Fluorescence. Metastable Molecules

The fluorescence rate Vf is usually of the order 10 — lO s . However, it may sometimes be considerably lower. This is possible when the excited state is meta-stable. 

It should be mentioned as an example that, upon absorption of the resonant 2 536.5 A mercury line by mercury vapour, there arises the initial state Hg ( Pi) of a mean lifetime 1.55 10 s [27, 28]. After this time, the excited atoms return to the ground state emitting monochromatic 2536.5 A light. However, during its lifetime, the excited atom may undergo collisions and be transferred either to the ground Sq, or to the metastable state pQ which is 0.218 eV below the primary excited state. The mean lifetime of a metastable pQ mercury atom is 7.7 10 s [297]. 

The quenching of mercury resonant fluorescence by nitrogen shows that the second process is much more probable. 

The transition to the metastable state Pq is directly evident from the absorption of the 4046.6 A line by mercury vapour (connected with the transition 6s6p Po on 

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All excited molecules will not undergo conversion to the metastable colored form, so that will generally be less than unity. Competing deactivating processes for the excited molecules include fluorescence, phosphorescence, permanent chemical reaction and internal conversion processes in which the excitation energy ultimately appears as thermal energy in the system. 

Solutions of unsubstituted monoheterocyclic compounds, pyridine, pyrrol, furan and thiopene are non-fluorescent. According to Kasha and Reid17, the near ultra-violet absorption spectra of N-heterocyclic compounds like pyridine, pvrazine and phenazine include n tt transitions and the excited state undergoes, with high probability, a radiationless transition to a lower triplet metastable state and hence few molecules remain in the excited state long enough to fluoresce. The presence of molecules in the triplet state has been shown by their phosphorescence. 

For typical aromatic chromophores the excited state properties of the singlet (S = 0) and triplet (S = 1) manifolds are often treated separately because of their drastically different properties. The former states account for all important absorption and fluorescence features of the molecule and are very short-lived (lifetimes of 10 s are typical). The latter states are metastable with respect... 

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