Dipole forbidden transitions

A < 640 nm (or 1.9 < E < 2.5 eV), weak absorption takes plaee, and is associated with electric dipole-forbidden transitions between the one-electron HOMO level w ith /i symmetry and the one-electron Uu LUMO level. 

W-Li absorption edges for WO3 and for several WOx-ZrOa samples calcined at 1073 K are shown in Figure 7. The absorption edge reflects the excitation of W 2s electrons by X-ray photons. A pre-edge feature at -5 eV is caused by 2s to 5d dipole-forbidden transitions, which become detectable in non-centrosymmetric compounds because of d-p orbital mixing [28]. 

Resonance Raman spectroscopy has been applied to studies of polyenes for the following reasons. The Raman spectrum of a sample can be obtained even at a dilute concentration by the enhancement of scattering intensity, when the excitation laser wavelength is within an electronic absorption band of the sample. Raman spectra can give information about the location of dipole forbidden transitions, vibronic activity and structures of electronically excited states. A brief summary of vibronic theory of resonance Raman scattering is described here. 

The term III scattering (equation 8) is the weakest in the three scattering mechanisms, as shown by two derivative terms (M ) in the electronic transition integrals. Clearly, for a dipole forbidden transition (M° = 0) the only non-zero term is term III. The term in scattering results in binary overtone and combination transitions of vibronically active modes. It is noted that no fundamental transition survives. 

Figure 4 Exciton band structures in dimers with several orientations of transition dipoles. Dotted arrows indicate dipole forbidden transitions. (Adapted from Kasha [17]).

Here, e and e represent slow and fast electrons, respectively, and the superelastic collision may preferentially produce the A resonant excited state because A — A is a strong dipole transition. (Note that inelastic cross sections for electron-atom collisions are generally larger for dipole-allowed transitions compared to dipole-forbidden transitions.) Since the ground state density N is high, the electron excitation of A following Eq. (7) is fast ... 

Let us consider a dipole forbidden transition, for example, the IS to 2S transition. This transition has a probability that is zero to first order and is dependent only upon the interatomic separation. In this case ... 

Figure 3.3. UV (—) and CD spectra (—) of octahydrobenzoquinoxaline. Band , which stems from a dipole-forbidden transition, is only seen in the CD spectrum (by permission from Snatzke, 1982).

The dipole forbidden transitions, S-S and S-D transitions of alkali atoms, are induced by the collisions with the rare gas atoms [82]. This phenomenon is called collision-induced absorption. The position and the profile of the collision-induced absorption sensitively reflect the interatomic potentials of the ground and excited states of the alkali-noble gas system and the (induced) transition moments between them. The SAC-CI method elucidated the detailed mechanism of the collision-induced absorption spectra of CsRg (Rg = Ne, Ar, Kr, Xe) system [83,84]. 

Thus the dipole-forbidden transition to the state can become vibrationally allowed, borrowing intensity from the interfering Bj state via modes of b2 symmetry. 

FIGURE 11.2 Calculated Doppler speetrum of a dipole-forbidden transition of a single trapped ion oscillating at frequencies co/ln = 0.7 MHz and ajlii = 1 MHz and cooled down to the Lamb-Dicke regime sueh that the Lamb-Dieke parameters are 0.1 in both directions (the laser wavevector has the same projeetion along the coordinates axes and O ). (Figure courtesy of Caroline Champenois, PIIM, Marseille.)... 

Laser-spectroscopic experiments with single ions confined in a trap have proved that such information can be obtained. The original idea was proposed by Dehmelt [1230] and has since been realized by several groups [1231-1233]. It is based on the coupling of an intense allowed transition with a weak dipole-forbidden transition via a common level. For the example of the Ba+ ion (Fig. 9.46) the metastable 5 T>5/2 level with a spontaneous lifetime of r = (32 5) s can serve as a shelf state. Assume that the Ba ion is cooled by the pump laser at A. = 493.4 nm and the population leaking by fluorescence into the 5 1)3/2 level is pumped back into the 6 Pi/2 level by the second laser at A. = 649.7 nm. If the pump transition is saturated, the fluorescence rate is about 10 photons per second with the lifetime r(6 Pi/2) = 8 ns. If the metastable 5 T>5/2 level is populated (this can be reached,... 

The allowed transition 6 5i/2-6 Pi/2 serves as an amplifying detector for a single quantum jump on the dipole-forbidden transition 5 Ds/2 6 5i/2 [1232]. In... 

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