Infrared spectra of matrix-isolated uranium oxides. III. Low-frequency modes

Infrared absorption spectra between 50 and 350 cm−1 have been obtained of argon matrices containing UO3 and UO2. The frequencies of all three expected low‐frequency vibrational modes of U16O3 have been identified and measured (±0.1 cm−1): ?3(A1)=186.2, ?5(B1)=211.6, and ?6(B2)=151.5 cm−1. The effects of 18O‐substitution have been measured and interpreted to confirm these assignments. All results are consistent with a C2v, T‐shaped structure for the UO3 molecule. The bending mode of U16O2 was measured at 225.2±0.5 cm−1. The measured frequencies of U16O18O and U18O2, the measured relative absorbances, and the observed mid‐infrared spectrum support the assignment for UO2. In addition, broad lower frequency bands were observed that may be due to impurity‐activated phonon modes of solid argon.