Localized high spin states in transition-metal dimers: X-ray absorption spectroscopy study

X-ray absorption spectroscopy provides direct evidence for localized valence electrons in ${\text{Cr}}_2^{+}$, ${\text{Mn}}_2^{+}$, and ${\text{CrMn}}^{+}$ dimer cations. Bonding in these transition-metal molecules is predominantly mediated by $4s$ electrons. This behavior is markedly different from other $3d$ transition-metal dimers with open $3d$ subshells and can be ascribed to the highly stable $3d^5\left({}^{6}S\right)$ configuration of the $3d$ subshell in chromium and manganese atoms and ions. In ${\text{Cr}}_2^{+}$, ${\text{Mn}}_2^{+}$, and ${\text{CrMn}}^{+}$, $3d$ electron localization indicates local high spin states.

Figure 1

(Color online) Surprisingly similar x-ray absorption spectra of atomic chromium (Ref. 18), ${\text{Cr}}_2^{+}$, and ${\text{CrMn}}^{+}$ taken at the chromium $2p$ edge. The spectra are dominated by $2p\to 3d$ transitions and can be reproduced by atomic multiplet calculations assuming a $3d^5$ initial-state configuration. This directly shows localization of $3d$ electrons. $2p\to 4s,4d$ transitions do not contribute significantly to the spectra.

Figure 2

(Color online) X-ray absorption spectra of atomic manganese (Ref. 21), ${\text{Mn}}_2^{+}$, and ${\text{CrMn}}^{+}$ taken at the manganese $2p$ edge. As for chromium, the spectra are dominated by $2p\to 3d$ transitions and can be reproduced by atomic multiplet calculations assuming a $3d^{5}4s^1$ initial-state configuration, again indicating localization of $3d$ electrons. Differences in the spectra of atomic manganese and ${\text{Mn}}_2^{+}$ are due to different photon energy resolution.