In microwave rotational spectroscopy, absorption spectra arise from molecular rotation and
correspond to transitions between the rotational energy levels associated with a given vibrational state of a
particular electronic state. The rotational transitions, which fall in the microwave region, are induced
through the interaction of the molecular electric dipole with the electric vector of the radiation. The
microwave region extends roughly from102 𝜇𝑚 to 104 𝜇𝑚. The article explains the pure rotational
absorption spectra of diatomic molecules such as HCl, HBr, HI, CO, … etc. In order to explain the absorption
spectra, diatomic molecules are treated as rigid rotor and non-rigid rotor.