Why fluorophores show this typical excitation curve? Why not show same level of absorption at all wavelengths below the indicated level? I understand they show some absorption at higher frequency, why not same? An excitation-emission spectra of any fluorophore looks like this.
Fluorescence is a process in which the fluorescent ‘dye’ emits energy in the form of visible light while transitioning from electronically excited states to the ground state. The basic principle behind absorption is that light is absorbed by a molecule when the energy of the incident photon matches the energy difference between two electronic states.
In the process of excitation, light of some specific wavelength falls on the dye and if the incident photons contain similar or slightly more energy than is necessary for an electronic transition, those photons are absorbed. The excess energy is usually converted into vibrational energy in the same excited electronic state. However, if the photons have insufficient energy to promote a transition, no absorption occurs. On the other hand, if the photon has too high energy it is not absorbed because the excess energy can not be dissipated by only vibrational level transitions to the same electronic state. That is why only the range of frequencies that can bring the ground state to the same excited electronic state (with different vibrational levels.) can excite a that molecule. This is typically a distribution of frequencies, known as the absorption spectra.
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