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Rotational Energy levels

  1. Jan 10, 2013 #1
    1. The problem statement, all variables and given/known data

    So I'm stuck on part (ii) where we have to solve the rotational energy levels.

    Problem: http://screencast.com/t/9rZStJdG3wJ [Broken]

    The answer is ii) 0, 3.84 cm-1, 11.52 cm-1

    2. Relevant equations

    Equations: http://screencast.com/t/ubFEheGBz [Broken]

    3. The attempt at a solution

    Calculate the reduced mass then divide by avagardos number and x1000 to get it into kg per molecule - 1.139*10^-26

    Then I use this value to work out I and get 1.457*10^-46

    Then, B can be calculated I got 603.52 for this

    Next, D = 187.7

    Put all this into the equation for E and I end up with 460.24 but the actual answer for this case (j=1) was 3.84cm^-1.

    Please let me know if theres anything I can clear up.
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Jan 10, 2013 #2


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    Hello, dscot.

    I agree with your value for I. But, I'm getting much smaller values for B and D. Can you show explicitly your calculations?
  4. Jan 10, 2013 #3
    Hi TSny,

    Sure no problem :)

    B = (6.63*10-34 / ( 8 * ∏ * 1.457*10-46 * 3*108
    = 603.52

    D = 4*(603.52)3 / 2169.72 = 460.24

  5. Jan 10, 2013 #4


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    Did you leave out one of the ##\pi##'s in the denominator of B?

    Think about the units for your answer for B. In the SI system, the unit of length is the meter.
  6. Jan 10, 2013 #5

    Sorry I did miss it out when copying it from my notes.

    As far as I can tell everything is in meters which I'm now guessing is wrong :)

    h, I and C are in meters I think I must be missing something obvious?

  7. Jan 10, 2013 #6


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    You worked in the SI system, so you're getting B in inverse meters rather than inverse centimeters. That's ok, since you can easily convert from one to the other.
  8. Jan 10, 2013 #7
    Ah, I see so just divide by 100?

    When working out D do I just leave We in cm-1 in that case?

  9. Jan 10, 2013 #8


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    Yes and Yes :smile:
  10. Jan 11, 2013 #9
    Great! Thanks so much TSny :)

    I have a similar problem regarding rotational energy levels that seems to be confusing me: Can somebody please tell me whats meant here by calculating the spacing?- a(i) - http://screencast.com/t/BK8BsW7r [Broken]

    We have these formulas available: http://screencast.com/t/ha9UvNfXHG2 [Broken]

    I think we may have to use that population given in the question

    Nj/Nd = gj exp( -ej/KT ) ?
    Last edited by a moderator: May 6, 2017
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