Unimolecular radiative lifetime

Thus the measured unimolecular radiative lifetime is the reciprocal of the sum of the unimolecular rate constants for all the deactivation processes. The general form of the equation is given by... 

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

Two kinds of unimolecular decay lifetimes can be described. The first is the true radiative lifetime, i.e., the reciprocal of the rate constant for the disappearance of a species which decays only by fluorescence or phosphorescence. Since values of true fluorescence lifetimes may be calculated from the relationship between these quantities and the / numbers (vide supra) of the corresponding absorption bands, these values are (or at least approximations of them) are, in a sense, available. The second kind of lifetime is the reciprocal of an observed first order rate constant for decay of an excited state which may be destroyed by several competing first-order processes (some of which may be apparent first order) operating in parallel. We suggest that the two kinds of lifetime be distinguished by the systematic use of different symbols, as utilized by Pringsheim (4). 

A. General Remarks on Unimolecular Decompositions of Chemically Activated Radicals. A major portion of the molecular beam studies of fluorine atom chemistry (12-19,37) has been concerned with a class of reactions characterized by the formation of a transient species from bimolecular association of the reactants and whose lifetime is long compared to its rotational or vibrational periods. The formation of such a long lived complex implies that the reactants experience a net attraction and consequently the potential energy surface for the reaction possesses a deep well of course, the total energy of the system is greater than that required to dissociate the intermediate, either to reactants or products, so that in the absence of a third body or relatively improbable photon emission (radiative lifetime > 10 sec), the intermediate must decay prior to detection. Under certain favorable conditions where a large... 

The reaction dynamics of few excited complexes are known however, the opportunities provided by pulsed lasers promise to make this research area one of major emphasis of mechanistic studies. Such methods are necessary because few transition-metal complexes exist as electronically excited states in RT solutions with lifetimes exceeding 1 fjis, and many are shorter lived. Several competing processes lead to ES decay nonra-diative deactivation to the ground state (GS), radiative deactivation (i.e., emission) to the GS, unimolecular reaction to products (such as ligand substitutions or redox decomposition) or bimolecular electron transfer or energy transfer with another species, Q, in solution. These processes are indicated in Eqs. (a)-(e) for a hypothetical complex [MLJ" + ... 

ES2 has its own decay processes including radiative (k, termed phosphorescence) and non-radiative deactivation (k ), unimolecular reaction to product(s) (kp) and bimolecular quenching by energy or electron transfer (kq) to another species. For this model the ES2 lifetime is defined by... 

The copper(I) complex Cu(dmp)2 (dmp = 2,9-dimethyl-l,10-phenanthroline) displays MLCT luminescence in ambient temperature CH2CI2 solutions [85]. This emission has been shown to be quenched by various Lewis bases (B), and the mechanism proposed is addition of B to the MLCT state at the metal center to give an exciplex which decays rapidly (Eq. 6.39). The validity of this mechanism was tested by comparing, in the presence and absence of Lewis base quenchers, the pressure effects on the emission lifetimes of Cu(dmp)2 vvith those on the emission lifetimes of the bulkier 2,9-diphenyl-phen analog Cu(dpp) [86]. The lattice ions should not be as susceptible to reaction of the copper center with B. For both ions, emission quantum yields are small (<10 ) at ambient T and unimolecular photoreactions are not observed, so the pressure sensitivity of x reflects non-radiative deactivation mechanisms. 

Reaction products of the radical chain process (carbonyl groups, double bonds) again aet as ehromo-phores. At low eoneentrations, the HPD is a pseudo-unimolecular step. The lifetime of peroxyl radi-... 

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