Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Rotational spectrum - equidistance

  1. Dec 11, 2009 #1
    Difference between two rotational terms is given by


    If we put values of [tex]J[/tex] in this expression we get that otational spectrum is equidistant.

    [tex]T_r[/tex] - rotational term
    [tex]J[/tex] - rotational quantum number

    But from this picture spectrum isn't equidistant.


    Can you tell me where is the problem? Thanks!
    Last edited: Dec 12, 2009
  2. jcsd
  3. Dec 12, 2009 #2


    User Avatar
    Science Advisor

    I can't view the picture. But the equation you cite is valid for the rigid rotor approximation. Real systems tend not to act like that except for low rotation numbers.
  4. Dec 12, 2009 #3
    I put the other picture
    http://www.mwit.ac.th/~physicslab/hb...mol/rotlev.gif [Broken]

    Look at this picture. You have rotation and vibration levels. Yes I assume that two moleculs system - rigid rotor approximation.
    Last edited by a moderator: May 4, 2017
  5. Dec 13, 2009 #4


    User Avatar
    Science Advisor

    Right well what's the question? Why the approximation fails at higher rotational states?

    Simply "centrifugal force" - the interatomic distance increases at higher rotation speeds/states, you get a different B.
  6. Dec 13, 2009 #5
    The centrifugal force is not the case in this picture



    For [tex]J=0[/tex] [tex]T_r=0[/tex]

    For [tex]J=1[/tex] [tex]T_r=2B[/tex]

    For [tex]J=2[/tex] [tex]T_r=6B[/tex]

    For [tex]J=3[/tex] [tex]T_r=12B[/tex]


    It isn't equidistant if I calculate like this. And in picture which you see is this terms. It looks like contradiction if you look this post and my first post!
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Threads - Rotational spectrum equidistance Date
I Rotational Spectra Nov 17, 2017
B Light determination for emission spectrum Feb 26, 2017
A Tools to transform primitive cell orientation Nov 13, 2016
I Monoclinic cell axes rotation Aug 17, 2016
A Octahedral rotation angle in Perovskite Jun 27, 2016