Single molecule processes and relaxation

Of particular importance are the times which the molecule spends in the various states. Absorption occurs in around 10 s. This can be considered to be instantaneous and is roughly a factor of ten times faster than the molecular vibrations. The time in the singlet excited state is of the order of nanoseconds, while the time in the triplet sate can be anything from microseconds, through milliseconds, to 

Since the emission intensity is proportional to the number of molecules in the emitting state, we can observe the time dependence of the fluorescence or phosphorescence and thereby study fluorescence (or phosphorescence) relaxation. If n is the number of molecules in the excited state  

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