Herzberg-Teller mechanism

Fig. 29. Energy level diagram for Pt(2-thpy)2 dissolved in n-octane. The electronic origin line I (0-0 transition) is not observed at zero magnetic field. The corresponding triplet substate I is radiatively deactivated via vibronic (HT = Herzberg-Teller) mechanisms. (Compare Fig. 13.) The emission lifetime given refers to T=1.3 K. The emissions from the triplet substates II and III are intense at the electronic origins and show vibrational Franck-Condon (FC) satellites. (Compare Fig. 14.) The lifetime of the Sj state of 50 fs is determined from the homogeneous linewidth of the Sq Si electronic transition at T = 1.3 K. (Compare Fig. 28)...

Corio, R, J.C. Rubim, and R. Aroca (1998). Contribution of the Herzberg-Teller mechanism to the surface-enhanced Raman scattering of iron phthalocyanine adsorbed on a silver electrode. Langmuir 14, 4162. 

Chemical enhancement is typically explained by the CT mechanism. When a molecule is adsorbed on a metal surface, new electronic states are formed due to chemisorption. The new electronic states may serve as resonant intermediate states in Raman scattering. If the Fermi level of the metal is located between the Highest Occupied Molecular Orbital (HOMO) Lowest Unoccupied Molecular Orbital (LUMO) in energy, CT excitations may likely occur at lower energy than intrinsic intramolecular excitations of the adsorbate [55-58]. According to Albrechts notation [55], in the CT mechanism via Albrechts A term Franck-Condon term) only the totally symmetric modes are resonantly enhanced when the laser excitation is close to an allowed electronic transition, and only one excited state is involved. The resonance Raman effects for vibrational modes that are non-totally symmetric, are usually observed when these modes couple two excited states of the chromophore. The product of the symmetry of both excited states should be equal or contain the non-totally symmetry. This mechanism is known as the Herzberg-Teller mechanism or B mechanism in Albrechts notation. 

In summary, all fundamentals observed as satellites in Fig. 24a (Table 9) seem to result from these vibronic (Herzberg-Teller) mechanisms discussed in (1) and (2) and shown in Fig. 26. It may be assumed that mechanism (1) might be more important for low-energy metal-ligand (M-L) vibrations than for high-energy internal ligand vibrations, since movements of the heavy metal, which carries the dominant soc, occur only for the M-L vibrational modes. 

The radiative deactivation from the lowest excited state 11) of [Os(bpy)3] in a high-symmetry environment occurs exclusively by Herzberg-Teller mechanisms. Thus, the emission spectra are dominated by vibrational satellites, while the intensity at the electronic origin nearly vanishes. The vibrational modes can be well correlated to IR-active modes. These satellites represent so-called false origins. There are strong indications that radiative deactivations are in part induced by the still not well-studied processes of spin-orbit-... 

The classic cases of the Herzberg-Teller mechanism relate to coupling between two electronic states of different symmetry. An important example of this case occurs when electric dipole transitions of one of the two states are forbidden (e.g., the Laporte-forbidden d-d and f-f transitions). In this case, the forbidden transition may acquire absorption intensity by Herzberg-Teller mixing with an allowed transition via a nontotaUy symmetric mode of appropriate symmetry (the irreducible representation of the active mode must be contained in the direct product of the irreducible representations for the two states coupled by the Herzberg-Teller mechanism). We shall illustrate our results in Chapter 7 by evaluating the vibronic induced d-d transitions in transition metal complexes. 

The 10a,1 band, as well as many bands built on it, and also the 10a , band obtain their intensity by the Herzberg-Teller vibronic coupling mechanism discussed in Section 7.3.4.2(b). 

The vibronic theory of the RR effect distinguishes two major intensity enhancement mechanisms that play a dominant role in the RR spectra (i) Franck-Condon (FC) principle and (ii) Herzberg-Teller (FIT) vibronic coupling, which have quite different properties. The FC scattering mechanism involves displacement of the potential minima of the ground and excited electronic states along a vibrational normal... 

