Energy randomization

The rates of dissociation of the molecular ion are determined by the probabilities of the energy randomly distributed over the ion becoming concentrated in the particular fashions required to give several activated complex configurations yielding the dissociations. 

Attention should be drawn to the fact that thermal energy randomization occurring after a particle has crossed the activation barrier is not perfect, so that return jumps may not be neglected. This can be taken into account by introducing a curved dividing hypersurface S which the jumping particle crosses more than once. Corrections (backjumps) of up to 10% are predicted [C.P. Flynn (1987)]. 

Bamford et al. (45) have recently studied the photolysis of H2CO near the S- origin. The complete rotational distribution has been obtained for CO, which was detected by vacuum ultraviolet laser induced fluorescence. The distribution has a peak at J" = 42 and highly nonthermal, suggesting that energy randomization does not occur during dissociation. The population in CO J" < 20 is absent. The vibrational population of v" = 1 is 14 5% as large as that of CO (v" = 0). The C0(v" = 1) has nearly the same rotational distribution as C0(v" = 0). 

Pressure effect on the product distribution in supercritical media would resolve the problem. If the reaction proceeds via the competitive concerted/ stepwise mechanism, the reaction under a higher pressure is expected to give more exo isomer because the activation volume is considered to be smaller for concerted process than the stepwise one and hence more concerted reaction is expected under a higher pressure. If, on the other hand, bimodal lifetime distribution of trajectories is the origin of the stereoselection, the product ratio is expected to approach to unity under high-pressure conditions, since energy randomization is more effective under a high pressure. 

The linear Boltzmann plot obtained in the photodissociation of H20 in the first continuum merely reflects the rotational FC distribution for the zero-point bending motion in the electronic ground state. It has nothing to do with energy randomization in a long-lived intermediate complex ... 

Bates (1984) interpreted the temperature dependence of 02 + 202 - O + 02 in terms of the energy randomization rate in the complex. 

As a consequence of these suppositions, CID is an ergodic ion activation method, which allows a redistribution of the energy in the vibrational modes of the ion because the dissociation rate is slower than the rate of energy randomization. In these conditions, the... 

Alternatively, the initial preparation process (e.g., collisional excitation, photoexcitation, or chemical activation) may not provide a random initial distribution. Then if the initial decay rate is rapid, i.e., faster than the rate of energy randomization, then its value may depend on the details of the initial... 

The initial rate of IVR is extremely fast. Only 67 Is after the initial excitation of the front ring, energy appears in the baek ring. This does not mean, of eourse, that the energy randomizes on this time seale, but it indieates that loeal elustering of energy in a molecule for an extended period of time is impossible. The methode of fast Fourier Transforms was used to identified the modes that partieipated in the IVR. Only four normal modes, out of the available 174 modes, were exeited and partieipated in the initial phase of IVR. Even though it took 60 fs for the exeitation to move from the front to the back of the molecule, total relaxation was obtained however, only after few ps. 

We can indeed claim that this is an example of photoselective laser chemistry. The competition between relaxation and reaction of photoex-cited electrons in clusters represented in Fig. 14(b) is reminiscent of the competition in many laser-induced chemical processes, stimulated by the selective absorption of one or more photons, such as photodissociation, photoionization, isomerization, and so forth in polyatomic molecules, where the coupling of many vibrational modes provides energy randomization and relaxation on picosecond time scales. 

Results of the earliest study of this system (20) showed that randomly sampled trajectories reacted through both channels at rates that were slower than expected, with a greater deviation for the C— channel. Excitation biased in favor of C—modes further retarded both C—C and C—reaction channels. These observations led to the conclusion that our classical model molecule was intrinsically non-RRKM. Intramolecular energy randomization was restricted on the time scale of dissociation. C—bonds were the worst offenders. 

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