MATRIX RAMAN SPECTRA OF THE ALKALI SUPEROXIDE MOLECULES $M^{+}O_{2}^{-}$ AND THE IONIC MODEL

Abstract

The Raman spectrum of the ${Li^{+}O_{2}}^{-}$ molecule has been observed from samples of oxygen codeposited with lithium atoms at $15^{\circ} K$ . An intense band at $1093 cm^{-1}$ exhibited appropriate oxygen isotopic shifts for assignment to the symmetric oxygen-oxygen vibrational mode. Intense bands at 1094, 1108, 1110, and $1114 cm^{-1}$ have been observed following matrix reactions of Na, K, Rb, and Cs atoms, respectively, with $Ar/O_{2} = 100$ samples. Experiments using $^{16}O_{2}$, $^{16}O^{18}O$, $^{18}O_{2}$ isotopic mixtures produced triplets indicating equivalent oxygen atoms in these $M^{+}O_{2}^{-}$ molecules. In all cases no bands were observed corresponding to the interionic $M^{+}--O_{2}^{-}$ vibrations, which were very strong in the infrared. The observed Raman intensities support the ionic model $ M^{+}O_{2}^{-}$ with $O_{2}^{-} $frequencies increasing with increasing polarizability of $M^{+}$ as the antibonding electron density of $O_{2}^{-} $ is correspondingly decreased.