Vibrational coherence transfer

In order to discuss vibrational coherence transfer, it is necessary to use two coordinates and two momenta, as shown in Fig. 1. 

It has been found that the short-range interaction model can be applied to study the vibrational relaxation of molecules in condensed phases. This model is applied to treat vibrational relaxation and pure dephasing in condensed phases. For this purpose, the secular approximation is employed to Eq. (129). This assumption allows one to focus on several important system-heat bath induced processes such as the vibrational population transition processes, the vibrational coherence transfer processes, and the vibronic processes. 

The rate constants of the vibrational coherence dynamics such as vibrational coherence transfer can be derived by using the same interaction model used in Eq. (130). For example, the rate constants of the vibrational coherence transfer processes from bv + 1 ->bv + 1 to bv -+bi/ and from bv —l 1 to bv ->bv are, respectively, given by... 

Chudoba C, Riedle E, Pfeiffer M and Elsaesser T 1996 Vibrational coherence in ultrafast excited-state proton transfer Cham. Phys. Lett. 263 622-8... 

B. Kohler 1 would like to ask two questions to Prof. Zewail. First, in your investigation of the electron transfer reaction in a benzene- complex, the sample trajectory calculations you showed appear to suggest that the charge transfer step may induce vibrationally coherent motion in h-. Have you tested this possibility experimentally My second question concerns your intriguing results on a tautomerization reaction in a model base-pair system. In many of the barrierless chemical reactions you have studied, you have been able to show that an initial coherence created in the reactant molecules is often observable in the products. In the case of the 7-azaindole dimer system your measurements indicate that reaction proceeds quite slowly on the time scale of vibrational motions (such as the N—H stretch) that are coupled to the reaction coordinate. What role do you think coherent motion might play in reactions such as this one that have a barrier ... 

A more general approach is required to interpret the current experiments, Jean and co-workers have developed multilevel Redfield theory into a versatile tool for describing ultrafast spectroscopic experiments [22-25], In this approach, terms neglected at the Bloch level play an important role for example, coherence transfer terms that transform a coherence between levels i and j into a coherence between levels j and k ( /t - = 2) or between levels k and l ( f - j - 2, k-j = 2) and couplings between populations and coherences. Coherence transfer processes can often compete effectively with vibrational relaxation and dephasing processes, as shown in Fig. 4 for a single harmonic well, initially prepared in a superposition of levels 6 and 7. The lower panel shows the population of levels 6 and 7 as a function of time, whereas the upper panels display off-diagonal density matrix ele-... 

To summarize, Jean shows that coherence can be created in a product as a result of nonadiabatic curve crossing even when none exists in the reactant [24, 25]. In addition, vibrational coherence can be preserved in the product state to a significant extent during energy relaxation within that state. In barrierless processes (e.g., an isomerization reaction) irreversible population transfer from one well to another occurs, and coherent motion can be observed in the product regardless of whether the initially excited state was prepared vibrationally coherent or not [24]. It seems likely that these ideas are crucial in interpreting the ultrafast spectroscopy of rhodopsins [17], where coherent motion in the product is directly observed. Of course there may be many systems in which relaxation and dephasing are much faster in the product than the reactant. In these cases lack of observation of product coherence does not rule out formation of the product in an essentially ballistic manner. 

V. Engel Prof. Neusser, you mentioned the technique of Stimulated Raman Rapid Adiabatic Passage STIRAP, which allows for the coherent transfer of vibrational population selectively. Is the technique not another very efficient and experimentally verified scheme of coherent control ... 

Figure 5.4, one can easily understand why the interfacial electron transfer should take place in the 10-100 fsec range because this ET process should be faster than the photo-luminescence of the dye molecules and energy transfer between the molecules. Recently Zimmermann et al. [58] have employed the 20 fsec laser pulses to study the ET dynamics in the DTB-Pe/TiC>2 system and for comparison, they have also studied the excited-state dynamics of free perylene in toluene solution. Limited by the 20 fsec pulse-duration, from the uncertainty principle, they can only observe the vibrational coherences (i.e., vibrational wave packets) of low-frequency modes (see Figure 5.5). Six significant modes, 275, 360, 420, 460, 500 and 625 cm-1, have been resolved from the Fourier transform spectra of ultrashort pulse measurements. The Fourier transform spectrum has also been compared with the Raman spectrum. A good agreement can be seen (Figure 5.5). For detail of the analysis of the quantum beat, refer to Figures 5.5-5.7 of Zimmermann et al. s paper [58], These modes should play an important role not only in ET dynamics or excited-state dynamics, but also in absorption spectra. Therefore, the steady state absorption spectra of DTB-Pe, both in...

Recent experimental studies on interference effects in solution, and on collisional vibrational energy transfer between molecules in solution, provide some insight into the molecular time scales of these relaxation events. For example [171], the time scale for transfer of population to die vibrational modes in liquid CH3OH is on thd order of 5 to 15ps [172], Further, studies of the preparation of coherent superpositions of states in solution show that phase coherences of molecules exist in solution for time scales greater than 100 fs [173, 174], -- i... 

The light-matter interactions of the Raman FID experiment are illustrated in Fig. 3a. Light pulses are needed at two frequencies Laser (L) and Stokes (S), with their frequency difference adjusted to the vibrational transition energy. An initial pair of Laser and Stokes pulses (pair I) excites the vibration through a Raman interaction. The density matrix of the vibration is transferred from the pure ground state (pm) to a coherent superposition of the v = 0 and v = 1 states (poi)-... 

Following the initial proposal, fifth-order time domain Raman spectroscopy received considerable attention both theoretically (11-20) and experimentally (21-32). For the case of intermolecular motions, the majority of the experimental efforts have involved probing the intermolecular modes of liquid CS2, a standard system in nonresonant Raman spectroscopy due to its very large polarizability and the wealth of available experimental results. Experimental efforts to probe intramolecular vibrations are fewer in number, with the only published example probing modes of liquid CH3C1 and CCU (24). It was quickly realized that, owing to the direct transfer of the first vibrational coherence to the second, the experiment offered substantially more information than had initially been... 

Since the pulse time is so short (see Sec. 3.6.2.2.3) one can coherently excite many vibrational modes at a time and monitor relaxation processes in real time. The first reported femtosecond time-resolved CARS experiments (Leonhardt et al., 1987 Zinth et al., 1988) showed beautiful beating patterns and fast decays of the coherent signal for several molecular liquids. The existence of an intermolecular coherence transfer effect was suggested from the analysis of the beating patterns (Rosker et al., 1986). Subsequent studies by Okamoto and Yoshihara (1990) include the vibrational dephasing of the 992 cm benzene mode. A fast dephasing process was found that is possibly related to... 

Vos, M. H., Jones, M. R., and Martin, J. L, 1998, Vibrational coherence in bacterial reaction centers spectroscopic characterisation of motions active during primary electron transfer. Chem. Phys., 233 179nl90. 

This evolution will be modified in the presence of colhsions by what was previously defined as primary processes vibrational and rotational relaxation within the i> and j/) manifolds, dephasing of initially prepared coherent states and (in principle) coherence-transfer effects. The basis of zero-order states (pure-spin states in the case of intersystem crossing) has been chosen for this discussion, because it provides clear separation of intramolecular and intermolecular effects the j) and /> states are mutually coupled by the intramolecular (spin-orbit) coupling u, while collisions may uniquely couple 5> and j > ( /> and / >) states within each manifold. 

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