An anomalous thermalization effect induced by optical excitation of Er 3+ ions in nanocrystals of Y 2 O 2 S is investigated. Due to the absence of low-energy phonon modes in the 20−40 nm crystals we studied, phonon relaxation is restricted, and as a result, the intensity of hot bands that originate from the upper crystal field levels in the 4 I 15/2 ground state increases rapidly as temperature decreases below 8 K. This unusual increase in hot band intensity is interpreted satisfactorily by calculations of temperature dependent multiphonon relaxation rates in nanoparticles.Our theoretical analysis provides a fundamental understanding of confinement effects on the spectroscopic properties of rare earth ions in nanocrystals. This analysis applies also to the previously observed unusual hot bands in 4−6 nm particles of Eu 2 O 3 .
An interpretation of the low-temperature absorption spectra of AnF4 (An=U, Np, Pu) is presented. Using an effective operator Hamiltonian with orthogonalized free-ion operators and initializing crystal-field parameter values based on a superposition model calculation for An4+ sites with C2 symmetry, good agreement between the model calculations and experimentally observed absorption band structure could be obtained. Correlations with published magnetic and heat capacity measurements are discussed.
An accurate semiclassical method to predict ground-state tunneling splittingsWe report optical-spectroscopic studies of the ground-state splitting of the nominal S-state ion Cm3+ in LaCI 3 . A total splitting of 1.97 ±O.05 cm -1 for the 8 Y 712 ground state of this system has been measured for the first time. Both electric and magnetic dipole transitions were observed. Energy levels of all four Kramers doublets of the 8 Y 7/2 manifold were resolved by means of the site-selective excitation and emission spectra of 8 Y 7/2++6g;7/2 and 8 Y 7/2++695/2 transitions. Our results show that the lowest component of the ground manifold is a JL=5/2 state dominated by M = I ± 7/2) in contrast to a previously reported JL = 1/2 state. The crystal-field model, established through analyzing optical spectra of the excited states, predicts a groundstate splitting with the right ordering but greater magnitude. In comparison with the lanthanide S-state ion Gd3+, much stronger spin--orbit coupling and reduced electrostatic interaction in concert with greater crystal field interaction significantly influence the characteristics of the ground state of the actinide S-state ion. Contributions from within the 5f7 configuration account for most of the observed splitting.
The theory of multiphonon vibronic coupling to electronic transitions is applied in analysing fluorescence spectra of EuZf in BaFCI, which consist of the 4f7(6P7 ,) + 4f7(8S712) and 4f65d + 4f7 transitions, and the 4f74f65d excitation spectrum of Ce-+ in YP04. The 4f electrons are weakly coupled to lattice vibration modes so that only weak one-and twophonon sidebands are observable in the 4f4f optical transitions, whereas the electronphonon coupling is significantly stronger for a 5d electron. Accordingly, intensive multiphonon vibronic transitions overwhelmingly dominate the 4f65d + 4f7 spectrum. It is shown that the extended Judd-Ofelt theory for weak vibronic coupling in the framework of the M-process is equivalent to the Huang-Rhys theory for the A-process. In the analysis of experimental data, contributions from local ligand modes and lattice acoustic modes are separated, and the coupling strength is evaluated, in terms of the Huang-Rhys parameter S, for the 4f4f and 5d4f vibronic transitions. 4-
The x-ray excited optical luminescence (XEOL) from a variety of rare-earth ions was used as a detection mode for the collection of L-edge x-ray absorption fine-structure (XAFS) data. In order to understand the source of the observed optical signal, advantage is taken of the known luminescent response of f ions in a variety of transparent host materials. Whereas some samples exhibit an optical response that is indistinguishable from the transmission XAFS data, other samples show marked differences between the data obtained with the two different detection schemes. The unexpected optical luminescence of a Gd2O3 sample is traced to a Eu impurity. An optical spectrum of 0.4% Tb in Gd2O2S, excited by x-ray photons at the Gd edge, is used to demonstrate that the optical signal may arise from an ion different from the absorbing ion. The implications of this energy transfer are discussed in terms of the suitability of XEOL as a detection scheme for XAFS spectroscopy.
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