Quantum yield and electron-transfer reaction of the lowest excited state of uranyl ion

Abstract

The pulsed photoreduction (using N/sub 2/ laser photolysis technique) of uranyl ion, UO/sub 2//sup 2/+, both in its ground state and in the lowest excited state (at pH 2 and in 2 M H/sub 3/PO/sub 4/ aqueous solutions) has been studied. We found that *UO/sub 2//sup 2/+ is quenched by ruthenium tris (bipyridine) ions, Ru(bpy)/sub 3//sup 2/+, and Ru(bpy)/sub 3//sup 3/+ is formed; namely, *UO/sub 2//sup 2/+ is an electron-acceptor species. It is shown here that the rate of decay of *UO/sub 2//sup 2/+ in the presence of Ru(byp)/sub 3//sup 2/+ exactly corresponds to the formation rate of Ru(bpy)/sub 3//sup 3/+, as measured by the bleaching of the Ru(bpy)/sub 3//sup 2/+ ground-state absorption. The excited state of Ru(bpy)/sub 3//sup 2/+ is found to be quenched by UO/sub 2//sup 2/+, and again Ru(bpy)/sub 3//sup 3/ is formed, UO/sub 2//sup 2/+ being an electron acceptor in its ground state. The quantum yield of the formation of the lowest excited state of UO/sub 2//sup 2/+, multiplied by the yield of electron transfer, between *UO/sub 2//sup 2/ and Ru(bpy)/sub 3//sup 2/ is found to be 0.5 at pH 2 and unity in 2 M H/sub 3/PO/sub 4/ solution. In the presence of a negativelymore » charged polyelectrolyte, such as poly(vinyl sulfate), PVS, the value of this product is reduced to an upper limit of 0.1. It is shown that the lowest excited state of UO/sub 2//sup 2/ is unaffected by the presence of PVS. The back reaction, UO/sub 2/ + Ru(bpy)/sub 3//sup 3/ ..-->.. UO/sub 2//sup 2/ + Ru(bpy)/sub 3//sup 2 +/, is a second-order reaction, k/epsilon wave length = 450 nm) = 1.05 x 10/sup 5/ s/sup -1/ cm at pH 2 and 1.78 x 10/sup 2/ s/sup -1/ cm in 2 M H/sub 3/PO/sub 4/ solution. This reaction is accelerated in the presence of O/sub 2/ or PVS. The yields and rates of formations and reactions of the photochemical products are discussed.« less