The experimental
2p binding energy distributions show characteristic size-specific patterns
that are well reproduced by the corresponding electronic density of states
obtained from density functional theory modeling. These results demonstrate
that 2p binding energies in silicon clusters are dominated by initial state
effects, i.e., by the interaction with the local valence electron density, and
can thus be used to corroborate structural assignments
Large orbital magnetic moments of small, free cobalt cluster ions Co n with n lt; 9
The size dependent electronic structure and separate spin and orbital magnetic moments of free Co cluster ions have been investigated by x ray absorption and x ray magnetic circular dichroism spectroscopy in a cryogenic ion trap. A very large orbital magnetic moment of per atom was determined for Co, which is one order of magnitude larger than in the bulk metal. Large orbital magnetic moments per atom of amp; 8201; amp; 8201; amp; 8776;1 were also found for Co, Co, and Co. The orbital contribution to the total magnetic moment shows a non monotonic cluster size dependence The orbital contribution increases from a local minimum at n amp; 8201; amp; 8201; amp; 8201; amp; 8201;2 to a local maximum at n amp; 8201; amp; 8201; amp; 8201; amp; 8201;5 and then decreases with increasing cluster size. The 3d spin magnetic moment per atom is nearly constant and is solely defined by the number of 3d holes which shows that the 3d majority spin states are fully occupied, that is, 3d hole spin polarization is 10
Experimental and theoretical near edge x ray absorption fine structure studies of NO
Experimental near edge x ray absorption fine structure NEXAFS spectra of the nitrosonium NO ion are presented and theoretically analyzed. While neutral NO has an open shell, the cation is a closed shell species, which for NEXAFS leads to the simplicity of a closed shell spectrum. Compared to neutral NO, the electrons in the cation experience a stronger Coulomb potential, which introduces a shift of the ionization potential towards higher energies, a depletion of intensity in a large interval above the amp; 960; amp; 8727; resonance, and a shift of the amp; 963; amp; 8727; resonance from the continuum to below the ionization threshold. NEXAFS features at the nitrogen and oxygen K edges of NO are compared, as well as NEXAFS features at the nitrogen edges of the isoelectronic closed shell species NO , N2, and N2
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