Vibronic borrowing

Such vibronically allowed transitions are said to derive their intensity through vibronic borrowing. 

Al(HE), Ga(HE) as well as In(HE) porphyrin are typical porphyrins incorporated with a tervalent metal ion Characteristic Q and B bands in the visible and near-ultraviolet region, respectively, arise from the (7T,7T ) excitations in the porphyrin ring with only minor perturbation from the outershell electrons of the central metal ion. The Q band is of forbidden character, however, the Q band can borrow the intensity by vibronic couplings from the allowed B band (30). The intensity of the Q(1,0) band is much less sensitive to the peripheral substituents, the axial ligands and the central metal ions, while that of the Q(0,0) band without excitation in the skeletal vibrational modes is rather sensitive to various substituents. 

The experimental evidences for these conclusions derive mainly from low-temperature spectra of isolated molecules in rigid matrices.62,63 The diffuseness observed ranges from a few tenths of a cm-1 to thousands of cm-1. In the case of a relatively simple molecule like naphthalene that has two nearby states (S at 31,680 cm-1, and S2 at 34,420 cm-1) a careful study of the second state shows many relatively sharp lines emerging from a diffuse background.64,65 It appears that these sharp transitions can be associated with vibrational levels of the lower electronic state, and that the intensity is borrowed from the second state by the mixing of vibronic states having different electronic parentage in the Born-Oppenheimer representation. 

Reber C (2004) Allowed and Forbidden d-d Bands in Octahedral Coordination Compounds Intensity Borrowing and Interference Dips in Absorption Spectra 107 145-158 Reinen D (2004) The Angular Overlap Model and Vibronic Coupling in Treating s-p and d-s Mixing - a DFT Study 107 159-178... 

The other enhancement method is to use molecular electronic resonances [19, 22, 23]. A most interesting topic in this enhancement method is so-called molecular near-held effect reported by Kano et al. [34]. This effect, explained by intermolecular vibronic coupling, makes HRS spectroscopy highly sensitive to intermolecular interactions one can observe HRS modes of solvent molecules adjacent to the solute dye molecules via the intensity borrowing from the dye molecules. That is, solvent-solute interactions are selectively observed without impairment from signals of the bulk solvent. It is almost impossible to observe a similar effect in resonance RS spectroscopy because of the unavoidable contribution from Franck-Condon type resonance. 

The first term in Equation (1.47) is identical with the expression derived in the last section for electronically allowed transitions. It is presently assumed to be very small or zero. (A/o f - 0 for symmetry-forbidden transitions.) The second term results from vibronic mixing and represents a first-order vibronic contribution to the transition moment. It is seen that in this description the forbidden transition 0->f steals or borrows intensity from the allowed transition 0- b. If A/o f is exactly zero all observed components of the electronic transition will be polarized along the direction of the transition dipole moment A o b. The 0- 0 transition (v = v = 0) will have zero intensity and only vibrational levels of overall symmetry given by the direct product of symmetries of the states % and % will appear. 

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