Coherent vibrational motion

Figure 6 shows the results for the more challenging model. Model IVb, comprising three strongly coupled vibrational modes. Overall, the MFT method is seen to give only a qualitatively correct picture of the electronic dynamics. While the oscillations of the adiabatic population are reproduced quite well for short time, the MFT method predicts an incorrect long-time limit for both electronic populations and fails to reproduce the pronounced recurrence in the diabatic population. In contrast to the results for the electronic dynamics, the MFT is capable of describing the almost undamped coherent vibrational motion of the vibrational modes. 

Figure 14. (a) Potential-energy surfaces, with a trajectory showing the coherent vibrational motion as the diatom separates from the I atom. Two snapshots of the wavepacket motion (quantum molecular dynamics calculations) are shown for the same reaction at / = 0 and t = 600 fs. (b) Femtosecond dynamics of barrier reactions, IHgl system. Experimental observations of the vibrational (femtosecond) and rotational (picosecond) motions for the barrier (saddle-point transition state) descent, [IHgl] - Hgl(vib, rot) + I, are shown. The vibrational coherence in the reaction trajectories (oscillations) is observed in both polarizations of FTS. The rotational orientation can be seen in the decay of FTS spectra (parallel) and buildup of FTS (perpendicular) as the Hgl rotates during bond breakage (bottom). 

Wavepacket motion is now routinely observed in systems ranging from the very simple to the very complex. In the latter category, we note that coherent vibrational motion on functionally significant time scales has been observed in the photosynthetic reaction center [15], bacteriorhodopsin [16], rhodopsin [17], and light-harvesting antenna of purple bacteria (LH1) [18-20]. Particularly striking are the results of Zadoyan et al. [21] on the... 

D. M. Neumark We are interested in generating coherent vibrational motion in negative ions, which typically do not have bound excited electronic states. Does your Impulsive Stimulated Raman Scattering (ISRS) scheme work if the excited state is not bound ... 

Several recent reviews have presented broad overviews of ultrafast time-resolved spectroscopy [3-6], We shall concentrate instead on a selected, rather small subset of femtosecond time-resolved experiments carried out (and to a very limited extent, proposed) to date. In particular, we shall review experiments in which phase-coherent electronic or, more often, nuclear motion is induced and monitored with time resolution of less than 100 fs. The main reason for selectivity on this basis is the rather ubiquitous appearance of phase-coherent effects (especially vibrational phase coherence) in femtosecond spectroscopy. As will be discussed, nearly any spectroscopy experiment on molecular or condensed-phase systems is likely to involve phase-coherent vibrational motion if the time scale becomes short enough. Since the coherent spectral bandwidth of a femtosecond pulse often exceeds collective or molecular vibrational frequencies, such a pulse may perturb and be perturbed by a medium in a qualitatively different manner than a longer pulse of comparable peak power. The resulting spectroscopic possibilities are of special interest to these reviewers. 

When a femtosecond laser pulse passes through nearly any medium, coherent vibrational excitation (in general, initiation of coherent wavepacket propagation) is likely [33, 34]. One- or two-photon absorption of a visible or ultraviolet pulse into an electronic excited state can result in phase-coherent motion in the excited-state potential [35]. Impulsive stimulated Raman scattering can initiate phase-coherent vibrational motion in the electronic... 

In the last few years Nelson and co-workers [63-65] have presented a new approach to light scattering spectroscopy, named impulsive stimulated light scattering (ISS), which seems to be able to detect one particle rotational correlation functions. In ISS, one induces coherent vibrational motion by irradiating the sample with two femtosecond laser pulses, and... 

Ruhman, S., Banin, U., Bartana, A., and Kosloff, R., Impulsive excitation of coherent vibrational motion ground surface dynamics induced by intense short pulses, J. Chem. Phys., 101, 8461, 1994. 

As discussed in detail in Ref. 34, the vibrational dephasing process represents the origin of the irreversible time evolution of the electronic population (Fig. 2). The initial quasi-periodic recurrences of P it) reflect the driving of electronic population by initially coherent vibrational motion in the tuning modes i/i and z/ga. The vibrational dephasing process destroys the coherence of vibrational motion and thus irreversibly traps the electronic populations. 

The examples collected for this survey of femtosecond nonadiabatic dynamics at conical intersections illustrate the interesting interplay of coherent vibrational motion, vibrational energy relaxation and electronic transitions within a fully microscopic quantum mechanical description. It is remarkable that irreversible population and phase relaxation processes are so clearly developed in systems with just three or four nuclear degrees of freedom. 

U. Banin, A. Bartana, S. Ruhman, and R. KoslofF, Impulsive Excitation of Coherent Vibrational Motion Ground Surface Dynamics Induced by Intense Short Pulses , J. Chem. Phys. 101, 8461 (1994). 

Figure 8.3 A schematic two-dimensional view of the potential energy surface and wave-packet dynamics in the ultrafast photodissociation of Hgl2 [adapted from Voth and Hochstrasser (1996), Zewail (1996)]. The transition state for the I + Hgl reaction is along the bisector, dashed line, with the lowest barrier at the bottom of the potential along that line. The UV excitation creates a localized wave-packet along the bisector. The center of the packet is displaced from the saddle point to a compressed configuration along the symmetric stretch. During the dissociation the wave-packet bifurcates, as shown, and each component is followed in the figure. It shows the coherent vibrational motion in the Hg—I well. ...

The excited state P decays with a (1/e) time constant of --200 ps at room temperature (Breton et al., 1990) at low temperatures similar decay times are observed. The overall kinetics in the Qy absorption region are the same as those in the stimulated emission (Breton et al., 1990), indicating that P decays back to the ground state in this time (see Introduction). At the timescale of a few picoseconds there is no overall kinetic evolution in the stimulated emission. However, at low temperature, the kinetics of the stimulated emission are modulated by oscillations (Fig. 1), which presumably reflect coherent vibrational motion of the protein, as discussed below. 

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