REACTIONS OF PULSED-LASER EVAPORATED URANIUM ATOMS WITH MOLECULAR-OXYGEN - INFRARED-SPECTRA OF UO, UO2, UO3, UO2+, UO22+, AND UO3-O2 IN SOLID ARGON

Uranium atoms from the Nd:YAG laser ablation of a uranium target were codeposited with molecular oxygen and excess argon at 12 K. Infrared spectra following the U+O2 reaction revealed a wide range of reaction products. The 776.0 cm-1 band due to UO2 was the strongest product absorption, strong UO3 bands were observed at 852.5 and 745.5 cm-1, and a weak UO absorption appeared at 819.8 cm-1. These product absorptions are in agreement with earlier work, which evaporated UO2 from a tungsten Knudsen cell at 2000-degrees-C. The O-16(2)/O-18(2) reaction gave only (UO2)-O-16 and (UO2)-O-18, which verified an insertion mechanism. New product absorptions were observed at 952.3, 892.3, and 842.4 cm-1. The 842.4 cm-1 absorption due to the UO3-O2 complex and the 892.3 cm-1 band assigned to the charge-transfer complex (UO2+)(O2-) grew markedly at the expense of the other uranium oxides during annealing the matrix to allow diffusion and reaction of O2. With 25% O-16(2), 50% (OO)-O-16-O-18 and 25% O-18(2) samples, the 952.3 cm-1 band became a sharp triplet at 952.3, 936.5, and 904.5 cm-1 and exhibited an isotopic ratio appropriate for a linear OUO species. Agreement of this band with uranyl ion spectra suggests assignment to a (UO22+) complex. Mechanisms of formation of charged species are discussed.