On the Physical Nature of Uranyl Charge Transfer Vibronic Interactions
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On the Physical Nature of Uranyl Charge Transfer Vibronic Interactions X. Y. Chen1, L. F. Rao2, and G. K. Liu1*, 1 Chemistry Division, Argonne National Laboratory, Argonne, IL 60439 2 Glenn T. Seaborg Center, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 ABSTRACT We address the electronic properties of uranyl ions in solids and solutions with an emphasis in theoretical understanding of charge transfer vibronic transitions and luminescence dynamics the O-U-O species. A general theory of ion-phonon interaction has been modified for modeling and simulating multi-phonon vibronic spectra. Spectroscopic data for uranyl ions in crystals and solutions have been analyzed to achieve a predictive understanding of the uranyl-ligand vibronic interactions. By adjusting the Huang-Rhys ion-phonon interaction parameters, an excellent agreement between theory and experiment has been accomplished for uranyl ions in the ligand environments we studied. Our modeling and simulation provide insights into the physical nature of uranyl vibronic interaction and its influence on spectroscopic properties, which are commonly utilized in characterizing photochemical properties of uranyl in complexes. INTRODUCTION From uranium to americium, the lighter elements in the actinide series often bind with two oxygen anions to form actinyl ions. The uranyl ion (UO22+) with a bond distance in the range of 175-185 pm is extraordinarily stable in solutions as well as in crystals and glasses. The environmentally sensitive optical spectra of uranyl species, arising from O-U-O charge transfer (CT) transition, have been widely used for characterization of local structure and coordinations around the uranyl. It is also well known that vibronic states originating from the charge transfer-vibronic interaction are quite localized due to the short bond distance as well as high vibrational energy (750-900 cm-1). As a result, the absorption and emission spectra of the UO22+ ion in single crystals such as Cs2UO2Cl4 indeed exhibit extremely sharp lines [1], in which the progressions are exclusively induced by the totally symmetric local mode. Unlike the vibronic spectra of f-d or f-f origins for lanthanide and actinide, the CT vibronic spectra of UO22+ apparently do not include the contributions from coupling to the lattice acoustic modes other than to the local stretching mode. As a result, the uranyl vibronic spectra are simpler than that coupled to the f-f and f-d transitions [2-6]. Although, optical spectroscopy and laser-induced fluorescence are widely used in actinide chemistry and speciation, the nature of photochemistry and photophysics of uranyl complexes is currently lacking a fundamental understanding in general. A dimensionless Huang-Rhys parameter (S), which is closely related to the transition mechanism of optical centers in the host, is generally used to represent the extent of electron-phonon coupling [7]. Theoretical work on the basis of Franck-Condon (FC) transition was reported for comparison with experimental results from rare ea
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