Coherent motion

Figure Bl.14.9. Imaging pulse sequence including flow and/or diflfiision encoding. Gradient pulses before and after the inversion pulse are supplemented in any of the spatial dimensions of the standard spin-echo imaging sequence. Motion weighting is achieved by switching a strong gradient pulse pair G, (see solid black line). The steady-state distribution of flow (coherent motion) as well as diffusion (spatially...

Under steady-state flow conditions (coherent motion), a Taylor series can be applied to describe the time-dependent position of the fluid molecules ... 

In a Poiseuille flow of a Newtonian liquid the coherent motion gives rise to the dominant effect, rendering contributions from the stochastic dispersion term... 

While the idea of LF explained the H2 data quite well [28], we were surprised by the magnitude of the oscillations in our I2 data [16], as, unlike H2,12 is not vibrationally cold at room temperature - the conditions for our experiment. Generally, thermal motion is detrimental to observing coherent motion. Thus, we took a long time scale run to get a more accurate measurement of the frequency of the vibrations, shown in Fig. 1.5. These data also exhibit a vibrational revival, from which the anharmonicity of the potential well can be determined. Indeed, the vibrational frequency accurately matched that of the ground state. 

Systematic TRSHG studies on alkali-atom adsorbed metal surfaces by Matsumoto and coworkers provided a deep insight on how coherent motions are created under very different electronic configurations [15, 77, 78]. The results showed that the coherent phonon generation critically depends on the surface and bulk electronic structure of the substrate. 

The ultrafast laser pulse excites many vibrational modes of the metal carbonyl simultaneously (i.e., the vibrational modes are phase-coupled). Are the subsequent ligand motions coherent or do ultrafast dephasing processes hinder coherent motions ... 

In a classical Bohr orbit, the electron makes a complete journey in 0.15 fs. In reactions, the chemical transformation involves the separation of nuclei at velocities much slower than that of the electron. For a velocity 105 cm/s and a distance change of 10 8 cm (1 A), the time scale is 100 fs. This is a key concept in the ability of femtochemistry to expose the elementary motions as they actually occur. The classical picture has been verified by quantum calculations. Furthermore, as the deBroglie wavelength is on the atomic scale, we can speak of the coherent motion of a single-molecule trajectory and not of an ensemble-averaged phenomenon. Unlike kinetics, studies of dynamics require such coherence, a concept we have been involved with for some time. 

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. H. Zewail The observation of coherent motion in the benzene-iodine system should be related to the l2 motion and hopefully with better time resolution we should be able to resolve it. As for the base-pair experiment, the key motion is that of the N N stretch and N—H asymmetric motions, and our time scale of observation was appropriate for the dynamics to be observed. 

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. 

There are two different temperature regimes of diffusive behavior they are analogous to those described by Holstein [1959] for polaron motion. At the lowest temperatures, coherent motion takes place in which the lattice oscillations are not excited transitions in which the phonon occupation numbers are not changed are dominant. The Frank-Condon factor is described by (2.51), and for the resonant case one has in the Debye model ... 

Motion and color are processed by different cortical areas. A difference in the luminance of a moving stimulus is required for coherent motion perception (Ramachandran and Gregory 1978). Cells found in V3 respond to lines of different orientation and also to motion (Tovee 1996). They do not respond to color. V3 is believed to process dynamic form. Some cells of V3 are able to discount the movement of the eye. They only respond to a stimulus that moves relative to the eye. These cells also receive information about the eye position (Zeki 1999). Cells in V3 are connected to layer 4B of VI. V3 is also connected to the thick stripes of V2. 

Figure 1. Schematic representation of a cubic trapped ion cell commonly used in FTMS. Coherent motion of ions in the cell induces an image current in the receiver plates. The time domain signal is subjected to a Fourier transform algorithm to yield a mass spectrum.

Coherent Motion In-phase movement of ions within a trapped ion... 

FTMS cell to induce coherent motion of the ions. 

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