Incoherent transfer

When the dissipative relaxation time is shorter than the exciton transfer time phase coherence of the exciton wavefunction can no longer be maintained and a probabilistic interpretation is more appropriate. 

The probability, Pm f), that the exciton is on the mth molecule at time t is determined by the rate equation, 

Equation (9.32) has been widely used to model exciton transport in conjugated materials. See Movaghar et al. (1986) and Meskers et at. (2001) for an example of this approach. In Section 9.2.4 we briefly describe the more general density matrix formalism to model exciton transport. 

1 Forster transfer The rate for direct exciton transfer between a donor molecule, D, and an acceptor molecule, A, is determined by the Fermi Golden Rule expression, 

We interpret the first delta function as representing the absorption of energy by the acceptor molecule, while the second delta function represents the emission of energy by the donor. 

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Both homonuclear and heteronuclear versions of relayed nOe experiments are known. The homonuclear relayed NOESY experiment involves both an incoherent transfer of magnetization between two spins H and H/ that are not coupled but close in space, and a coherent transfer of magnetization between two spins H(and H that are /-coupled together. The magnetization pathway may be depicted as... 

In a heteronuclear nOe experiment, the first step may be identical to the homonuclear nOe experiment (i.e., involving an incoherent transfer of coherence from H to H/), while the second step could involve a coherence transfer from H to C nucleus by an INEPT sequence. These methods suffer from poor sensitivity and have therefore not been used extensively. 

Oil and 0)2, and (b) 2D shift-correlation spectra, involving either coherent transfer of magnetization [e.g., COSY (Aue et al, 1976), hetero-COSY (Maudsley and Ernst, 1977), relayed COSY (Eich et al, 1982), TOCSY (Braunschweiler and Ernst, 1983), 2D multiple-quantum spectra (Braun-schweiler et al, 1983), etc.] or incoherent transfer of magnedzation (Kumar et al, 1980 Machura and Ernst, 1980 Bothner-By et al, 1984) [e.g., 2D crossrelaxation experiments, such as NOESY, ROESY, 2D chemical-exchange spectroscopy (EXSY) (Jeener et al, 1979 Meier and Ernst, 1979), and 2D spin-diffusion spectroscopy (Caravatti et al, 1985) ]. 

Huber D. L. (1981) Dynamics of Incoherent Transfer, Top. Appl. Phys. 49, 83-111. 

The incoherent transfer for t > 10 ps has first a multi--exponential character which becomes at t 25 ps a single--exponential one. The single-exponential transfer for t > 25 ps is the most important one as it gives the most significant contribution to 7)0j (about 90 %). In this regime, the fluorescence intensity and other observables have single--exponential character. The corresponding life time equals... 

We also notice that, unlike the case of coherent transfer - where the transfer rate is oc Jda and hence Rda - for incoherent transfer the rate is oc Jda and hence Rj3 . It is customary to define a Forster radius, Rp,aX which the Forster transfer rate is equal to the radiative emission rate, kp = /tr. Then,... 

In terms of dressed states and the collective modes, the present system is similar to a single-mode system. As a consequence of quantum beats, modes 82,3 decouple and only mode 8 mediates into the interaction. Incoherent transfer of population is carried out through the channel... 

Figure 5.75. Pulse sequences used for relayed NOE (A) an incoherent transfer of magnetization occurs, followed by a coherence transfer of magnetization (B)a coherence transfer takes place first, and is then followed by an incoherent transfer of magnetization.

T. Holstein, S.KLyo, R. Orbach Excitation Transfer in Disordered Systems. - D.L. Huber Dynamics of Incoherent Transfer. - RM.Sel-zer General Techniques and Experimental Methods in Laser Spectroscopy of Solids. -W.M. Yen, RM.Selzer High Resolution Laser Spectroscopy of Ions in Crystals. -MJ. Weber Laser Excited Fluorescence Spectroscopy in Glass. AH Francis, KKopel-man Excitation Dynamics in Molecular Solids. 

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