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Absorption of radiations in UV- visible range (λ 1nm to 800nm) causes electronic excitations. The electron are excited from their ground state to higher excitations states by absorbing the proper radiation. Since UV-visible spectroscopy involves electronic transitions, it is often called electronic spectroscopy. The intensity of absorption of radiations as a functions of wavelength of light absorbed I plotted and the graph gives an idea of electronic transition taking place in the molecules. From these electronic transition information is obtained about the structure of the molecule.


Most of the organic compounds absorb UV light having wave length of 200-800 nm. The coloured compounds usually absorb the light of wavelength 400-800 nm but they may also absorb the light of shorter
 wavelengths, the radiations having wavelength shorter than 200 nm are also absorbed by atmospheric gases. Therefore, in order to study the transitions brought about by these radiations in a molecule, the study should be conducted in vacuum.

Most UV-visible spectrophotometer are double beam instruments as shown in schematically in Fig .1.1. The caption of this figure briefly describes the working of the spectrophotometer. For recording an UV spectrum . The sample is dissolved in a suitable organic solvent and placed in a cell while pure solvent is placed in another similar cell called the reference cell. The most common solvents used for recording UV spectra are methanol, ethanol, hexane and water, which are transparent to UV radiation.

Fig.1.1 In the ultravilot spectrometer, the UV source produce a continuum of light in the UV region, the monochromator  selects one wavelength of light, which split into two beams. one beam passes through the sample cell, while the other passes through the reference cell. the detector measures the ratio of the two beams, and the chart recorder plots this ratio as a function of wavelength.  


 The light source is usually a hydrogen lamp, and the optical and cells are made up of quartz because most other clear material absorb UV radiation.

The absorption of energy is recorded as absorbance (not transmittance).

Two empirical law has been formulated about absorption intensity.

1) Lambert’s law: Lambert’s law states that the fraction of the                                                     incident monochromatic radiation absorbed by a homogeneous medium is independent of the intensity of the incident radiation.

2) Beer’s law: Beer’s law states that the absorption of a monochromatic                                radiation by a homogeneous medium is proportional to the number of absorbing molecules.
The absorption at particular wavelength is defined by the equation:

                   A = log I/I0
Where, A = absorbance
               I0 = intensity of the            reference radiation
                I = intensity of the beam coming out of sample     
                                  cell.

The absorbance by a compound at a particular wavelength increases with an increasing number of molecules undergoing transitions. Therefore, absorbance depends upon the electronic structure of the compound and also upon the concentration of the sample and the length of the sample cell. For this reason energy absorption is reported as molar asborptivity ε, also known as molar extinction coefficient, rather than as the actual absorbance. Often UV spectra are reported to show ε or log ε instead of A, as the ordinate. The log ε value is specially useful when value for ε are very large.


                                    
   C
                                    A    l
Therefore:                      
                                    A    Cl

                                    A = constant Cl
                                    A = ε . Cl
                                    ε = A/Cl
where
                                     ε = molar absorptivity
                                     A = absorbance
                                     C = concentration in mole/litre
                                     l  = cell length in cm
                                     
If  absorbance, concentration and the cell length are known, the molar absorptivity can be calculated.
It is a valuable information that can be used in connection with UV spectra . Spectra are often redrawn bry plotting ε against λ, expressed in nm. (one nanometer equals to 10-9  meter ). The molar absorptivity ranges from 10 to 100,000. The typical UV spectrum plot is shown in Fig. 1.3. UV absorption bands are thus characterized by the wavelength of the absorption maxima (λmax) and  ε.

Fig.1 .2 Typical UV spectrum