Lanthanide Systems

Webster and Drickamer [255] studied the Dj Fj emission of Eu + (4f ) doped at several concentrations in LajOjS as a function of pressure. The energy level diagram of Eu + is shown in Fig. 19. Emission spectra were measured upon excitation into the charge transfer state. At ambient pressure, emission occurs from the Dq 12 states to several of the Fj states. Upon increasing pressure up to 110 kbar, the emission intensity from the i states was observed to decrease steadily. On the contrary, emission intensity from the D2 state increased between 0 kbar and 20 kbar, remained approximately constant between 20 kbar and 80 kbar, and decreased above 80 kbar. The decrease in D2 intensity coincided with the onset of new emission from the state. In addition to intensities, Webster and Drickamer also measured the emission Hfetimes from the Di 2,3 states. The values of the lifetimes depended on the Eu + concentration, but the trends with pressure were uniform. The emission lifetime remained nearly constant with pressure while the D2 emission Hfetime increased between 0 kbar and 20 kbar and remained constant above 20 kbar. The D3 lifetime could be measured above 70 kbar and was observed to increase up to 110 kbar. 

The decreased emission intensity and constant hfetime with pressure indicate that pressure is influencing the population of the state rather than its non-radiative decay rate. Webster and Drickamer developed a Dj excitation model based on feeding rates from the charge transfer state involved in the excitation process. According to the model [256,257], the Dj states are populated through transfer of excitation energy from the charge transfer state and depopulated by back transfer. Upon excitation, transfer occurs sequentially to the 

0 states. The rate of back transfer from a given Dj state depends on the activation barrier separating the state and the charge transfer state. At ambient pressure, the activation energies from the Dq, 1,2,3 states to the charge transfer state are 6300 cm5100 cm , 3000 cm , and ioOO cm , respectively. The low activation barrier associated with the 03 state is responsible for its efficient depopulation and lack of emission intensity at room temperature. 

Webster and Drickamer measured the variation of the energy of the charge transfer state with pressure and showed that it increased by 2000 cm between 0 kbar and 110 kbar. The increased charge transfer energy leads to increased activation barriers with the Dj states. Since the activation barrier associated with the D3 state is the smallest at ambient pressure, back transfer from the state will be most strongly inhibited with pressure. As a result, the population of the state progressively increases with pressure. Above 70 kbar, the back transfer rate becomes sufficiently small that emission from the state is observed. The initial increase in the lifetime and intensity of the D2 emission with pressure indicates that the D2 state is also thermally depopulated at room tempera- 

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Bray KL (2001) High Pressure Probes of Electronic Structure and Luminescence Properties of Transition Metal and Lanthanide Systems. 213 1-94 Bronstein LM (2003) Nanoparticles Made in Mesoporous Solids. 226 55-89 Bronstrup M (2003) High Throughput Mass Spectrometry for Compound Characterization in Drug Discovery. 225 275-294... 

Enforced by the preorganized bimetallic array, a more symmetrical cyanato bridge has been observed in (106). Triple /r-l,l,l-NCO linkages have been reported for heterobimetallic nickel/ lanthanide systems.423... 

The analytical data on these small hydrocarbon lanthanide systems indicated that a variety of reactions were occurring. Evidence was... 

One of the few advantages of the weak exchange interactions present in most lanthanide systems is that the Zeeman energy associated with typical laboratory... 

Other uranium binary systems de-mixing in the liquid state are U-Pb and U-Bi and several uranium-lanthanides systems which are characterized by nearly complete immiscibility in the liquid and solid state. 

Figure 13 compares the results of the calculations with Eq. (22), for the AnN and AnAs systems with the experimental lattice parameters and with the corresponding lanthanide systems. This latter comparison evidences, also, in these compounds, that the presence of a departure from a monotonous, almost linear curve, as found for lanthanides, is a clear sign of metallic 5 f bonding. 

Mixed valence phenomena, such as studied by photoelectron spectroscopy in lanthanide systems, are expected to become important especially (but not only) in the second half of the actinide series. It is to be expected that much of the photoelectron spectroscopic effort will be in the future devoted to the study of these phenomena in actinides, especially as soon as measurements on hazardous actinides will become more feasible. 

Bray KL (2001) High Pressure Probes of Electronic Structure and Luminescence Properties of Transition Metal and Lanthanide Systems. 213 1-94... 

Riehl, J. P. Muller, G. Circularly polarized luminescence spectroscopy from lanthanide systems. In Handbook on the Physics and Chemistry of Rare Earths, Gschneidner, K. A. Biinzli, J.-C. G. Eds. North-Holland Publishing Company Amsterdam, 2005 Vol. 34, pp 289-356. 

James P. Riehl and Gilles Muller, Circularly polarized luminescence spectroscopy from lanthanide systems 289... 

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