Intensities forced electric dipole

After X-ray irradiation of thermally annealed NaCl crystals, a small percentage of divalent europium ions are converted into trivalent europium ions (Aguilar et al, 1982). This is shown by the appearance of weak and narrow absorption lines at around 460 nm and 520 nm, related to the Fq D2 and Fq Di transitions of Eu + ions, respectively. For our purposes, this example allows us to compare the different band features between (RE) + and (RE) + ions Eu + ions show broad and intense optical bands (electric dipole allowed transitions), while Eu + ions present narrow and weak optical lines (forced electric dipole transitions). 

Similar results have been reported for Eu3+ in glasses (J4) germanate glasses where the Eu3+ c.t. band is situated at 38462 cm i show a more intense forced-electric-dipole emission than phosphate glasses, where the c.t. band lies at 49020 cm i. These excimples illustrate the influence of the c.t. state upon the Eu3+ 4/- 4/ emission. 

The crystal field model may also provide a calciflation scheme for the transition probabilities between levels perturbed by the crystal field. It is so called weak crystal field approximation. In this case the crystal field has little effect on the total Hamiltonian and it is regarded as a perturbation of the energy levels of the free ion. Judd and Ofelt, who showed that the odd terms in the crystal field expansion might connect the 4/ configuration with the 5d and 5g configurations, made such calculations. The result of the calculation for the oscillator strength, due to a forced electric dipole transition between the two states makes it possible to calculate the intensities of the lines due to forced electric dipole transitions. 

The first known work on the intensities of the intra 4f—4f transitions is that of Van Vleck [53]. This was followed by the work of Jurbner and coworkers [54,55]. In 1962 Judd [56] and Ofelt [57] independently proposed the theory of forced electric-dipole transition which enabled the compilation of oscillator strengths for lanthanide aquo ions. 

Empirical correlation of intensities of absorption bands with the structure of complexes in solutions have been made for lanthanide complexes. It has been recognized that forced electric-dipole transitions of low intensities, in some cases lower in intensity than those of magnetic-dipole transitions, may indicate that the ligand field has point group symmetry with a center of inversion. This criterion has been used in the determination of the ligand field by symmetry of Eu3+ aquo ion [202], The absorption band intensity ratios have been used to show the octahedral structure [49] of lanthanide hexahalide complexes, LnXg. ... 

Forced Electric Dipole Transitions. In more recent work, Judd (15) has given further attention to the problem of intensities. According to this work, under certain symmetry restricted circumstances, the Hamiltonian for the interaction of a lanthanide ion with its neighbors can contain spherical harmonics with fc = 1 if the electrons of the rare-earth ion produce an electric field at the nucleus that exactly cancels that... 

The model which has been most widely applied to the calculation of vibronic intensities of the Cs2NaLnCl6 systems is the vibronic coupling model of Faulkner and Richardson [67]. Prior to the introduction of this model, it was customary to analyse one-phonon vibronic transitions using Judd closure theory, Fig. 7d, [117] (see, for example, [156]) with the replacement of the Tfectromc (which is proportional to the above Q2) parameters by T bromc, which include the vibrational integral and the derivative of the CF with respect to the relevant normal coordinate. The selection rules for vibronic transitions under this scheme therefore parallel those for forced electric dipole transitions (e.g. A/ <6 and in particular when the initial or final state is /=0, then A/ =2, 4, 6). 

Note that the excited state has a component. It turns out that strong transition intensities in the lanthanides occur due to a transition termed "forced-electric-dipole" transitions, where AJ s 2. This rule holds in all cases where strong intensities have been observed. Thus, for our case, the transition involves the 6H13/2 state, because of the AJ = 2 restriction. 

This is nicely confirmed by a study of some Eu -activated phosphates and vanadates with zircon structure (Blasse and Bril, 1969). The observed ratio of electric to magnetic dipole emission of the Eu " luminescence in these hosts is correlated with the position of the lowest excitation (and absorption) band of these materials and the intensity ratio. This absorption band is a c.t. transition in which either europium or vanadium or both are involved. It has, therefore, been proposed that the parity-forbidden 4f-4f transitions of the Eu " ion borrow intensity from the lowest strong absorption band (either host lattice absorption or charge-transfer absorption within the centre) and not from the 4f-5d absorption band. In conclusion we find that for intense forced electric-dipole emission from Eu two conditions must be fulfilled, viz. absence of inversion symmetry at the Eu " crystallographic site and c.t. transitions at low energies. 

Generally, the room temperature emission spectra of Ln species show incompletely resolved stmcture within the peaks. However, an advantageous attribute of luminescent Ln complexes is the dependence of this emission spectral form on the specific coordination environment of the ion. This sensitivity arises from the selection rules associated with intraconfigurational (4f-4f) electronic transitions the selection rules for forced electric dipole transitions are relaxed due to 5d and 4/orbital mixing. In reality the majority of the complexes included for discussion here are non-centrosymmetric, low symmetry species and the relative intensities of the 4/-4/transitions are generally determined by the induced electric dipole transition selection rules. It should also be noted that visibly emissive Eu also possesses a magnetic dipole transition, F, whose intensity is relatively independent of the coordination environment [1,9]. 

Usually we call neutral molecule as polar one if it has considerable permanent electric dipole moment /i°. The total dipole moment should include also an induced one, aR (a is a polarizability of the molecule, R is the intensity of electric field interacting with molecule), and may be presen ted as /i = /<° + a . Permanent part of dipole moment for nonsymmetrical organic molecules usually accepted to be essentially larger than induced one that is why orientational forces or interactions of permanent electric dipoles are the most important in polar solutions [1,2,4,12, 39]. 

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