Crystal field components

The assumption of a large crystal-field interaction for Pu5+ spectra makes it necessary to conclude that while certain aspects of earlier free-ion estimates (37) are valid, the "assignment" of free-ion states to observed absorption bands was premature. Much of the structure must be due to crystal-field components of many free-ion groups that overlap in energy or to vibronic satellites similar to those encountered in CS2UCI6 (33). Thus, while the present computations would agree with earlier work in interpreting the levels observed in... 

Under the action of a crystal field component of lower symmetry each state of Ok will split up further. Under tetragonal symmetry (D ), Tig and Eg decompose as follows ... 

The 4f-> 5d transitions of nearly all Ln + ions have been observed in CaF2 (9). Luminescence from these transitions has been studied in detail for Eu2+ 10), Sm + (77) and Yh + (12). In good approximation these spectra can be ascribed as transitions between the 4/ ground state and the d crystal-field components of the state. The influence of the surroundings on these transi-... 

Compositions CsMgj xNixCl3 show Ni2+ emission at about 5000 cm-1 [40]. The emission band shows vibrational structure yielding an S value of about 2.5. From this value Qo—QZ is found to be 0.7 A, which gives dr=0.24 A for the change in the Ni-Cl distance. This emission is due to a transition from one of the crystal-field components of the first excited state 3nt ground state (3d8, Oh notation). The lifetime of the excited state is 5.2 ms. The luminescence is quenched above 200 K. 

Lochhead and Bray (1995) studied Eu3+ doped sodium disilicate glass with a high-pressure fluorescence line-narrowing technique. This technique was used to characterize the local structure of the Eu3+ ions up to a pressure of 21 GPa. For the crystal-field analysis they assumed a C2v site symmetry which allowed for a complete splitting of the crystal-field components. The crystal-field strength was determined according to eq. (11). The effect of pressure... 

Luminescence lifetime depends upon radiative and nomadiative decay rates. In nanoscale systems, there are many factors that may affect the luminescence lifetime. Usually the luminescence lifetime of lanthanide ions in nanociystals is shortened because of the increase in nomadiative relaxation rate due to surface defects or quenching centers. On the other hand, a longer radiative lifetime of lanthanide states (such as 5Do of Eu3+) in nanocrystals can be observed due to (1) the non-solid medium surrounding the nanoparticles that changes the effective index of refraction thus modifies the radiative lifetime (Meltzer et al., 1999 Schniepp and Sandoghdar, 2002) (2) size-dependent spontaneous emission rate increases up to 3 folds (Schniepp and Sandoghdar, 2002) (3) an increased lattice constant which reduces the odd crystal field component (Schmechel et al., 2001). 

The absorption spectra of Pr(III) in solutions in the visible region have four bands due to the transitions from the ground state to 3P2, 3Pi, +[l6, 3Po and D2 levels. Relatively more complete spectra of Pr(III) in dilute DCIO4, ethyl acetate and molten mixture of L4NO3 and KNO3 have been obtained [139], So far, it has not been possible to resolve the crystal field components in the absorption spectra of Pr(III) in solution. The aqueous absorption spectrum of Pr3+ ion is depicted in Fig. 8.17. 

The excitation mechanism of the anti-Stokes emission is as follows. First the ion is excited into the Dq level. Although the Fo- Do transition is highly forbidden (J = 0- J = 0), it has a rather high absorption strength in LaOCl due to the strong linear crystal-field component at the La CEu ) site. The lifetime of the Do level is long (of the order of milliseconds). A second photon is now absorbed, which raises the system to the charge-transfer state. This is an allowed transition. More-... 

Pr with a Uf value of 824 cm. , as compared with a predicted value of 820 cm. . In addition to the peaks shown in Figure 2, a number of weak bands were observed, ranging from 403 to 3910 cm. . The absorption at 403 cm." probably arises from the asymmetric Pr—F stretching vibration, but the origin of the other peaks (probably crystal-field components) has not been ascertained as yet. An interesting feature of the above data may... 

In our model, the emission originates from the T2 level at all temperatures, and no polarization of the emission is expected. It is, however, unlikely that non-cubic crystal field components are totally absent. Such a crystal-field component would lift the degeneracy of the T2 level and cause polarized emission at very low temperatures. 

The analysis of crystal-field components has remained at the single-particle level introduced by Bethe ( ). Crystal-field parameters for the actinide ions in lanthanum trichloride are shown in Table III. They are approximately twice as large as the values found for the lanthanides. Although the values... 

Under the influence of the crystal field, these states split, as shown for a trigonal field, into crystal field components. The crystal field strength of AI2O3 is 20,000 cm 1 as shown, and this accounts for the exact energies... 

Figure 3.14 gives the emission spectrum as a function of temperature. At 4.2 K there is line emission from P7/2 (and a weak vibronic structure). At 33 K the thermally activated emission from the higher crystal-field components of P7 2 appears, together with a broad band due to the 4f 5d 4f transition. This band has a zero-phonon line, indicated 0. At 110 K the band dominates. 

The small pressure shifts of f —> f transitions (see Sect. 3.2.2) imply only small changes in the relative energies of 4f" states with pressure. As a result, electronic crossovers involving 4f" states of lanthanides are uncommon and limited to closely spaced crystal field components within a given term [193,262]. Much larger shifts, however, are observed for the 5d states of lanthanides (Sect. 3.2.3) and it becomes possible to observe 5d-4f electronic crossovers in lanthanide systems. 

The Aicm are called crystal-field components. The parametrized crystal-field Hamiltonian is valid for a wide range of single electron-lattice interactions the decoupling represented by eq. (34), however, breaks down when nonelectrostatic effects such as overlap of neighboring ions (ligands), exchange, and covalency are included. In interpreting experimental data, the Bt are usually treated as variable parameters in a least-squares fit. 

An electrostatic model of crystal-field interactions can be developed in which the crystal-field components A n defined by eq. (32) are expanded in a multipolar series (Hutchings and Ray, 1%3). It is assumed that the lattice consists of a series of points, one for each ion in a lattice, that have multipolar moments associated with them that are defined by... 

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