UV SPECTROSCOPY/ UV radiation, properties/ principle and types of electronic transition

                                    UV SPECTROSCOPY

    UV spectroscopy is the measurement of attenuation of a beam of light after it passes through a sample or after reflection from a sample surface. Absorption measurement can be a single wavelength or over an extended spectral range.
      
Uv radiation
      The region beyond red is known as infra-red while that beyond violet is known as ultra ultra-violet. The wavelength of UV radiation starts at the blue end of visible light (4000A)  and at 2000A.

Properties of UV spectroscopy

• UV stands for ultraviolet spectroscopy.

• It is also known as ultraviolet-visible spectrophotometry.

• Most organic molecules and functional groups are transparent in the portions of the electromagnetic spectrum that we call the ultraviolet (UV) and visible (VIS) regions—that is, the regions where wavelengths range from 190 nm to 800 nm.

• It is also known as absorption spectroscopy.

•  In this region of the electromagnetic spectrum, atoms and molecules undergo electronic transitions.

Principle of UV spectroscopy
         
          Molecules containing π-electrons or nonbonding electrons (n-electrons) can absorb the energy in the form of ultraviolet or visible light to excite these electrons to higher anti-bonding molecular orbitals. This spectroscopy obeys Beer-Lambert law.
                                               which states that: when a beam of monochromatic light is passed through a solution of an absorbing substance, the rate of decrease of intensity of radiation with a thickness of the absorbing solution is proportional to the incident radiation as well as the concentration of the solution.
             
                                  The expression of Beer-Lambert law is-
                                                 A = log (I0/I) = Ecl

Types of electronic transitions
           In U.V spectroscopy molecules undergo electronic transitions involving σ,π, and n electrons. Four types of electronic transitions are possible.

1.   π-π*

1.   n-π*

1.   σ-σ*

1.   n-σ*

                                            

types of electronic transition