Dipole forbidden

It should also be remembered that the selection mles derived here are relevant to the free molecule and may break down in the liquid or solid state. This is the case, for example, with the electric dipole forbidden 4q transition in ethylene, where V4 is the torsional vibration shown in Figure 6.23. It is not observed in the infrared specttum of the gas but is observed weakly in the liquid and solid phases. 

Unlike linear optical effects such as absorption, reflection, and scattering, second order non-linear optical effects are inherently specific for surfaces and interfaces. These effects, namely second harmonic generation (SHG) and sum frequency generation (SFG), are dipole-forbidden in the bulk of centrosymmetric media. In the investigation of isotropic phases such as liquids, gases, and amorphous solids, in particular, signals arise exclusively from the surface or interface region, where the symmetry is disrupted. Non-linear optics are applicable in-situ without the need for a vacuum, and the time response is rapid. 

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. 

The induced absorption band at 3 eV does not have any corresponding spectral feature in a(co), indicating that it is most probably due to an even parity state. Such a state would not show up in a(co) since the optical transition IAK - mAg is dipole forbidden. We relate the induced absorption bands to transfer of oscillator strength from the allowed 1AS-+1 (absorption band 1) to the forbidden 1 Ak - mAg transition, caused by the symmetry-breaking external electric field. A similar, smaller band is seen in EA at 3.5 eV, which is attributed to the kAg state. The kAg state has a weaker polarizability than the mAg, related to a weaker coupling to the lower 1 Bu state. 

Conjugated polymers are centrosymmetric systems where excited states have definite parity of even (A,) or odd (B ) and electric dipole transitions are allowed only between states of opposite parity. The ground state of conjugated polymers is an even parity singlet state, written as the 1A... PM spectroscopy is a linear technique probing dipole allowed one-photon transitions. Non linear spectroscopies complement these measurements as they can couple to dipole-forbidden trail-... 

HRELS = high-resolution, electron-energy-loss spectroscopy. " Surf. Sci. (in press). Ref. (123). Ref (101). Softened pCHj surface-mode. Weak band observed around 1500 cm could be a surface-dipole-forbidden, Pfc mode. Hidden under intense SCHj mode of free C2H4 in the matrix. " One of these bands belongs to Ni2(C2H4)2. 

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]. 

XAS data comprises both absorption edge structure and extended x-ray absorption fine structure (EXAFS). The application of XAS to systems of chemical interest has been well reviewed (4 5). Briefly, the structure superimposed on the x-ray absorption edge results from the excitation of core-electrons into high-lying vacant orbitals (, ] ) and into continuum states (8 9). The shape and intensity of the edge structure can frequently be used to determine information about the symmetry of the absorbing site. For example, the ls+3d transition in first-row transition metals is dipole forbidden in a centrosymmetric environment. In a non-centrosymmetric environment the admixture of 3d and 4p orbitals can give intensity to this transition. This has been observed, for example, in a study of the iron-sulfur protein rubredoxin, where the iron is tetrahedrally coordinated to four sulfur atoms (6). 

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. 

Carotenoids are characterised by two low lying singlet excited states. The S2( Eu) state has a high absorption dipole strength and is very weakly fluorescent ( F 10 -10 ) [200, 201-204], The state is dipole forbidden... 

E and E. Transitions from the ground A, state to the E state is dipole allowed, but transitions to all the triplet states are spin-forbidden. The Aj Aj, and the Ai A2 transitions are electric dipole-forbidden but become allowed by vibrcnic coupling to the E level. A vibration of t or symmetry makes the former absorption allowed, while the latter is allowed only by a vibration of e symmetry. 

The characteristic absorption and emission spectra of lanthanide compounds in the visible, near ultra-violet and infra-red is attributed to transitions between 4/ levels due to the fact that they present a sharp line with oscillators strengths typically of the order of 10 . These transitions are electric dipole forbidden but became allowed as forced electric dipole transitions. 

The density of the actual energy loss is, of course, the derivative of the expression in Eq. (4) The Ashley approximation greatly enhances the validity of the computed cross section for electronic excitation. However, as only the DOSD is used, excitations to states that are dipole forbidden are not included. This may not be a serious limitation unless the incident energy is very low. An additional advantage of the approximation is that it is conveniently used in the condensed phase. 

By reference to the MO scheme in Fig. 2, the bands at X < 400 nm have been assigned to sharp and intense parity-allowed transitions between occupied (bonding) and empty (antibonding) MOs. Such excitations include Au(HOMO)— fJg(LUMO + 1) and hg—> Optical transitions between the HOMO(Au) and LUMO(f/u), which are electric dipole forbidden, occur via excitation of a vibronic state with appropriate parity symmetry and account for the broad and low intensity band at X > 400 nm. 

UV Spectrum. The lowest energy electronic transition (HOMO-LUMO) is no n( (y leading to the nn state, which has A2 symmetry in the local C2v point group. The transition to this state is symmetry forbidden (it is quadrupole allowed but dipole forbidden) and will be expected to be weak. 

The efficiency of photon capture to form the l(nn ) state is very low since the excitation is electric dipole forbidden in the local C2V point group of the carbonyl. Direct... 

Welsh suggested correctly that similar transitions take place even if the molecular pair is not bound. The energy of relative motion of the pair is a continuum. Its width is of the order of the thermal energy, Efree 3kT/2. Radiative transitions between free states occur (marked free-free in the figure) which are quite diffuse, reflecting the short lifetime of the supermolecule. In dense gases, such diffuse collision-induced transitions are often found at the various rotovibrational transition frequencies, or at sums or differences of these, even if these are dipole forbidden in the individual molecules. The dipole that interacts with the radiation field arises primarily by polarization of the collisional partner in the quadrupole field of one molecule the free-free and bound-bound transitions originate from the same basic induction mechanism. 

Specifically, the collision-induced absorption and emission coefficients for electric-dipole forbidden atomic transitions were calculated for weak radiation fields and photon energies Ha> near the atomic transition frequencies, utilizing the concepts and methods of the traditional theory of line shapes for dipole-allowed transitions. The example of the S-D transition induced by a spherically symmetric perturber (e.g., a rare gas atom) is treated in detail and compared with measurements. The case of the radiative collision, i.e., a collision in which both colliding atoms change their state, was also considered. 

Forbidden transitions. If the transition is dipole forbidden in the isolated molecule, an induced electronic absorption line or spectrum may appear as was seen above in oxygen and in mixtures of oxygen with other gases, p. 357ff. 

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