Knowledge Bank

The F center in calcium oxide consists of two electrons trapped at a vacant $O2-$ site. Calculations indicate that the ordering of the electronic energy levels from the ground state is $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}A1g$, $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}T1u$, $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}A1g$, $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}T1u$. The $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}T1u$ level presumably lies just below the conduction band. An absorption band due to the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}A1g$ $\to$ $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}T1u$ transition is observed at 400 nm at $300\circ K$, and a characteristic long-lived orange emission band at 601 nm is apparently due to the $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}T1u$ $\to$ $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}A1g$ transition. Investigations of the level scheme of this center have been made by laser-excited infrared and visible luminescence and by pseudo-two photon absorption techniques. A luminescence band centered at approximately 600 $cm-1$ at $80\circ K$ is tentatively assigned to the $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}A1g$$\to$ $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}T1u$ transition. Attempts have been made to observe the inverse transition, $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}T1u$ $\to$ $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}A1g$, by maintaining the population of the long lived $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}T1u$ state with laser excitations at 457.9, 488.0 and 514.5 nm during the absorption measurements. The effects observed in the absorption and luminescence experiments will be discussed in terms of a configuration coordinate diagram of the F center in CaO.