Transition dipole-allowed

Ceo samples [66]. At higher photon energies, dipole-allowed transitions ean occur, and the optical absorption increases dramatieally. 

Figure 15-3 shows the optical absorption spectrum of a MEH-PPV/C60 film with different C, content compared to the optical absorption spectrum of the components alone. The peak at 2.5 eV is identified as the n-n absorption of MEH-PPV and is clearly observed along with the first dipole-allowed transition in C(l0 (at 3.75 eV). The spectrum is a simple superposition of the two components. Further-... 

The dipole oscillator strength is the dominant factor in dipole-allowed transitions, as in photoabsorption. Bethe (1930) showed that for charged-particle impact, the transition probability is proportional to the matrix elements of the operator exp(ik r), where ftk is the momentum transfer. Thus, in collision with fast charged particles where k r is small, the process is again controlled by dipole oscillator strength (see Sects. 2.3.4 and 4.5). 

At resonance with an electric dipole allowed transition, the Stokes resonance Raman scattering, I(tt/2), associated with a single totally symmetric mode and its overtones is proportional to... 

Some results are given in Figure 1 for [MnC ]", and related ones have been obtained for [ReS ]", [MoS ]2" and [WS ]2" (J -7), for each of which the lowest electric-dipole-allowed transition (1t2 Uj) is clearly vibronically structured (a situation which is brought about by the fact that ouj > r). The best fit 6 value derived for each ion (0.05-0.09 %) indicates that, in the T2 state, the change undergone is rather larger than that typical of a one-electron reduction of the ion e.g. 0.03 A for [MnO (8). 

The average energy flux in the evanescent wave is given by the real part of the Poynting vector S = (c/47t)ExH. However, the probability of absorption of energy per unit time from the evanescent wave by an electric dipole-allowed transition of moment pa in a fluorophore is proportional to lnfl - El2. Note that Re S and pa E 2 are not proportional to each other they have a different dependence on 0. 

In a similar way, we can approximate the transition probability for a magnetic dipole allowed transition by... 

We see that the electric dipole allowed transitions are, in general, much more intense than the magnetic dipole allowed transitions. In fact, the magnetic dipole contribution to an optical transition of a center dominated by an electric dipole character is usually completely masked by the much more intense J electric dipole transitions. 

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). 

It was not until quite recently that the presence of octahedral lanthanide(III) complexes with CN = 6 has been demonstrated. In 1966 Ryan and Jergensen were able to prepare the anionichexahalide complexes of the lanthanides, [MXe] " in non-aqueous (nitriles) solvents [33, 34). Complexes of the type [(C6H5)3PH]3 [MXe] and [C5H5NH]3 [MXe], where M =Pr, Nd, Sm, Eu, Dy, Er, Yb and X- = Cl, Br were obtained as solids. The electronic (/- -/) transitions in these and in hexaiodide, [Mle] -, complexes are extremely weak, except for the magnetic dipole allowed transitions [35). Both electronic and electron transfer spectra (4f- 5d) indicated the presence of octahedral MX species. 

In Figures 9.8 and 9.9, the absorption line shapes as a function of the detuning from the Fj -Fq resonance are shown. The case of unstructured continua is presented in Figure 9.8, in which the situation is shown when only the IFq) level (the one with the dipole allowed transition to the ground state) is broadened (Fq = 0.05 X 10 a.u.). No detuning 82 = 0) of the center of the strong pulse... 

As tetrahedral closed-shell molecules, all dipole-allowed transitions of these complexes will be to an excited state with 1T2 symmetry. 

Schipper52) discussed the LICD in terms of group theory using Murakami and Hatano s experiments51. This new spectroscopic method permits the direct observation or discrimination of electric or magnetic dipole-allowed transitions. 

Specifically, the collision-induced absorption and emission coefficients for electric-dipole forbidden atomic transitions were calculated for weak radiation fields and photon energies Ha> near the atomic transition frequencies, utilizing the concepts and methods of the traditional theory of line shapes for dipole-allowed transitions. The example of the S-D transition induced by a spherically symmetric perturber (e.g., a rare gas atom) is treated in detail and compared with measurements. The case of the radiative collision, i.e., a collision in which both colliding atoms change their state, was also considered. 

It is well known that the tetrahedral frame of the CH4 molecule is easily distorted. If the tetrahedral frame of CH4 were robust, the purely rotational infrared spectra of CH4 would not exist. However, even at temperatures as low as room temperature, the CH4 molecule features hundreds of very weak, dipole-allowed rotovibrational lines at frequencies from 42 to 208 cm-1, the so-called groundstate to groundstate (gs—>gs) transitions. Moreover, more than 1500 weak, dipole-allowed transitions exist within the polyad system v /v — 1/2/1, at frequencies from 14 to 500 cm-1 [42]. These allowed transitions arise from distortions of the tetrahedral frame by rotation and the internal dynamics of the CH4 molecule, due to the coupling of normal modes of the flexible CH4 frame. Collisional frame distortion should probably be associated with unresolved gs— gs and similar polyad bands. Some evidence of such collision-induced bands of CH4 in CH4-X complexes has been pointed out [39-41]. Besides these collision-induced bands that presumably are due to collisional frame distortion of CH4, fairly significant, unexplained collision-induced bands also exist that are shaped by rotovibrational transitions of the collisional partner X = H2, N2, or CH4, and by double transitions of the bimolecular CH4-X complex [39-41]. 

The generalization to the control of the dynamics of a molecule with n electronic states is straightforward. For the purpose of deducing the control conditions we will examine the extreme case in which every possible pair of these electronic states is connected via the radiation field and a nonzero transition dipole moment. If the molecule is coupled to a radiation field that is a superposition of individual fields, each of which is resonant with a dipole allowed transition between two surfaces, the density operator of the system can be represented in the form... 

The above relations define the conditions for concurrent control of population and energy transfers between all of the states of the system that are connected by dipole allowed transitions. It is unlikely that a situation that complicated will ever be encountered. In the n-state molecule language, typically, not all pairs of states of the molecule are connected with nonzero transition dipole moments. In the skeleton spectrum language, there is usually only a small subset of dipole coupled doorway states. In both cases, of course, when only some pairs of states are coupled with nonzero transition dipole moments, the appropriate control conditions are simplified. 

In connection with certain forms of spectroscopy (e.g., circular dichroism and magnetic circular dichroism) it is necessary to know what electronic transitions are magnetic dipole allowed. The operators for this have the symmetry properties of r, and Rz. For a molecule of Tlt symmetry, determine what pairs of states could be connected by a magnetic dipole allowed transition. 

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