In molecular vibrational infrared spectroscopy, absorption spectra arise from molecular
vibration and correspond to transitions between the vibrational energy levels associated with a given
electronic state of the molecule. The vibrational transitions, which fall in the near infrared region, are
induced through the interaction of the molecular electric dipole with the electric vector of the
electromagnetic radiation. The near infrared region extends roughly from 1ππ to 102ππ. The article
explains the pure vibrational absorption spectra of diatomic molecules such as HCl, HBr, HI, CO, β¦ etc. In
order to explain the vibrational spectra, diatomic molecules are treated as harmonic oscillator and
anharmonic oscillator. In the harmonic oscillator model, we get only one absorption band at the
wavenumber valueπΜ ππ π corresponding to frequency of oscillationπππ π while in the actual experimental data,
there are many absorption bands corresponding to wave numbers slightly lesser than πΜ ππ π, 2πΜ ππ π, 3πΜ ππ π,
β¦β¦..The occurrence of these additional bandsis attributed to the selection rule βπ£ = Β±2, Β±3, Β±4, β¦β¦The
additional bands are having lesser intensity and are called overtone bands.
