Excitation Energy Dependence in Near-infrared Photoluminescence Spectra and Dynamics of PuO 2 2+ in Cs 2 U(Pu)O 2 Cl 4

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1104-NN08-08

Excitation Energy Dependence in Near-infrared Photoluminescence Spectra and Dynamics of PuO22+ in Cs2U(Pu)O2Cl4 John M Berg, and Marianne P Wilkerson Los Alamos National Laboratory, PO Box 1663, Los Alamos, NM, 87545 ABSTRACT Recently we reported the observation of near-infrared photoluminescence from metalcentered 5f electronic excited states of PuO22+ doped into polycrystalline Cs2U(Pu)O2Cl4.[1] Photoluminescence dynamics following pulsed excitation show complicated decay patterns suggesting that multiple luminescent states are involved. Here we report the results of two recent sets of experiments showing that photoluminescence processes depend significantly on the energy of photoexcitation. In the first case, decay kinetics following excitation at a lower energy are missing an in-growth term that is present when exciting at higher energy. In the second case, we have observed that lower excitation energy produces significantly reduced number of emission transitions than higher excitation energy. Both observations suggest that higher energy excitation populates feeder states that decay to emitting states, causing signal from the latter to have an in-growth followed by a decay characteristic of their intrinsic lifetimes, whereas lower energy excitation leads to more direct population of luminescent states. INTRODUCTION

Relative Intensity

The photoluminescence of Cs2U(Pu)O2Cl4 following excitation by visible light includes transitions throughout the near-infrared spectral region, as we have recently reported and as can be seen in Figure 1.[1] These transitions are assumed to be principally due to decay between 5f electronic states and to include numerous vibronic sidebands. This paper reports new data that will help to clarify the assignments of the transitions.

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Figure 1: Photoluminescence spectrum of Cs2U(Pu)O2Cl4 at 75 K following excitation at 15,920 cm-1.

All excitations in the visible range produced essentially similar luminescence spectra, though modulated in intensity as the excitation was tuned across absorption peaks reported by Gorskov and Mashirov.[2] Luminescence decay kinetics following pulsed (10 ns) excitation reveal considerable complexity however. At least three overall decay rates were measured, and many of the transitions do not begin immediately but show distinct in-growth terms. At least four clearly distinct combinations of in-growth and decay rates were noted from among the observed near-infrared transitions. This implies that there are multiple luminescent states contributing to the observed spectra, some being populated by potentially complicated decay processes from the higher-energy states that are directly excited by the light pulse. Our earlier results were suggestive but not sufficient to determine the initial and final states in the observed transitions. To move closer to definitive assignments, it is important to identify which luminescence transitions terminate on the ground electronic s