Activation:

The absorption of a photon of appropriate energy causes the molecule to get excited from the ground state to one of the excited states. This process is known as excitation or activation and is governed through Franck-Condon principle. According to this principle, an electronic transition takes place so fast (~10^-15 s) in which the molecule does not get an opportunity to execute a vibration, that is when the electrons are excited the internuclear distance does not change. The basis for the principle is in which the nuclei are extremely massive as compared to the electrons and thus move very slowly.

The implication of Frank -Condon principle is in which the transition from the ground state to excited state could be represented through vertical arrows in the Jablonski diagram. An electronic transition takes place from the state of lowest vibrational energy of an electronic ground state to any of the vibrational stages of the excited electronic states as shown through the vertical upward arrows within the diagram. The excited state achieved would depend on the energy of the photon absorbed. In the diagram the two excited states as S₁ and S₂ have been displays. These are acquired through the absorption of the radiation in the region of wavelength λ₁ and λ₂ correspondingly. Once the molecule gets excited through the absorption of radiation, it does not stay excited indefinitely; a number of processes bring it back to the ground state. This is known as deactivation. Let us learn about various means of deactivation of excited states.