Fig. 17. Vibronic coupling mechanisms (Herzberg-Teller couplings). The purely electronic transition between the excited state I (triplet substate) and the ground state 0 (Sq) is spin and symmetry forbidden, i.e. no intensity is found at the electronic origin I. Two mechanism are proposed. Coupling route (a) is probably more important for vibrations of metal-ligand character, while mechanism (b) preferentially induces satellite intensities by internal ligand vibrations. The electronic state S is a singlet, for which an electronic transition is dipole forbidden to the electronic ground state Sq. On the other hand, the state S , represents a singlet that carries sufficient transition probability. For detailed explanations see the text...

The mechanisms that induce the intensities of the vibrational satellites in the 1.3 K emission of state I are assigned to be the same as discussed in Sect. 4.2.4.1 for the perprotonated compound, i. e. all vibrational satelfites that correspond to fundamentals represent false origins, they are Herzberg-Teller induced. (Compare Sect. 4.2.2.)... 

In centrosymmetric molecules, HRS gains intensity via Herzberg-Teller term (the first vibronic B-term), indicating that IR-active modes and silent modes are enhanced. In the case of non-centrosymmetric molecules, however, Franck-Condon mechanism (A-term) dominantly contributes to the enhancement. Moreover, the mutual exclusive rules between HRS and RS are broken, and hence, some of RS-active modes selectively appear in the spectra. In the case of plasmonic enhancement, the spectral appearance is more sensitive to molecular orientations at the metal surface because of the surface selection rules [25]. 

Vibrational Contributions Contribution of vibrational modes has been described for TPA [5-9, 11-17, 19, 22, 23, 31, 37, 61, 235, 309, 343-345] and for other nonlinear optical processes [346]. One classical example is the 1A j -1 B2u TP transition of benzene, the so-called green band. This electronic transition is allowed due to a vibronic coupling mechanism [346]. Semiempirical [60, 61] as well as ab initio response theory calculations using the Herzberg-Teller expansion [344] demonstrate the role of vibronic coupling. Such contributions can either enhance an allowed transition or intensify a symmetry-forbidden transition. 

Fig. 5. The Herzberg-Teller scattering mechanism. A and B illustrate the first-order contributions to fundamental scattering for non-totally symmetric modes where the excited-state potential curves are unshifted relative to those of the ground state. C and D show the dominant second-order contributions to overtone scattering.

Such modes are Herzberg-Teller active, i.e., they can transfer transition dipole moment between electronic states, since d /dQ can be expressed in terms of states vibronic coupling in Section IV. Thus if g and 4) belong to the representations Ag and B2U of D21, respectively, only nontotally symmetric modes of symmetry big and b g are resonance-enhanced since they can transfer transition moment from transitions and Bi Ag, respectively, to the transition... 

The first term of Eq. 8.46 is the FC term, while the remaining terms arise from the Herzberg-Teller (HT) borrowing mechanism of electronic transitions. By inspection of Eq. 8.33, one may notice that the antisymmetric anisotropy vanishes for pure EC transitions but is in general different from zero when the HT effect is... 

Different triplet sublevels of the same complex can be radiatively deactivated by different vibronic mechanisms, like Herzberg-Teller (HT) and/or Franck-Condon (FC) activity. 

Dominance of Radiative Deactivation by Franck-Condon Active Modes. The observed vibrational satellites can be induced by different mechanisms. In particular, they may result from Franck-Condon (FC) and Herzberg-Teller (HT)... 

Due to the relatively strong forbiddenness of the purely electronic transition 11) <-> 10), for example, displayed in the long emission lifetime at T = 1.2 K of Tj = 230 ps, vibronic coupling (Herzberg-Teller, HT, coupling) becomes an important mechanism of radiative deactivation. This mechanism provides intensity to specific vibrational satellites by vibrationally induced admixtures of... 

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