Knowledge Bank

MCSCF and MCSCF/CI computations have been carried out for the ground and several excited states of $O2$-both in vacuo and in simulated ionic lattices. The potential energy curves are fitted to modified Morse functions. The $a4\Sigma a\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}$ state is unique in representing a stable anionic excited state in the gas phase, while the $A2\Pi u$ state is metastable. From stability plots the $a4\Sigma u\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}$ state is unstable with respect to electron autodetachment at bond lengths where it lies above the ground state of $O2$. The effect of an ionic host lattice on the spectroscopic parameters is simulated by an array of point charges in which the anion is oriented . All electronic states examined are electronically stable in these lattices. However the spectroscopic constants, particularly of the more highly excited states, differ markedly from those in the gas phase. The $\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}\Sigma u\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}$ states exhibit an avoided crossing with no analog in the gas phase. A pronounced crystal-field splitting is found for all $\Pi$ states in the lattice. The computed spectroscopic properties of the $A\leftrightarrow X$ transition are compared with experimentally known absorption and luminescence spectra that have been attributed to this system in alkali halide hosts