Introduction to High Resolution Spectroscopy 

Introduction Rotational Vibrational Miscellaneous

Microwave spectrum Of A Non-Rigid Rotor Diatomic Molecule

In microwave spectroscopy, rotational transitions are allowed between adjacent J levels - the selection rule is:

delta j = plus or minus 1

where an increase in J corresponds to absorption and a decrease in J to emission. The transition energy is the difference in the energy between the two levels. If one level has a rotational quantum number of J then absorption is to the level with quantum number J+1. For the non-rigid rotor, the energy of these levels is:


The energy difference is then:

Input a value into the calculator below for the rotational constant and centrifugal constant (both in cm-1) and press "calculate" to work out the transition energies (in cm-1). Typically, D is of the order of 0.001. Unlike the rigid rotor where the separation is constant, the separation of the lines decreases as J increases (and the centrifugal distortion increases). Using large values of J actually causes the separation to become negative.

Once you have covered all of the available resources for rotational spectroscopy, you should test your knowledge and understanding with the self test.

non-rigid rotor transition energy calculator

rotational constant B (cm-1) centrifugal constant D

    J                   energy                    transition and transition energy (cm-1)         separation

© A.J. Bridgeman, School of Chemistry 2014 | Last Modified 30/01/08
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