Mean radiative lifetime

Spin Orbit Energies of the 2P% States of the Halogen Atoms and Mean Radiative Lifetimes for Magnetic Dipole Emission (following Garstang13)... 

B. Spontaneous Emission and the Mean Radiative Lifetimes of the Halogen Atoms in the np5 2Pyi States... 

It can be seen from Table II that unless experimental conditions are carefully chosen, the contribution from emission to the over-all decay of X(np5 2Py2) can usually be neglected as more rapid kinetic processes result in the removal of the excited atoms. Further, as the mean radiative lifetime is long, it is possible to detect these atoms by absorption spectroscopy. (See Section VII.B, general kinetic equation for the removal of 2Pyt atoms.)... 

D. Experimental Determination of the Mean Radiative Lifetime of I(52Py2)... 

The term in Eq. (49) which describes radiative decay is relatively small under conditions where stimulated emission does not occur. Thus, only when all other contributions to the over-all decay are accurately determined will this term be accessible. At present only time-resolved atomic emission studies provide the required precision.11 The mean radiative lifetime has been obtained by observing the variation in the first-order decay coefficient for this emission as a function of inert gas pressure (Fig. 11). The slope of the graph so obtained yields the diffusion coefficient, while the intercept represents the sum of... 

At high viscosities or low temperatures, dielectric relaxation time xj may be larger than the mean radiative lifetime t/ of the molecule. This may decrease the O—O separation between absorption and emission. On the other hand, at high temperatures solvent relaxation may be promoted thermally decreasing xd and O—O separation may again decrease. A maximum value for Av (O—O) is expected at some intermediate temperatures. Besides the relaxation effects, the O—O separation can also be affected by environmental modification of the potential energy surfaces. 

If one adopts the correct point of view that the complete wave function of any state of a diatomic molecule has contributions from all other states of that molecule, one can understand that all degrees of perturbation and hence probabilities of crossover may be met in practice. If the perturbation by the repulsive or dissociating state is very small, the mean life of the excited molecule before dissociation may be sufficiently long to permit the absorption spectrum to be truly discrete. Dissociation may nevertheless occur before the mean radiative lifetime has been reached so that fluorescence will not be observed. Predissociation spectra may therefore show all gradations from continua through those with remnants of vibrational transitions to discrete spectra difficult to distinguish from those with no predissociation. In a certain sense photochemical data may contribute markedly to the interpretation of spectra. 

Constants or lifetimes of the excited states are important parameters since the reactivities of these energy states depend on them. Rate constants of various photophysical and photochemical processes can be adduced from quantum yield data only if the mean radiative lifetimes (t0) are known. The defining relationships are (Section 5.3)... 

SPECIES AND STATE MEAN RADIATIVE LIFETIME (ji sec) PRINCIPAL transition 2 (in A) NAME OF TRANSITION APPROX. AEif (eV) REF. 

The following crude estimate of Pj. is usually made. Let Tq be the collision duration and Tr the mean radiative lifetime of the quasi-molecule. Then... 

Quantum mechanics also yields an expression for the Einstein spontaneous emission coefficient (the reciprocal of the mean radiative lifetime) [8] ... 

This uniform distribution is achieved by having the exciting electron beam coaxial with the atomic beam. Early experiments with transverse electron beam excitation led to atom recoil and serious systematic effects due to non-uniform velocity distributions over the beam cross-section. The mean radiative lifetime is determined by taking the ratio of the number of metastable atoms in the same velocity interval at the two spatially separated detectors since... 

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