Anharmonicity strong

Consider the wave packet populating just one vibrational level. This occurs for only a short period of time (the length of the femtosecond pulse). Then we can think of vibration occurring in a classical fashion. The wave packet travels along the vibrational level until it reaches the other extremity when it may be reflected and continue to travel backwards and forwards along the level. Because of the strongly anharmonic nature of the vibration the wave packet is broadened, as shown, as r increases. 

For aromatic hydrocarbon molecules, in particular, the main acceptor modes are strongly anharmonic C-H vibrations which pick up the main part of the electronic energy in ST conversion. Inactive modes are stretching and bending vibrations of the carbon skeleton. The value of Pf provided by these intramolecular vibrations is so large that they act practically as a continuous bath even without intermolecular vibrations. This is confirmed by the similarity of RLT rates for isolated molecules and the same molecules imbedded in crystals. 

Mclnnes EJL (2006) Spectroscopy of Single-Molecule Magnets. 122 69-102 Merunka D, Rakvin B (2007) Anharmonic and Quantum Effects in KDP-Type Ferroelectrics Modified Strong Dipole-Proton Coupling Model. 124 149-198 Meshri DT, see Singh RP (2007) 125 35-83... 

B. The Main Mechanism Strong Coupling Theory of Anharmonicity... 

At last, because of the whole strong anharmonicity involved in the H-bond system, the possibility of electrical anharmonicity cannot be a priori ignored [20],... 

The cornerstone of the strong anharmonic coupling theory relies on the assumption of a modulation of the fast mode frequency by the intermonomer distance. This behavior is correlated by many experimental observations, and it is undoubtly one of the main mechanisms that take place in a hydrogen bond. Because the intermonomer distance is, in the quantum model, represented by the dimensionless position coordinate Q of the slow mode, the effective angular frequency of the fast mode may be written [52,53]... 

Note that founder theoretical treatment of the strong anharmonic coupling has been done by Marechal and Witkowski [7] in the simplest case, obtained by neglecting the terms in p0,/0, and ga. 

Now, recall that for weak hydrogen bonds the high-frequency mode is much faster than the slow mode because 0 m 20 00. As a consequence, the quantum adiabatic approximation may be assumed to be verified when the anharmonic coupling parameter aG is not too strong. Thus, neglecting the diabatic part of the Hamiltonian (22) and using Eqs. (18) to (20), one obtains... 

Bratos and Hadzi have developed another origin of the anharmonicity of the fast mode X-H -Y, the Fermi resonance, which is supported by several experimental studies [1,3,63-70], Widely admitted for strong hydrogen bonds [67], the important perturbation brought to the infrared lineshape by Fermi resonances has also been pointed out in the case of weaker hydrogen bonds [53,71-73]. 

In this section we shall give the connections between the nonadiabatic and damped treatments of Fermi resonances [53,73] within the strong anharmonic coupling framework and the former theory of Witkowski and Wojcik [74] which is adiabatic and undamped, involving implicitly the exchange approximation (approximation later defined in Section IV.C). 

In the strong anharmonic coupling framework, the fast mode potential IJ is, according to Eq. (8),... 

When neglecting the strong anharmonic coupling—that is, in the situation of a pure Fermi coupling (no hydrogen bond)... 

There are two kinds of damping that are considered within the strong anharmonic coupling theory the direct and the indirect. In the direct mechanism the excited state of the high-frequency mode relaxes directly toward the medium, whereas in the indirect mechanism it relaxes toward the slow mode to which it is anharmonically coupled, which relaxes in turn toward the medium. 

The pure quantum approach of the strong anharmonic coupling theory performed by Marechal and Witkowski [7] gives the most satisfactorily zeroth-order physical description of weak H-bond IR lineshapes. 

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