Difference Between Spectrum Of A Rigid And A Non-Rigid Rotator

 Difference Between Spectrum Of A Rigid And A Non-Rigid Rotator

A rigid rotator is an idealised rotating object with a set shape and size in physics. A non-rigid rotator, on the other hand, is a rotating object that is susceptible to deformation. These two types of rotators’ spectra differ in several significant ways.

Energy Levels:

A rigid rotator has energy levels that are evenly spaced, but a non-rigid rotator does not. Due to the fact that a rigid rotator’s energy solely depends on its moment of inertia and rotational state’s quantum number. In contrast, the energy of a non-rigid rotator is influenced by both the distribution of mass inside the object and its moment of inertia.

Line Spacing:

Each line in the rotational spectra of a rigid rotator corresponds to a transition between two neighbouring energy levels, and the line spacing is uniform. The spacing between successive lines in the spectrum of a non-rigid rotator, in contrast, is not uniform and is determined by the moment of inertia of the molecule at that particular energy level.

Band width:

Broader than a rigid rotator’s, a non-rigid rotator’s rotational spectra have a wider bandwidth. This is due to the overlapping spectral lines caused by the non-uniform line spacing, which broadens the spectra.


In general, due to the additional degrees of freedom brought about by molecular vibrations and deformations

  • Rigid rotators’ spectra are simpler and more regular
  • While non-rigid rotators spectra are more complex and irregular.