Intramolecular dynamics control

The coherence properties of laser radiation provide an opportunity to exert some external control over intramolecular dynamics. Control over photofragmentation product branching ratios has been achieved in both time and frequency domain experiments. 

The unzipping procedure reveals the diagnostically significant trends in fractionation widths and patterns illustrated by Figure 3. These trends can lead to qualitative insights into IVR mechanisms and can suggest optimal schemes for external control over intramolecular dynamics. The unzipped polyads can also yield quantitative least-squares refinements of anharmonic coupling constants, from which any dynamical quantity based on (Q, 0 may be calculated. 

Controlling atomic and molecular quantum states with the help of laser light has been an intensively studied field in the last years (see, e.g., [Tan-nor 1985 Judson 1992 Rice 2000 Shapiro 2003]). The ability to produce and shape femtosecond laser pulses [Zewail 1994 Manz 1995] made it possible to create specially tailored wave packets and to manipulate their dynamics in order to reach pre-assigned goals. The techniques of quantum control have been applied to the problem of steering chemical reactions [Tannor 1985 Assion 1998 Shapiro 2003] as well as intramolecular dynamics [Aver-bukh 1993 Goodson 2000],... 

There are many useful tools to help focus attention on specifiable parts of the behavior of a many-body system, and many of these were listed in Section 9.1.4. The concepts of bright state, doorway state, and dark state, the experimental tools of state-selective excitation and detection, and the analysis tools of wavepackets and survival and transfer probabilities, can provide insights into the causes, preferred pathways, and possibilities for control of intramolecular dynamics. 

This analysis is complicated, owing to the relatively high dimensionality of the phase space of two coupled 2-dimensional isotropic anharmonic bending oscillators. But the analysis is rich in insights, some of which may guide the formulation of novel schemes for external control over intramolecular dynamics. 

Computational studies have indicated that chaotic behavior is expected in classical mechanical descriptions of the motion of highly excited molecules. As a consequence, intramolecular dynamics relates directly to the fundamental issues of quantum vs classical chaos and semiclassical quantization. Practical implications are also clear if classical mechanics is a useful description of intramolecular dynamics, it suggests that isolated-molecule dynamics is sufficiently complex to allow a statistical-type description in the chaotic regime, with associated relaxation to equilibrium, and a concomitant loss of controlled reaction selectivity. 

Bar, I. and S. Rosenwaks (2001). Controlling bond cleavage and probing intramolecular dynamics via photodissociation of rovibrationally excited molecules. Int. Rev. Phys. Chem. 20,711. 

Experimental methods of femtochemistry are based on the achievements of femtosecond spectroscopy (see Section 3.2.11). Three main directions of this new area can be distinguished dynamics of intramolecular process and transition state during chemical transformation kinetics of superfast chemical reactions and control of the intramolecular dynamics and elementary chemical act. These three directions are briefly described in the next sections. The examples are taken from the review by A. Zewail. 

In analyzing dielectric relaxation spectra in condensed matter, two different concepts of molecular dynamics are of great significance. The first, which is predominantly applicable to liquids, is to consider the liquid as a dense gas with very frequent collisions, the rate of which is so high that the reorientation of a molecule is completely controlled by the rate. Thus, in this case the rotational dynamics becomes similar to the Brownian translational diffusion and it is known as rotational diffusion. The second approach, which is more frequently used to describe dynamics in molecular crystals and intramolecular dynamics, emphasizes reorientation of a molecule in the presence of an orientation-dependent potential with well-defined minima. In this case, the molecule resides for finite time intervals in different potential wells, jumping between them from time to time. Since the time of jump is very short in comparison with the time of residence in the well, the concept is known as reorientation by instantaneous jumps. On infinite increase of the number of potential wells and the corresponding decrease of the angular distance between them, this approach reduces to rotational diffusion. ... 

In the sections below a brief overview of static solvent influences is given in A3.6.2, while in A3.6.3 the focus is on the effect of transport phenomena on reaction rates, i.e. diflfiision control and the influence of friction on intramolecular motion. In A3.6.4 some special topics are addressed that involve the superposition of static and transport contributions as well as some aspects of dynamic solvent effects that seem relevant to understanding the solvent influence on reaction rate coefficients observed in homologous solvent series and compressed solution. More comprehensive accounts of dynamics of condensed-phase reactions can be found in chapter A3.8. chapter A3.13. chapter B3.3. chapter C3.1. chapter C3.2 and chapter C3.5. 

In the excited state, the redistribution of electrons can lead to localized states with distinct fluorescence spectra that are known as intramolecular charge transfer (ICT) states. This process is dynamic and coupled with dielectric relaxations in the environment [16]. This and other solvent-controlled adiabatic excited-state reactions are discussed in [17], As shown in Fig. 1, the locally excited (LE) state is populated initially upon excitation, and the ICT state appears with time in a process coupled with the reorientation of surrounding dipoles. 

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