Argon matrix Raman spectrum of LiO2. Bonding in the M+O2− molecules and the ionic model

The Raman spectrum of LiO₂ in argon matrices has been observed using 4880 Å excitation. The intraionic (O–O)⁻ mode was observed as a sharp intense band at 1097 cm⁻¹; the symmetric interionic $({\mathrm{Li}}^{+}{\mathrm{–O}}_2^{-})$ mode was recorded as a weaker broad feature at 694 cm⁻¹. Oxygen‐18 and lithium‐6 substitution confirmed these vibrational assignments. The trend of increasing ${\mathrm{O}}_2^{-}$ vibrational frequency with increasing alkali atomic weight in the ${\mathrm{M}}^{+}{\mathrm{O}}_2^{-}$ molecules is rationalized using the Rittner ionic model of polarizable ion pairs.