Vibrational Rotational Spectra : Molecular Physics

Some of the salient features of vibrational-rotational spectra are as follows :
  • Vibrational-rotational spectra arise from the transition between vibrational energy states associated with the same electronic state of the molecules.
  • These spectra are observed in the near-infrared region.
  • These spectra are observed only for the molecule which has the permanent dipole moment, i.e. heteronuclear diatomic molecules. e.g. HCl, HBr, etc.
  • These spectra are observed in absorption.
  • The internuclear distance changes when the nuclei of these molecules vibrate related to each other. Thus these molecules possess an oscillating dipole moment.

Vibrational-Rotational Spectra for HCl Molecule :

In practice, vibrational-rotational spectra are observed in absorption. The vibrational-rotational spectra of HCl molecule can be explained in the following points clearly :
  • Spectra of a molecule like HCl has an intense band called the fundamental band.
  • The spectrum consists weak bands called overtones that appear at wave number approximately double, triple, quadruple,….  of fundamental bands.
Vibrational Rotational Spectra
Fig. Fundamental bands in HCl
 
  • On close examination, the fundamental band is consist of two closed maxima which is an isotope effect.
Vibrational Rotational Spectra
Fig. Isotope effect
  • Under the high resolution, it is found that each band consists of a series of lines that are not equidistant.
  • A line is found missing in center of the band and is called null line or zero line.
  • The line seems to converse towards higher wave number side and band is said to be degraded, toward the low wave number side. i.e. towards red.

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