Luminescence: A Science Related To Spectroscopy

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Luminescence is a science closely related to spectroscopy, which is the study of the general laws of absorption and emission of radiation by matter. Luminescence is a process that corresponds to emission of electromagnetic radiation beyond thermal equilibrium.
“Cold light” the generation of light in a non thermal way.Light that is not generated at high temperature and occurs at low temperature in the form of cold body radiation.
Main processes of luminescence
A) Absorption of excitation energy for a particular system or material and stimulation of the system into an excited state. B) Transformation and transfer of the excitation energy emission of light . C) Relaxation of the system into an unexcited condition.

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PL canoccur in gas, liquid and solid phases. An energy level diagram as in Figure 1.1 can illustrate the radiative and non-radiativetransitions that lead to the observation of molecular photoluminescence. The spin multiplicities of a given electronic state can either a singlet @aired electrons) or a triplet (unpaired electrons). The ground electronic state is normally a singlet state and is designated as SO. Excited electronic states are either singlet (SI. S2) or triplet (TI) states. When the molecule absorbs light an electron is promoted within 10-14 - 10.'' seconds from the ground electronic state to an excited state that posses the same spin multiplicity as the ground state. Thisexcludes a triplet-excited state, as the final state of electronic absorption because the selection rules for electronic transition dictates the spin state should be maintained upon excitation. A plethora of radiative and non-radiative processes usually occur following the absorption light en routc to the observation of molecular luminescence.
Non-radiative relaxation processes
(1)Vibrational relaxation: -Excitation usually occurs to higher vibrational level of the target-excited state. The excited molecules normally relax rapidly to the lowest vibrational level of the excited
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The radiative transition SI+ So in Figure 1.1 represents fluorescence. Since fluorescence transitions are spin allowed they occur very rapidly and average lifetimes of the excited states responsible for are typically less than 1w6 s. Electronic transitions between states of different spin multiplicity are spin forbidden, however it becomes more probable when spin orbit coupling increases. The net result of spin orbit coupling is the mixing of excited singlet and triplet states. This mixing removes the spin forbidden nature of the transitions between pure singlet and pure triplet states. Therefore if intersystem crossing populates the triplet-excited state then luminescence might occur from the triplet state to the ground state. Phosphorescence refers to the emission of light associated with a radiative transition from an electronic state that has a different spin multiplicity from that of ground electronic state. The radiative transition TI -+ So in Figure 1.1 represents the phosphorescence.

Absorbance molecule returns to the ground or lower energy state via a non-radiative transition such as vibration, collision with other molecules, etc. These give off the energy absorbed rather than the emission of light.
Some energy is lost through various processes (e.g. non-radiative transitions)
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