Predissociation rates

With this simplification, Gray, Rice, and Davis obtained reasonably accurate values for the predissociation rate constant as a function of initial vibrational excitation. The rate constant thus obtained is larger than that from exact trajectory calculations by about a factor of two. By contrast, the RRKM theory would give a rate constant that is about three orders of magnitude larger than is observed. 

When the effect of intramolecular energy transfer is taken into account, more accurate rate constants can be obtained. We first compare the rate constants associated with the intramolecular bottleneck from the MRRKM theory with those from the Davis-Gray turnstile approach. As seen in Table III, they are in reasonable agreement. Hence, the Davis-Gray theory and the MRRKM theory predict similar overall reaction rates. This is demonstrated in Table IV. Table IV also shows that the predissociation rate constants would have been overestimated by a factor more than 100 if the RRKM theory were to be directly applied. 

Intramolecular Bottleneck Predissociation Rate Constants (in cm ) for the T-Shaped HeT Molecule"... 

Predissociation Rate Constant of the MRRKM Theory (in cm ) for a Three-Dimensional Model of the HeE Molecule, Compared to the Available Experiment Observations ... 

Y.Kami and E.E.Nikitin. Vibrational predissociation rate from dynamics of the full collision a test of the Eandau method against the exact results, J. Chem. Phys. 100, 8065 (1994)... 

The classical effective predissociation rate, (f), out of the energy level E=E, can now be compared with the quantum predissociation rate k (s ,Ae) via their... 

Predictions, Comparisons, and Implications Regarding Potentials. Another point of considerable physical interest concerns the differences in the predissociation rates of H2-Ar, H2-Kr and H2 Xe. For the same bellweather level considered in Table III, widths (Pcc) calculated on the proper vibrationally-averaged surfaces for these three species are compared in Table IV. Following the arguments... 

Adams has shown recently that It Is possible for an RRKM calculation based on a reasonable parameterization of the system to reproduce the llnewldth-derlved lifetime for a vdU molecule photodlssoclatlon (25). However, If the llnewldth-derlved lifetimes t actually are the Inverse predissociation rates, then the above data show clearly that the fundamental statistical assumptions of such an approach are qualitatively Incorrect for these vdW systems. Consider the following statistical predictions ... 

The intensities in an optical spectra can be regarded as transition rates. Hence, the distributions under consideration apply equally well, for example, to the distribution of predissociation rates. Indeed, it is in this context that such fluctuations were first considered in nulear physics. 

Figure 2.3 Perturbations and predissociations affect absorption and emission line intensities in quite different ways. Two pairs of absorption and emission spectra are shown. The first pair illustrates the disappearance of a weakly predissociated line in emission without any detectable intensity or lineshape alteration in absorption. The second pair shows that emission from upper levels with slow radiative decay rates can be selectively quenched by collision induced energy transfer. The opposite effect, selective collisional enhancement of emission from perturbed, longer-lived levels, is well known in CN B2 +—X2 +(u = 0,v") emission spectra (see Fig. 6.14 and Section 6.5.5). (a) the CO B1S+—X1S+(1,0) band in emission (top) and absorption (bottom). The last strong lines in emission are 11(16) and P(18). Emission from levels with J > 17 is weak because the predissociation rate is larger than the spontaneous emission rate. (Courtesy F. Launay and J. Y. Roncin.) (6) The CO A ll—X1 + (0,0) band in emission (bottom) and absorption (top). The a 3 + —X1 +(8,0) band lines appear in absorption because the A1 FI a 3 + spin-orbit interaction causes a small amount of A1 character to be admixed into the nominal a 3 + levels. These a —X lines are absent from the emission spectrum because collisional quenching and radiative decay into a3II compete more effectively with radiative decay into X1 + from the long-lived a 3 + state than from the short-lived A1 state. In addition, collisions and radiative decay into a3II cause the P(31) extra line (E) (arising from a perturbation by d3A v = 4) to be weakened in emission relative to the main line (M). (Courtesy F. Launay, A. Le Floch, and J. Rostas.)...

Only the total lifetime r of a level can be measured, r is related to the rate of decrease of the number of molecules initially in a given level via both radiative and nonradiative routes. Let kr be the radiative rate constant (the probability per unit time that a molecule will leave the level as a result of emission of a quantum of light) and knr the predissociation rate (the dissociation probability per unit time). Recall that the pressure is assumed to be low enough that the rates are not affected by collisions. The number of molecules leaving the initial state during the time interval dt is given by... 

A relative magnitude for the predissociation rate can be obtained by measuring the fluorescence quantum yield (see Eq. (1.2.3)),... 

The effect of predissociation on spectral features depends on whether it is the initial or final state of the transition that is predissociated. Predissociation can be detected either by direct measurements of lifetimes (r), linewidths (T), or level shifts (5E), or indirectly by observation of fragments. Table 7.2 surveys the range of predissociation rates sampled by different methods. Erman (1979) has reviewed the experimental methods for characterizing predissociation phenomena. 

When the predissociation rate is so much larger than the radiative decay rate that the fluorescence quantum yield is too low to measure a radiative decay rate directly, it is possible to infer the decay rate of the parent molecule from the effect of a static magnetic field on the polarization of a photofragment (Buijsse and van der Zande, 1997). 

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