ANALYSIS THROUGH SPECTROSCOPY
Processes in Spectroscopy
The interaction of the light (electro-magnetic radiation) with a substance and the subsequent
energy transfer ends with three main processes namely:
Absorption:
The process by which the energy of the light (in the form of photons) is transferred to the
atom or molecule raising them from the ground state to an excited state
Fluorescence:
The absorbed energy is rapidly lost to the surroundings by collisions within the system and
relax back to the ground state. Sometimes the energy is not lost in this way but is re-emitted a
few milli seconds later, which is referred as fluorescence
Emission:
If the substances (atoms or molecules) are heated to high temperatures (in a flame or in an
electric discharge) the electrons are exited to higher energy levels. Later, they relax to the
ground state with the emission of radiation, the magnitude of which is more or less equivalent
to absorbed energy
Most of the analytical techniques are based on the light interactions with the substances and
utilise any of the above three associated processes. Substances interact with light differently
at various wavelengths and hence different types of analysis & instruments. The entire
spectrum of light can be represented as below. Since, light has both electrical and magnetic
components, this representation is referred as an ‘Electro-Magnetic Spectrum’:
The following is short comparison between Ultra Violet (UV), Visible (Vis) and Infra Red
(IR) ranges for the energy, frequency and wavelength:
Energy: UV > Vis > IR
Frequency: UV > Vis > IR
Wavelength: UV < Vis < IR
The symbol for the wavelength is “lambda” (λ) and the unit is either nanometer (nm) or
micrometer (or micron, μm). The symbol for frequency is “nu” (μ) and the unit is either hertz
or sec-1
A parameter closely related to frequency is the wave number, which has the symbol
“nu bar” ( υ ) and the unit is cm-1
There are two levels by which the substances can interact with the light as, atomic level and
molecular level and hence the corresponding techniques:
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