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In my class we have an exercise that I technically understand but that I can't get conceptually.

We have trois molecules (naphtalene, anthracene, tétracene). Considering each molecule as infinite quantum well of length 2L(naphtalene), 3L(anthracene) and 4L(tetracene), we would like to predict the wave length absorbed the electron of each molecules. To do so :

- We find the transition energy for each electron state : ΔE =

^{h}*(2n+1)/(8m*Lw)

- Then we find the wave lengths absorbed by each molecules for each transition with : λ = h*c/ΔΕ

So, the exercise is resolved. But my question is : when ALL the electrons at state n = 1 go to n = 2, and n = 2 go to n = 3, ... considering that light is always available, why molecules don't stop absorbing the wave length of the first transition states (consecutively, why the sample doesn't change its colour) through time. And why after a certain state, electron doesn't absorbe the wave length corresponding to it's transition (like in naphtalene, the last transition is from 4 to 5).

Thanks a lot for answers and sorry for my bad english. I hope i have been clear enough

Bye !