Chemical dynamics coherence mechanisms

The coherent motion initiated by an excitation pulse can be monitored by variably delayed, ultrashort probe pulses. Since these pulses may also be shorter in duration than the vibrational period, individual cycles of vibrational oscillation can be time resolved and spectroscopy of vibrationally distorted species (and other unstable species) can be carried out. In the first part of this section, the mechanisms through which femtosecond pulses may initiate and probe coherent lattice and molecular vibrational motion are discussed and illustrated with selected experimental results. Next, experiments in the areas of liquid state molecular dynamics and chemical reaction dynamics are reviewed. These important areas can be addressed incisively by coherent spectroscopy on the time scale of individual molecular collisions or half-collisions. 

Time-domain spectroscopies entail a major shift in emphasis from traditional spectroscopies, since the experimenter can control, in principle, the duration, shape, and sequence of pulses. One may say that traditional, CW spectroscopy, is passive—the experimenter attempts to study static properties of a particular molecule. Coherent pulse experiments are active in that, given a set of molecular properties (which may in fact be known from various spectroscopies), one tries to arrange for a desired chemical product, or to design a pulse sequence that will probe new molecular properties. The time-dependent quantum mechanics-wavepacket dynamics approach developed here is a natural framework for formulating and interpreting new multiple pulse experiments. Femtosecond experiments yield to a particularly simple interpretation within our approach. 

We first discuss the overall chemical process predicted, followed by a discussion of reaction mechanisms. Under the simulation conditions, the HMX was in a highly reactive dense fluid phase. There are important differences between the dense fluid (supercritical) phase and the solid phase, which is stable at standard conditions. Namely, the dense fluid phase cannot accommodate long-lived voids, bubbles, or other static defects, since it has no surface tension. Instead numerous fluctuations in the local environment occur within a timescale of 10s of femtoseconds. The fast reactivity of the dense fluid phase and the short spatial coherence length make it well suited for molecular dynamics study with a finite system for a limited period of time. Under the simulation conditions chemical reactions occurred within 50 fs. Stable molecular species were formed in less than a picosecond. We report the results of the simulation for up to 55 picoseconds. Figs. 11 (a-d) display the product formation of H2O, N2, CO2 and CO, respectively. The concentration, C(t), is represented by the actual number of product molecules formed at the corresponding time (. Each point on the graphs (open circles) represents a 250 fs averaged interval. The number of the molecules in the simulation was sufficient to capture clear trends in the chemical composition of the species studied. These concentrations were in turn fit to an expression of the form C(/) = C(l- e ), where C is the equilibrium concentration and b is the effective rate constant. From this fit to the data, we estimate effective reaction rates for the formation of H2O, N2, CO2, and CO to be 0.48, 0.08,0.05, and 0.11 ps, respectively. 

Chemical kinetics is all about bottlenecks. They determine the reaction mechanism on different timescales. We have seen that ET can be hmited by nonadiabatic transitions, solvent dynamics, intramolecular vibrational relaxation, translational diffusion, and conformational fluctuations either of the reactants themselves or of the embedding medium. The interplay between different mechanisms as well as nonequilibrium initial condition may result in rich kinetic behaviors, with strong nonexponentiality and coherence effects observed in recent experiments. 

The difference in the relaxation rates of ZQ and DQ coherences is the result of three principal mechanisms. These include the cross-correlation between the chemical shift anisotropies of the two participating nuclei, dipolar interactions with remote protons as well as interference effects due to the time-modulation of their isotropic chemical shifts as a consequence of slow mus-ms dynamics. The last effect when present, dominates the others resulting in large differences between the relaxation rates of ZQ and DQ coherences. Majumdar and Ghose have presented four TROSY-based experiments that measure this effect for several pairs of backbone nuclei. These experiments allow the detection of slow dynamic processes in the protein backbone including correlated motion over two and three bonds ". A suite of TROSY-based NMR relaxation dispersion experiments that measure the decay of DG and ZQ coherences as a... 

The kinetic simulations of the pulse combustor ignition can be carried out under conditions which closely approximate those in a continuously stirred tank reactor (cstr). In those calculations, hot product gases are steadily mixed with cold, unbumed reactants until the mixtures ignite. The reaction mechanisms used are valid for high temperatures, and the most important, sensitive reaction is reaction (3), and the combined influences of chemical kinetics, acoustics, and fluid dynamics can all be incorporated into a coherent practical design model [20]. 

Three novel approaches to the simulation of NA dynamics of large chemical systems have been presented [20-22]. The approaches extend the standard quantum-classical NA MD to incorporate quantum effects of the solvent subsystem that have been traditionally treated by classical mechanics. These effects include quantum trajectory branching (wave packet splitting), loss of quantum coherence directly related to the Franck-Condon overlap contribution to the NA transition probability, and ZPE of nuclear motion that contributes to the NA coupling and must be preserved during the equilibration of the energy released by the NA transition. 

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