Dissociation lifetime

Fig. 5. Effect of the dissociation rate on the ion image intensity distribution, (a) Simulated translational energy distribution, (b), (c) Image intensity distributions that would result from (a) if the dissociation lifetime was 0.1/rs and 15/l

All this pertains to the dissociation reaction (132) or its inverse (126). Since other dissociation reactions, inverse to (128)—(130), may becomes dominant in strongly p-type regions, the dissociation lifetime r2 may well be shorter here than in the weakly n-type region of the small step in Fig. 24. And if an H species exists, there could be a similar doping dependence of the lifetime of H2 in strongly n-type material here, however, relevant data are almost entirely lacking. 

C. Radiative and Unimolecular Dissociation Lifetimes of Chemically Activated Ions... 

Because of the ease with which they undergo photodissociation, aromatic iodides have been used extensively to generate free radicals.46 The rate constant for the dissociation of p-diiodobenzene triplet has been measured to be about 4 x 1012 to 4 x 1013 sec-1 which would give a lifetime of 2.5 x 10 13 to 2.5 x 10 14 sec.47 Obviously a sensitizer with this short a dissociative lifetime could not be quenched by an acceptor. 

The primary yield of dissociation at the low-pressure limit is unity at 3130 A, slightly less than unity, at 3340 A, and about 0.04 at 3660 A. The ( () values decrease at higher pressures of ketenc or added inert gas and the rate of decrease is much larger at longer wavelengths. The dissociation lifetimes arc 0.3, 4, and 850 nsec, respectively, at 3130, 3340, and 3660 A [Porter and Connelly (814)]. 

Fluorescence from ketene upon absorption of light in the near ultraviolet has not been observed. The quantum yield of fluorescence is less than 10 3. Since the radiative lifetime calculated from the integrated absorption cocflicicnt is 40 //see. the lifetime of the excited state must be less than 0.4 nsec. Since the lifetime of the initially formed excited state is much shorter than the dissociative lifetimes, an excited state responsible for dissociation must be different from the one initially formed at 3340 and 3660 A. 

By computer simulation of the concentration-time profiles of the reactants and products, Su et al. [67,68] derived approximate values for the rate constant fc7 = 1 x 10"14cm3 molecule-1 s-1 and the dissociative lifetime fc 7 = 1 s(25°C, 700Torr air). The more recent, extensive study by Barnes et al. [73] reported an order of magnitude larger values for both rate constants. Thus, the HOO + HCHO reaction represents the fastest known reaction of HOO with organic molecules at large and also a potentially important source of HC(0)0H in the atmosphere. 

B2) + hv (X < 320 nm) - OCD) + 02 ( a J Term symbols of the electronic states are indicated in brackets. Both 02 and O are in their electronic excited states. With more complex species, may make its dissociative lifetime very large and either of the competitive process de-excitation or chemical reaction may take place. 

I). The dissociation of actinomycin D Is shown to be dependent on the phosphate to drug ratio (P/D ratio). At high P/D (low degree of saturation) the dissociation lifetime Is about 900 sec. At higher degrees of saturation the lifetime Increases until a final value of about 1400 sec Is reached. Neither the concentration of... 

Dissociation lifetime of actinomycin D from Poly(dGdC) Poly(dGdC) as a function of phosphate/drug ratio (T 22°C) and temperature. 

SDS nor the pH affects the actinomycin D dissociation lifetime however, the dissociation time Is markedly dependent on both temperature (Table I) and salt concentration (Table II). 

Decreasing either the salt concentration or the temperature Increases the dissociation time, but In neither case does the dissociation change from a single exponential. The divalent metal Ion, Mg" ", Is more effective In lowering the dissociation lifetime than Na (Table II). This suggests that a divalent Ion Is more effective In lowering the activation barrier than a monocation. 

Figure 12.16 Proton dissociation lifetimes time of protonated 2-AP photoacid as a function of the molar fraction of organic co-solvent in water solutions. Ref. [116].

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