We take absorption to be an instantaneous event occurring at the timescale of 10^-15 s. While emissions are much slower phenomenon. Take the case of absorption and fluorescence only and let the transition in question is between S1 and S2 states. The transitions are between same states, so why this difference in timescales?
Dear Anish Rao,
You are confusing two different timescales here; one for the process to occur and one for the process itself. Spontaneous emission is a probabilistic process that is characterised by a lifetime on the order of picoseconds to nanoseconds (for fluorescence); however, the (Franck-Condon) transition between the two states involved is, like absorption, essentially instantaneous (occurring on the timescale of femtoseconds). In other words, it might take a while (say, a nanosecond) for emission to randomly occur but the process itself is instantaneous.
It is also worth pointing out that the emission lifetime simply gives the time at which the emission intensity has dropped to ~37% (1/e) of its original intensity after excitation; it is a convenient characteristic for describing the system but it does not mean that emission always takes that long to occur.
I hope that helps.
Darren
Dear Anish Rao,
You are confusing two different timescales here; one for the process to occur and one for the process itself. Spontaneous emission is a probabilistic process that is characterised by a lifetime on the order of picoseconds to nanoseconds (for fluorescence); however, the (Franck-Condon) transition between the two states involved is, like absorption, essentially instantaneous (occurring on the timescale of femtoseconds). In other words, it might take a while (say, a nanosecond) for emission to randomly occur but the process itself is instantaneous.
It is also worth pointing out that the emission lifetime simply gives the time at which the emission intensity has dropped to ~37% (1/e) of its original intensity after excitation; it is a convenient characteristic for describing the system but it does not mean that emission always takes that long to occur.
I hope that helps.
Darren
Dear Anish,
No problem, I'm glad I could be of assistance. To be honest, I think this is actually quite a common misconception.
By the way, I should have also pointed out that the emission lifetime gives the average (mean) time between excitation and emission. I realise this was not really part of your question but I wanted to clarify why lifetimes are useful parameters to use to describe a system.
All the best,
Darren
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