UV-VISIBLE SPECTROSCOPY
Electromagnetic Radiation & Spectroscopic Transition
↳ consists of orthogonal oscillation electric and magnetic fields
Main descriptive parameters :
• The wavelength (λ, m)
• The frequency (ν, s−1 or Hz)
ν = c/λ
• The wavenumber (⊽ , cm-1)
⊽ = 1/λ
E = hν = hc/λ= hc⊽
When the frequency ,ν of the radiation matches the energy gap between two quantum states in an atom or molecules , the absorption may occur
There are three types of absorption that can occur :
- stimulated absorption ( used for spectroscopy )
- stimulated emission ( used for laser )
- spontaneous emission ( fluorescence / phosphorescence )
UV-VISIBLE (electronic) SPECTROSCOPY
↳ measures transition between electronic energy level
- The most common transition are from the Highest Occupied Molecular Orbital (HOMO) to Lowest Unoccupied Molecular Orbital (LUMO)
If you know the actual values of the energy levels , you can calculate the energy gap and thus the frequency or even wavelength
Colored compound means that it has absorption bands in the visible region
- the UV-visible spectrum will shows the exact positions and intensities of these absorption
HOMO-LUMO band gap will have the highest peak (wavelength) as can be seen in the spectrum
Other transition to higher energy level can also occur at shorter wavelength
These are due to combined electronic and vibration transition
R M I V U X G
VISIBLE (700-350nm)
UV (350-200nm)
So typical range for UV-visible spectrum is between 200-700nm
Electric dipole transition mainly occur between orbitals of the same symmetry
When the HOMO is a σ orbital, we get σ→σ* transitions.
• These are generally high energy – see, for example, ethane
• They would be quite strong if we could measure them
• This is a region called the ‘vacuum ultraviolet’ because air absorbs in these regions.
NURBALQIS BINTI ZAILAN
Allah S.W.T.
you 
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