Solution:

A line-broadening mechanism is referred to as homogeneous when it broadens the line of each atom in the same way. In this case, the lineshape of the single-atom cross section and that of the overall absorption cross section are identical.

A line-broadening mechanism is said to be inhomogeneous when it distributes the atomic resonance frequencies over some spectral range.

Thus, in the discussion that follows, we consider the line-shape function in either absorption or emission, whichever is more convenient.

There are two mechanisms involved in causing electrons to have different energy states.

These are referred to as “homogeneous line broadening” and “inhomogeneous line broadening”.

Homogeneous Line Broadening:

• Sometimes called “natural broadening”, this occurs when all of the electrons in the material have the same resonant frequency. Even though all of the atoms in the excited state are equivalent there are differences in energy level due to “uncertainty”.

Inhomogeneous Line Broadening:

• Inhomogeneous Line Broadening takes place when the atoms in the medium have different resonance frequencies. This is the case with “Doppler Broadening” is due to the thermal vibration of atoms. It is also the case when impurities are present in the material.