Allowed electronic transitions

Fig. 4.19. Allowed electronic transitions to the K shell and corresponding X-ray lines after ionization of an atom (two forbidden transitions are also shown as dashed lines).

On the right side of Fig. 3-9 are represented the relative energies of the two Tig terms, the and 2g. The ground term is the from the 2g configuration. Spin-allowed electronic transitions (those between terms of the same spin angular momentum - but see also Sections 3.6, 3.7 and Chapter 4) now take place upon excitation from -> A2g, The d-d spectra of octahedrally... 

We note that three spin-allowed electronic transitions should be observed in the d-d spectrum in each case. We have, thus, arrived at the same point established in Section 3.5. This time, however, we have used the so-called weak-field approach. Recall that the adjectives strong-field and weak-field refer to the magnitude of the crystal-field effect compared with the interelectron repulsion energies represented by the Coulomb term in the crystal-field Hamiltonian,... 

Only electric-dipole- and spin-allowed electronic transitions in the proximity of the observed transitions are tabulated. Anticipated overlap with atomic absorptions. 

Emission of light due to an allowed electronic transition between excited and ground states having the same spin multiplicity, usually singlet. Lifetimes for such transitions are typically around 10 s. Originally it was believed that the onset of fluorescence was instantaneous (within 10 to lO-" s) with the onset of radiation but the discovery of delayed fluorescence (16), which arises from thermal excitation from the lowest triplet state to the first excited singlet state and has a lifetime comparable to that for phosphorescence, makes this an invalid criterion. Specialized terms such as photoluminescence, cathodoluminescence, anodoluminescence, radioluminescence, and Xray fluorescence sometimes are used to indicate the type of exciting radiation. 

Figure 6. Calculated potential energy curves for sextet states of FeO. The ground electronic Slate and excited states accessible by allowed electronic transitions from the ground state are shown. Points are calculated using TD-DFT at the B3LYP/6-311G(d,p) level. Sohd hnes are S states and dashed lines are II states, the vertical dashed hne indicates for the ground state. The experimental value of the dissociation energy is also diown for reference.

The term I scattering (equation 6) is dominant when the excitation laser wavelength is resonant with an allowed electronic transition (M° / 0, M° M ). The... 

The term II scattering (equation 7) from vibronic activity in allowed electronic transitions mainly results in fundamental transitions of non-totally symmetric vibrations. This term corresponds to the B and C terms of the Albrecht theory25. 

ADAIR EQUATION COOPERATIVE LIGAND BINDING HILL EQUATION PLOT KOSHLAND-NEMETHY-EILMER MODEL MONOD-WYMAN-CHANCEUX MODEL NEGATIVE COOPERATIVITY POSITIVE COOPERATIVITY HEMOGLOBIN ALLOTOPIC EFFECT Allowed electronic transitions,... 

TABLE 3.1 Allowed" Electronic Transitions of Diatomic Molecules Having Light Nuclei1 ... 

In a complex that possesses a center of symmetry, all states arising from a dn configuration have the g character inherent in the d orbitals. Since the dipole moment vectors belong to odd representations, all of the integrals such as / y/gXi//g dr are identically zero because the direct product of two g functions can never span any u representations. On this basis alone, we would predict that transitions between the various states arising from dn configurations in octahedral environments would have zero absorption intensity. In fact, these transitions do take place but the absorption bands are only —lO"3 times the intensity expected for symmetry allowed electronic transitions. Thus the prediction we have made is substantially correct, but at the same time there is obviously some intensity giving mechanism that has been overlooked. 

Unambiguous assignments of the band maxima of the allowed electronic transitions are only possible for the longest-wavelength bands of ions with Td and C3p symmetry. However, all bands of Table 1 of Chapter 16.1 can be assigned qualitatively to metal reduction transitions (from MOs with predominantly ligand character to ones essentially localized on the central atom). Empirically, this... 

A complex 7THG can result from one-, two-, or three-photon resonances. One-photon resonance occurs when the fundamental frequency co is close to an allowed electronic transition. Two-photon resonance occurs when 2co is close to a two-photon allowed electronic transition. For centrosymmetric molecules the two-photon selection rule couples states of like inversion symmetry, e.g. g <- g. For acentric molecules one-photon transitions can also be two-photon allowed. Three-photon resonance occurs when 3co is close to the energy of an electronic transition the same symmetry rules apply as for one-photon transitions. 

The energy stored in an excited state is dissipated by the unimolecular radiative and radiationless relaxations. For strongly allowed electronic transitions, Strickler and Berg have obtained an expression for the rate constant of the excited-state radiative decay (Equation 6.67).31... 

In addition, we can obtain the following allowed electronic transitions ... 

The CD induced in 2-(2,4,6-cycloheptatrien-l-ylidine)-4-cyclopentene-l,3-dione upon its inclusion in 6-cyclodextrin was used to obtain full assignment of the observed absorption bands [49]. It was concluded that the low energy band system was composed of three electronic transitions, two of which were polarized perpendicular, and one parallel, with respect to the long molecular axis of the solute. The higher energy band system was found to also consist of three allowed electronic transitions, but two of these were polarized parallel, and one perpendicular, to the long axis of the solute molecule. These deductions were supported by a series of CNDO/S-CI calculations performed on the system. A similar approach was taken to assign the polarizations of the various transitions associated with 2-thioxo-l,3-benzodithiole and 2-selenoxo-1,3-benzodithiole [50]. 

Next we recall that the strongly allowed electronic transition between the 2S+ ground state and the essentially repulsive 2S+ first excited state, induced by the... 

CT and II.(NN) states mix very little (if at all), because of a large energy gap (However, this situation is profoundly different in the triplet manifold, see Sect. 4). There are not many Re complexes in which the lowest allowed electronic transition is IL(NN). Atypical case is presented by the family of complexes with N,N = pyridylimidazol l,5-a pyridinc derivatives [35, 36], where the IL(N,N) state occurs below a MLCT/XLCT state. [Re(L)(CO)3(R-dppz)] 1 complexes are another example [28, 29],... 

On the basis of these calculations one can tentatively associate the low energy absorption of Ag,(440-474 nm) with the 2Sg+ 2EU+ dipole allowed electronic transition, i.e.,... 

Resonance occurs in Raman scattering when the incident energy is equal (or nearly equal) to an allowed electronic transition of the material under... 

Spin-allowed electronic transition An electronic transition which does not involve a change in the spin part of the wavefunction. 

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