Wave-packet dephasing

Milota F, Sperling J, Szocs V, Tortschanoff A, Kauffmann HF (2004) Correlation of femtosecond wave packets and fluorescence interference in a conjugated polymer towards the measurement of site homogeneous dephasing. J Chem Phys 120 9870... 

Just as the expansion in the zeroth-order states can describe the exact molecular eigenstates, likewise an expansion in the exact states can be used to prepare, for a short time, a zeroth-order state. If the perturbation V is small, and the model Hamiltonian Ho is a good approximation to //, then the initially prepared superposition of eigenstates will resemble a zeroth-order state. The dephasing of the exact molecular eigenstates in the wave packet superposition subsequently leads to an evolution of the initial zeroth-order electronic character, transforming into a different zeroth-order electronic state as a function of time. 

In n-conjugated polymers no zero-phonon lines have been observed in SSF spectra, indicating that there is always some chain relaxation in response to optical excitations [17, 59]. The magnitude of this effect should and, in fact, does depend on the rigidity of the chain. In PPV the residual Stokes shift is on the order of 100 cm-1, while in MeLPPP it is —20 cm-1 only [21]. In the time domain the former value corresponds to a dephasing on the order of 100 fs as has been verified by experiments on fs-wave-packet fluorescence interferometry on tail states of PPV [60]. 

As mentioned above, vibronic-coupling model Hamiltonians constructed by low-order Taylor expansions of the diabatic PE functions in terms of normal coordinates are particularly suitable for the calculation of low-resolution spectra of polyatomic molecules. In resonance Raman spectroscopy, for example, the usually extremely fast electronic dephasing in polyatomic s tems limits the time scale for the exploration of the excited-state PE surface by the nuclear wave packet to about 10 jjj... 

In this chapter we survey characteristic features of time-dependent quantum wave-packet dynamics on conically intersecting potential-energy (PE) surfaces. The focus will be on the fully microscopic description of nontrivial dynamical processes such as ultrafast internal conversion and photoisomerization, as well as vibrational energy redistribution and dephasing. The quantum dynamics calculations discussed in this chapter are... 

Fig. 3.51. Snapshots of wave packet propagation in Nas B for a 120 fs excitation at P = 16 021cm" demonstrating the dephasing of the 310 fs breathing mode (taken from [382])...

This usually is larger than the corresponding integral for scattering to b,( ) = 2 because, in the wave-packet picture, Xi(t) builds up overlap withxi,i(g) more quickly than it does f/iSn Xix(g) (Fig- 12.6). Dephasing of the wavepacket frustrates the rise of Cf2,ife)P (0)- In addition, the build up of ixi,ng) At)) must occur while the wavepackets for all the other modes retain good overlap with the initial,... 

Figure 1.8 Contrasting direct photodissociation in the gas phase and in a soivent [adapted from Schwartz eta/. (1994)]. As we shall discuss, the coming back together of the two fragments owing to the "fence" presented by the solvent may initially be coherent in that the wave-packet describing the relative motion has not yet dephased, see Problem H. On a longer (>picosecond) time scale the recombination will be diffusive. When the fragments are polyatomic a diffusive recombination means that the fragments will lose their relative orientation. They can even recombine to a different isomer of the parent.

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