Excitation Wavelength-Dependent Emission from Americium(III) in Mineral Acid Solutions

The photoluminescence behavior of trivalent americium, Am(III), in mineral acid solutions is reported under direct-metal excitation at seven high-energy wavelengths (378-512 nm) in the ultraviolet and visible regions. This study revealed a dependence of the Am(III)-based visible emission profile on the chosen excitation wavelength. For example, excitation at the prominent absorption band at 504 nm (5L6' ← 7F0') produces the two expected emission bands in the visible region: one at approximately 600 nm (5D1' → 7F0') and another at 690 nm (5D1' → 7F1'). When the excitation wavelength is shifted to 398 nm, targeting the 5D3', 5H7' ← 7F0' transitions, the emission occurs almost exclusively at 600 nm. This behavior is unexpected and potentially highlights fundamental knowledge gaps in the current understanding of the intraconfigurational 5f-5f electronic transitions that give rise to Am(III) photoluminescence. Although a full interpretation of the results presented here are limited by the incomplete understanding of Am(III) speciation in mineral acid systems, this study explores several potential explanations for the observed behavior and challenges the widely held assumption of a single Am(III) emissive species in these systems.