Abstract :

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 1μm to 〖10〗^2 μm. The article explains the pure vibrational absorption spectra of diatomic molecules such as HCl, HBr, HI, CO, … 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 valueω ̅_osc corresponding to frequency of oscillationν_osc while in the actual experimental data, there are many absorption bands corresponding to wave numbers slightly lesser than ω ̅_osc, 2ω ̅_osc, 3ω ̅_osc, ……..The occurrence of these additional bands is attributed to the selection rule ∆v=±2, ±3, ±4, ……The additional bands are having lesser intensity and are called overtone bands.