Chiroptical luminescence spectra of uranyl ion in cubic sodium tris(acetato)dioxouranate(1-) crystals

Abstract

Steady-state chiroptical luminescence measurements are reported for cubic crystals of Na(UO{sub 2}(CH{sub 3}COO){sub 3}). These crystals belong to the enantiomorphic space group P2{sub 1}3, with four molecules per unit cell, and each UO{sub 2}(CH{sub 3}COO){sub 3}{sup {minus}} coordination unit has a chiral tris-bidentate chelate structure of C{sub 3} symmetry. The UO{sub 2}O{prime}{sub 6} coordination clusters (where O{prime} denotes an acetate oxygen donor atom) also have chiral structures of C{sub 3} point-group symmetry, but they deviate only slightly from an achiral D{sub 3h} symmetry. The luminescence observed for Na(UO{sub 2}(CH{sub 3}COO){sub 3}) is assigned to transitions that originate from the lowest electronic excited state (II{sub g}) of UO{sub 2}{sup 2+} and terminate on the ground electronic state ({Sigma}{sub g}{sup +}). At least two types of UO{sub 2}{sup 2+}species contribute to this luminescence, but the luminescence spectra can be analyzed in terms of separate majority species (or bulk site) contributions and minority species (or defect site) contributions. The luminescence spectra show zero-phonon origin lines, one-phonon false-origin lines, and vibronic progressions in the symmetric stretching mode ({nu}{sub s}) of UO{sub 2}{sup 2+}. The false-origin lines and the progressions based on these lines are essentially unpolarized. However, the origin lines and their progressions exhibitmore » a very large degree of circular polarization, with emission dissymmetry factors of g{sub em} = 1.31 (majority species) and g{sub em} = 0.96 (minority species). The circularly polarized luminescence results for {Sigma}{sub g}{sup +} {l arrow} II{sub g} emission are compared to circular dichroism results for {Sigma}{sub g}{sup +} {r arrow} II{sub g} absorption, and the distribution of {Sigma}{sub g}{sup +} {leftrightarrow} II{sub g} electronic rotary strength among origin and vibronic lines is discussed within the context of vibronic optical activity theory.« less