Dear Yasir,
Please read the following text:
Ultraviolet–visible spectroscopy or ultraviolet-visible spectrophotometry (UV-Vis or UV/Vis) refers to absorption spectroscopy or reflectance spectroscopy in the ultraviolet-visible spectral region. This means it uses light in the visible and adjacent (near-UV and near-infrared [NIR]) ranges. The absorption or reflectance in the visible range directly affects the perceived color of the chemicals involved. In this region of the electromagnetic spectrum, molecules undergo electronic transitions. This technique is complementary to fluorescence spectroscopy, in that fluorescence deals with transitions from the excited state to the ground state, while absorption measures transitions from the ground state to the excited state
Luminescence is the emission of light by a substance. It occurs when an electron returns to the electronic ground state from an excited state and loses it's excess energy as a photon.
Luminescence spectroscopy is a collective name given to three related spectroscopic techniques. They are;
Molecular fluorescence spectroscopy
Molecular phosphorescence spectroscopy
Chemiluminescence spectroscopy
Fluorescence
Absorption of UV radiation by a molecule excites it from a vibrational level in the electronic ground state to one of the many vibrational levels in the electronic excited state. This excited state is usually the first excited singlet state. A molecule in a high vibrational level of the excited state will quickly fall to the lowest vibrational level of this state by losing energy to other molecules through collision. The molecule will also partition the excess energy to other possible modes of vibration and rotation. Fluorescence occurs when the molecule returns to the electronic ground state, from the excited singlet state, by emission of a photon. If a molecule which absorbs UV radiation does not fluoresce it means that it must have lost its energy some other way.
Phosphorescence
A molecule in the excited triplet state may not always use intersystem crossing to return to the ground state. It could lose energy by emission of a photon. A triplet/singlet transition is much less probable than a singlet/singlet transition. The lifetime of the excited triplet state can be up to 10 seconds, in comparison with 10-5 s to 10-8 s average lifetime of an excited singlet state. Emission from triplet/singlet transitions can continue after initial irradiation. Internal conversion and other radiationless transfers of energy compete so successfully with phosphorescence that it is usually seen only at low temperatures or in highly viscous media.
Chemiluminescence occurs when a chemical reaction produces an electronically excited species which emits a photon in order to reach the ground state. These sort of reactions can be encountered in biological systems; the effect is then known as bioluminescence. The number of chemical reactions which produce chemiluminescence is small. However, some of the compounds which do react to produce this phenomenon are environmentally significant.
A good example of chemiluminescence is the determination of nitric oxide.
Skoog, Douglas A.; Holler, F. James; Crouch, Stanley R. (2007). Principles of Instrumental Analysis (6th ed.). Belmont, CA: Thomson Brooks/Cole. pp. 169–173. ISBN 9780495012016.
Hoping this will be helpful,
Rafik
UV-Visible gives the absorption spectra of materials, band gap of materials, defects.
Luminescence spectroscopy is transition from excitation state to intermidiate state then falls to ground state with emission of diffrent color. These color are due to dopant materials or host defects.
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