Parity forbidden

Charge-transfer excitations from odd ligand levels to the even metal ys and y>3 levels clearly represent formally Laporte-allowed u - -g transitions, and consequently should be intense. Ligand to metal transitions involving even ligand orbitals are of course also possible, but would be parity forbidden and are therefore rather seldom observed. For many of the ions here treated though the data are derived from reflectance measurements and the intensity criterion is of limited value because of the increase in the scattering coefficient which usually occurs above about 25 kK. [c.f. (7)]. 

Finally, we should remember that f f transitions are parity-forbidden. However, most of them become partially allowed at the electric dipole order as a result of mixing with other orbitals that have different parity because of a noninversion symmetry crystal field (see Section 5.3). Thus, a proper choice of the crystal host (or the site symmetry) can cause a variety of (RE) + transitions to become forced electric dipole transitions. 

Electron configuration of Bp" is (6s) (6p) yielding a Pip ground state and a crystal field split Pap excited state (Hamstra et al. 1994). Because the emission is a 6p inter-configurational transition Pap- Pip. which is confirmed by the yellow excitation band presence, it is formally parity forbidden. Since the uneven crystal-field terms mix with the (65) (75) Si/2 and the Pap and Pip states, the parity selection rule becomes partly lifted. The excitation transition -Pl/2- S 1/2 is the allowed one and it demands photons with higher energy. 

Both absorption and emission spectra have been recorded for a variety of octahedral chromium(III) complexes. For the systems of interest here, A/B 2. Inspection of Figure 2 leads to the expectation of three spin-allowed, parity-forbidden transitions between the iA2g and the other quartet states and two spin- and parity-forbidden transitions between the iA2g and the 2Eg and 2T2g states. Aqueous solutions of Cr(H20)s3+ display three bands with e 15 at 17,400, 24,500, and 38,000 cm-1, assigned respectively to the transitions iA2g- iT2g,... 

For centrosymmetric complexes the intensities of the parity-forbidden d< d bands arise through vibronic interactions and consequently show substantial temperature dependence. It can be shown that for the ideal case of a si ngle harmonic vibration of frequency y coupled to the electronic system the intensity of a band should be given by105-106... 

Figures 9A and 9B are photofragment images of D+ following irradiation of D2 with 532-nm light. All of the features can be assigned to dissociation of different vibrational levels of D2 by nominally either one-, two, or three-photon absorption. Because two-photon absorption to the 2p

Modified Notation.—The Platt notation is applied mainly to aromatic molecules and based on the conceptually simple perimeter model description of electronic excitations (7). Ground states are labeled A, the excited states involved in certain very high intensity transitions are labeled B and the excited states produced in partially forbidden transitions (i.e., those in which selection rules are violated) are labeled L and C. The notation is derived from selection rules appropriate for imaginary monocyclic aromatic systems. States to which transitions are forbidden because of a large change in angular momentum are L states. Transitions to C states are parity forbidden that is, they violate the g g, u u selection rule. In common aromatics other than benzene these selection rules break down and transitions to L and C states occur but at lower intensities relative to B states. 

Relaxation of the rules can occur, especially since the selection rules apply strongly only to atoms that have pure Russell-Saunders (I-S) coupling. In heavy atoms such as lanthanides, the Russell-Saunders coupling is not entirely valid as there is the effect of the spin-orbit interactions, or so called j mixing, which will cause a breakdown of the spin selection rule. In lanthanides, the f-f transitions, which are parity-forbidden, can become weakly allowed as electric dipole transitions by admixture of configurations of opposite parity, for example d states, or charge transfer. These f-f transitions become parity-allowed in two-photon absorptions that are g g and u u. These even-parity transitions are forbidden for one photon but not for two photons, and vice versa for g u transitions [46],... 

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