Overtones and Combination Bands of Herzberg-Teller Active Modes

1 Overtones and Combination Bands of Herzberg-Teller Active Modes 

The absorption intensity of the 0-1 vibronic sideband is entirely borrowed by HT coupling. The observation that this 0-1 transition is of comparable intensity to the 0-0 transition suggests the feasibility of resonance Raman processes in which both the absorption and emission moments are borrowed. This second-order HT coupling displays itself in the appearance of first overtones and combination bands of non-totally symmetric modes. These are observed extensively in the haem proteins 35). 

In the vibrational parts of the diagrams, vertical lines between potential curves represent overlap integrals, which are diagonal in the vibrational quantum number for unshifted harmonic potentials of the same fundamental frequency. An inclined line with a circle represents a matrix element of the coordinate operator, Q, which changes the vibrational quantum number by 1. 

In the electronic parts of the diagrams, vertical arrows represent electric-dipole transitions effected by the transition-moment operators A dashed arrow represents the vibronic coupling inter- 

No overtones or combinations involving more than two quanta are observed, which is consistent with the absence of a 0-2 transition in absorption. The second-order bands in the resonance Raman spectrum of cytochrome c are shown in Fig. 20 (Section 4.8) where they appear in the 1650-3 300 cm region for excitation at 514.5 nm (point F). The excitation profiles of the second-order bands should peak only in the 0-1 absorption region, at positions consistent with the wavenumbers of the relevant fundamentals. This behaviour is expected since the appropriate parts of the scattering tensor show only 0—1 resonance conditions  

---