Intermolecular dephasing

Within the Mtuliiov approximation, the intermolecular dephasing constant associated with the coherence decay between i<->j and i ->j vibrational Raman transitions is expressed in terms of the population decay constant from each state and intermolecular pure dephasing constant as ... 

Here f 3( OniB) represents the interaction Hamiltonian between the molecule at site f (m) and the heat bath mode. From the general structure of the intermolecular pure dephasing constant, the intermolecular dephasing constant for the transition i <- j at f tmd i <- j at site m is expressed... 

When the heat bath modes are different between sites ( and m, these molecules are uncorrelated. In this case, the intermolecular dephasing constant consists of the sum of the intramolecular dephasing constants at the two sites,... 

McMorrow D and Lotshaw WT 1991 Dephasing and relaxation in coherently excited ensembles of intermolecular oscillators Cham. Phys. Lett. 178 69-74... 

Fig. 5 Radio frequency pulse sequences for measurements of Sj and Si in DSQ-REDOR experiments. The MAS period rR is 100 ps. XY represents a train of 15N n pulses with XY-16 phase patterns [98]. TPPM represents two-pulse phase modulation [99]. In these experiments, M = Nt 4, N2+ N3 = 48, and N2 is incremented from 0 to 48 to produce effective dephasing times from 0 to 9.6 ms. Signals arising from intraresidue 15N-13C DSQ coherence (Si) are selected by standard phase cycling. Signal decay due to the pulse imperfection of 15N pulses is estimated by S2. Decay due to the intermolecular 15N-I3C dipole-dipole couplings is calculated as Si(N2)/S2(N2). The phase cycling scheme can be found in the original figure and caption. (Figure and caption adapted from [45])...

In this chapter we will first discuss coherent anti-Stokes Raman scattering (CARS) of simple liquids and binary mixtures for the determination of vibrational dephasing and correlation times. The time constants represent detailed information on the intermolecular interactions in the liquid phase. In the second section we consider strongly associated liquids and summarize the results of time-resolved IR spectroscopy (see, e.g., Ref. 17) on the dynamics of monomeric and associated alcohols as well as isotopic water mixtures. 

Dephasing induced by the translational motion (VT coupling) via the repulsive (al) and attractive (a2) parts of the intermolecular potential... 

Dephasing induced by rotational motion in combination with the repulsive (bl) and attractive (b2) intermolecular interaction... 

A number of fifth-order coherent Raman spectroscopies have appeared in recent years (11,43-55), and they can show echo-like behavior (56-60). However, the interpretation of these echoes is more complex than the seventh-order echoes. Moreover, many of these experiments are aimed at low-frequency intermolecular motions only a few look at intramolecular vibrational dephasing (47,48). These experiments are not considered in detail in this chapter but are discussed by Blank et al. (Chapter 10) and Fourkas (Chapter 11). 

Table 2 Classification of Dephasing Mechanisms by Type of Intermolecular Potential, Type of Solvent Dynamics, and Speed of the Resulting Frequency Modulation...

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... 

Temporally two-dimensional non-resonant Raman spectroscopies, which are based on higher order susceptibilities, have been developed to distinguish homogeneous and inhomogeneous contributions to vibrational dephasing. The five-pulse correlation experiment was performed to study low frequency intermolecular vibrational dephasing of liquid CS2, and... 

The Voigt profile is frequently used to fit non-Lorentzian lines, but it must be remembered that it relies on the extreme hypothesis of frequency modulation by two distinct mechanisms, one determining the dephasing time and having a short correlation time T, T2, and the other being static on the time scale of T2 (t,. Tj). These conditions can be satisfied in solids (crystals or glasses), but only very rarely in a single-component liquid where Tj 1 -100 ps whereas for most intermolecular interactions is < 1 ps. 

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