Electric dipole allowed transition

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. 

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). 

Crystal field, or d-d, transitions are defined as transitions from levels that are exclusively perturbed d orbitals to levels of the same type. In other words, the electron is originally localized at the central metal ion and remains so in the excited state. When the system has ( symmetry, Laporte s rule says that an electric-dipole allowed transition must be between a g state and an u state, i.e., u - g. Since all the crystal field electronic states are gerade ( g ), no electric-dipole allowed transitions are possible. In short, all d-d transitions are symmetry forbidden and hence have low intensities. The fact that the d-d transitions are observed at all is due to the interaction between the electronic motion and the molecular vibration. We will discuss this (vibronic) interaction later (Section 8.10). 

Figure 2.14. The coupling scheme by which the spin-forbidden a 3A2(rc,7t ) <— X Ai(n2) transition borrows oscillator strength from the B 1A1(jt,jt ) <— X iAi(n2) electric dipole allowed transition.

Having the qualitative pattern of the VRT levels, one can turn to electric dipole allowed transitions. The allowed dipole transitions between states labeled by the irreps r2 and r2 of G8 are obtained from the relation393 ... 

Light absorbed by an atom or molecule excites it from the initial ground (or excited) state to a higher-energy excited state for low-intensity light, this occurs, provided that the various applicable quantum rules for the transition are satisfied (electric-dipole "allowed" transitions). If quantum rules "forbid" a transition, then the transition is either absent ("strongly forbidden transition") or very weak ("weakly allowed transition"). The "Jablonski"110 diagram (Fig. 3.16) depicts various forms of absorption and emission from... 

The optical spectra of conjugated polymers are dominated by an intense absorption due to the electric dipole allowed transition between the highest occupied and the lowest unoccupied electronic states. For solid samples the energy of this transition can lie anywhere between about 0.8 and 4 eV. Data for solid samples are listed in Table 9.1 and for solutions and solid samples in Table 9.2 on page 351. 

It is found experimentally that the intensity of both the magnetic dipole and vibronically induced electric dipole allowed transitions are principally determined by the orbital character of the electronic states. That is, transitions that are forbidden in the orbital basis but allowed in the spin-orbit basis in Table 1, are likely to be weak. 

In the second case above, since the states have the same symmetry, both are required to give electric-dipole-allowed transitions. The most important examples of this type, in the context of resonance Raman scattering, are the much studied metal-lo-porphyrin molecules which constitute the active sites of the haem proteins, notably haemoglobin and cytochrome c (Section 4.8). The visible and near ultraviolet absorption spectra of these systems show two -n - -n transitions of the porphyrin ring both are allowed with in-plane polarisation and excited-state symmetry. The lower... 

The purpose of this section is to draw attention briefly to two interesting examples of progressions involving both totally symmetric and non-totally symmetric modes. First, the resonance Raman spectra (50) of various salts of the [AuBr4]" ion (D4h symmetry) show progressions in the i i (Aig) Au—Br stretching frequency based on one quantum of V2 (Big) (up to 2 f j) and also based on a quantum of J 4( 2g) (up to 1 4 + 5 J i). Excitation was at 457.9 nm, which lies near the absorption maximum (22,500 cm ) of a broad band assigned (51) to a superposition of two electric-dipole-allowed transitions, both of the type a dx2 yj(Au) - 7r p(Br),... 

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