4/ - 5d transition

Short decay times can be obtained by using luminescent ions with allowed emission transitions. In the Held of inorganic materials the best examples arc the 5d 4/ transitions (t 10 ns) (Sects. 2.3.4 and 3.3.3) and the cross luminescence (t 1 ns) (Sect. 3.3,10) [11]). The afterglow is governed by the presence of traps in the host lattice as described in Sect. 3.4. 

It is observed that the 5d 4/transition of Ce " ion has a large A, where the d and/states have opposite parity, so that it is an allowed electric dipole transition. As a result, this transition has a very short decay time, e.g., Ce " in BaHfOs has a decay time of r = 20 ns. Activators may occupy more than one symmetry site in a crystal lattice, such as Eu " in (Y,Gd)203 Eu, in which the Eu ion occupies both the symmetric 85 site and the nonsymmetric C2 site [93-95]. Due to the difference in crystal field between the two sites, they have different A and thus different r. In this case, the decay time is the sum of two. Generally, the total probability A of a transition from the excited state to the relaxed state is given by ... 

Another well-known example of 5d 4/ transitions is Eu " (4 ) ion. The influence of host lattice to the energy of the 5d band is the same as Ce " ion, but the Eu " ion has 4/ levels. When the Eu ion is located in a weak crystal field, the energy of its 5d bands shifts to high energy, and the level of the 4/ configuration lies below it. Thus, sharp-line emission from the > 87/2 transition... 

Other 5d 4/transitions of lanthanide are available for Sm " (4/ ) and Tm (4/ ) ions. They are rare to observe and difficult to synthesize even under a... 

Figure 1.7 illustrates the overall factors of the host lattice affecting the emission properties of 5d 4/ transitions [3]. The nephelauxetic effect is the result of the covalency of the host lattice and ligand polarization. The polarization of nitrogen... 

Fig. 1.7 Schematic of the energy level shifts for 5d 4/transitions in octahedral coordination [3]...

Figure 7.11 shows the photoluminescence excitation and emission spectra of the olivine-type NaMgP04 Eu phosphor. The phosphor exhibits a strong broad red-emission band centering at 628 nm, which is due to the 5d 4/ transition of... 

Values for the spin Hamiltonian are given in Table XIV. The 5D state of d6 has three orbital states for the ground state in octahedral symmetry. Since these three states are connected by the spin-orbit coupling, the spin-lattice-relaxation time is quite short and the zero-field splitting very large. In a distorted octahedral field the large zero-field distortion makes detection of ESR difficult. In the case of ZnF2 the forbidden AM = 4 transition was measured and fitted to Eq. (164). 

The peaks a. a2 arise from the transitions to (4f5 2)(5d ) configuration, <23 to (4f7,2)(5d ). 4 to transitions to the mixed states of (4fs 2)(5d/,) and (4f7 2)(5dj). Therefore, the difference between peaks a and a2 originates from the multiplet splitting within (4fs 2)(5dfl) configuration, i.e., the interaction between the 4fs 2 electron and 5da electron, while the difference between peaks a, a2 and peak 03 originates from the spin-orbit splitting of the Pr 4f state. Peak 04 arises from the transitions to Pr 5d/, levels. 

The sharp drop in the core-hole electron attraction U compared to the bulk value (AU, Figure 3) around N = 4.5 and 6.5 for the basic and acidic supports respectively is obviously due to the onset of the insulator to metal transition. The size of AU (= 3.5 eV) for the very small clusters is consistent with previous estimates of U(5d5d) = 3-6 eV for the nearly filled 5d band transition metals when efficient metallic screening is shut off. Most interesting is the large difference in the behavior of the core-hole electron attraction for the basic vs. acidic supports. This difference dramatically reveals the importance of the support in determining the nature and onset of the metal-insulator transition. It provides direct experimental evidence for a metal-support interaction. 

Fig. 4.48. Absorption spectra of the Eu(5D() <-7Fq) transition for the dimetallic (9.9 x 10 3 M) and monometallic (2.7 x 10 3 M) complexes with p-r-butylcalix[8]arene, at 298 K, in DMF the shoulder at 577.9 nm corresponds to a reaction intermediate. Reproduced with permission from J.-C.G. Biinzli et al., in Calixarenes for Separations, ACS Symposium Series, eds G J. Lumetta, R.D. Rogers, A. Gopalan, Vol. 757, Ch. 14, American...

Figure 14. A simplified energy level diagram showing why there is significant difference in the line (band) widths for f-f vs. d-f transitions in Eu. As the crystal field is increased (A) the spacing of the 4/ states does not change due to their symmetry agreement. However, the 4pd state drops appreciably in energy. For low crystal fields the f-f emission band will be quite narrow, while for high fields the A/5d to 4/ transition will be very broad. See also Figures 1 and 7. Modified after Shionoya and Yen (1999).

How is the parity selection rule relaxed Vibrations have only a very weak influence. For interesting consequences of this influence the reader is referred to Ref. [13]. Of more importance are the uneven components of the crystal-field which are present when the rare earth ion occupies a crystallographic site without inversion symmetry. These uneven components mix a small amount of opposite-parity wave functions (like 5d) into the 4/wavefunctions. In this way the intraconfigurational 4/° transitions obtain at least some intensity. Spectroscopists say it in the following way the (forbidden) 4/-4/transition steals some intensity from the (allowed) 4/-5rftransition. The literature contains many treatments of these rare earth spectra, some in a simple way, others in considerable detail [1,16,17,18,19]. 

Under certain conditions 5d-4/cmission has also been observed for Pr (4/ ) and Nd " (4/ ). For example, LaB306 Pr shows band emission around 260 nm and I.aF3 Nd around 175 run. Due to the r A relation, the decay time of the latter is only 6 ns [11]. However, these ions have an alternative way to emit, viz. by an emission transition in the 4/" configuration. 

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 Sm ion (4/ ) can show 5d- 4/emission in the red. However, if the lowest level of the 4 f 5d configuration is at high energy, the intraconfigurational 4/ emission is observed. This runs parallel with the case of although the transitions are at a much longer wavelength. 

Transition-metal complexes, organometallic compounds, and catalysts containing metal ions with incomplete 3d, 4d, or 5d electron subshells. The detection of V(IV) (which has the ls 2j 2/ 3j 3p 3d configuration) in crude petroleum is one notable application of ESR spectroscopy. 

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