We report polarization-dependent on-resonance and off-resonance photoemission and x-ray absorption measurements to probe the character and the symmetry of the occupied and empty valence-band (VB) states of the four-layered misfit cobaltate ${\left[{\text{Bi}}_2{\text{Ca}}_2{\text{O}}_4\right]}^{\text{RS}}\cdot {\left[{\text{CoO}}_2\right]}_{1.67}$. These experiments bring clear evidence that the one-electron removal VB photoemission spectra, between the Fermi energy $\left(E_F\right)$ and $\sim 4.5\text{eV}$, contain $\text{Co}3d$ states, which are strongly hybridized with $\text{O}2p$ states in the binding energy region of $\sim 1.8–\sim 4.5\text{eV}$ below $E_F$. The states at the Fermi energy have $a_{1g}$ out-of-plane symmetry. In addition, by tuning the incident photon energy across the $\text{Co}2p_{3/2}\to \text{Co}3d$ $\left(a_{1g}\right)$ threshold and the $\text{O}1s\to \text{O}2p–\text{Co}3d$ $\left(a_{1g}\right)$ hybridized threshold, both found at $\sim 2\text{eV}$ below the absorption edge maximum, it is possible to observe a transition from a Raman regime to an Auger regime. From this transition the charge transfer dynamics from the $\text{Co}3d$ to the $\text{O}2p$ states and from the $\text{O}2p$ to the $\text{Co}3d$ states have been estimated to be $\sim 3$ and $\sim 3.3\text{fs}$, respectively. This combined information suggests an electron hopping process between neighboring $\text{Co}{a}_{1g}$ states across $E_F$ and mediated by $\text{O}2p$ states with suitable symmetry.

• Received 13 January 2009

DOI:https://doi.org/10.1103/PhysRevB.80.035114