J T Lau

The size dependent electronic structure and separate spin and orbital magnetic moments of free Co$_n^+$ ($n = 4\;\textrm{-}\;9$) 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 $1.4\pm0.1\;μ_{\mathrm B}$ per atom was determined for Co$_5^+$, which is one order of magnitude larger than in the bulk metal. Large orbital magnetic moments per atom of $\approx 1\;μ_B$ were also determined for Co$_4^+$, Co$_6^+$, and Co$_8^+$. The orbital contribution to the total magnetic moment shows a non-monotonic cluster size dependence: The orbital contribution increases from a local minimum at $n = 2$ to a local maximum at $n = 5$ 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 100%.

The $^{4}Φ_{9/2}$ ground state of the Ni$_2^+$ diatomic molecular cation is determined experimentally from temperature and magnetic-field-dependent x-ray magnetic circular dichroism spectroscopy in a cryogenic ion trap, where an electronic and rotational temperature of $7.4 \pm 0.2$ K was reached by buffer gas cooling of the molecular ion. The contribution of the magnetic dipole term to the x-ray magnetic circular dichroism spin sum rule amounts to $7\, T_z = 0.17 \pm 0.06$ $μ_B$ per atom, approximately 11 % of the spin magnetic moment. We find that, in general, homonuclear diatomic molecular cations of $3d$ transition metals seem to adopt maximum spin magnetic moments in their electronic ground states.

The $^6Π$ electronic ground state of the Co$_2^+$ 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, $^6Φ$, $^8Φ$, and $^8Γ$, for the electronic ground state of Fe$_2^+$ 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 $3d$ transition elements cannot generally be assumed to be connected by a one-electron process.