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

Homework Help: Rotational energies

  1. May 11, 2005 #1

    Pengwuino

    User Avatar
    Gold Member

    Ok so i have 5 wavelengths of the rotational spectrum of a certain molecule. I need to find the moment of inertia.

    I have the equation down to I=(Hbar * wavelength)/(hc)

    Do i just use the shortest wavelength to figure out the moment of intertia? No radius was given.
     
  2. jcsd
  3. May 13, 2005 #2

    ehild

    User Avatar
    Homework Helper

    How did you get this formula? Check it. The unit of I should be mass times length squared and you have "second".

    ehild
     
  4. May 13, 2005 #3

    Pengwuino

    User Avatar
    Gold Member

    The book gave me the equation and i figured what 'I' would be. Its an HCl module molecule.
     
  5. May 13, 2005 #4

    Doc Al

    User Avatar

    Staff: Mentor

    I don't know where that equation comes from--the units don't make sense.

    Treating the HCL molecule as a rigid rotor, the allowable rotational energy levels are:
    [tex]E_J = J (J +1) \frac {\hbar^2}{2I}[/tex]

    You should be able to relate the wavelengths to transistions between levels.
     
  6. May 13, 2005 #5

    ehild

    User Avatar
    Homework Helper

    To Pendwuino:


    It has to be known that only such transitions are allowed where J changes by +1 or -1. If you have absorption spectrum the spectrum lines correspond to the transitions from J to J+1. In an emission spectrum, it is the opposite, the molecule emits a photon while it gets back from the J+1-th rotational level to the J-th one.

    The energy of a the photon emitted is

    [tex]hf=((J+2)(J+1)-J(J+1))\frac{\hbar^2}{2I}=(J+1)\frac{\hbar^2}{I}[/tex]
    The emission spectrum of a two-atomic molecule consists of equidistant spectral lines, which correspond to transitions on to the levels J=0, J=1...and so on. The difference between the frequencies of two closest lines is
    [tex]\Delta f = \frac{h}{4\pi^2I}[/tex]
    You know the wavelength of the spectral lines. Calculate the frequencies from the wavelengths
    [tex]f=c/\lambda[/tex]. Sort the frequencies and calculate the difference between the subsequent ones. Take the average: and calculate I from it.

    ehild
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook