Molecule Types Linear Molecules <Prev Next>

Making a Linear Molecule Data File

A worked example of the process is available for the The Schumann-Runge Bands of O2.
A simple linear molecule file can be generated from File, New, Linear Molecule. This initializes the program for a transition in a molecule between two vibronic states with a band origin of 1000 cm-1and B' = B'' = 1 cm-1. To adjust the constants (and set the type of vibronic transition) use select View, Constants. Most constants and settings can be left unchanged at their default values (and indeed should be unless you need a specific effect) but the ones that must normally be changed are adjusted as follows:
  1. If the molecule has a centre of symmetry, click on "Linear Molecule" and set Symmetric = "True". You will also need to set the statistical weights in the same window. For diatomic molecules with nuclear spin I the values should be:

    		I integral
    		 I half integral
    SymWt (I+1)(2I+1) I(2I+1)
    AsymWt I(2I+1) (I+1)(2I+1)
    Note that you should leave nNuclei at 0 unless you specifically want to calculate hyperfine structure.
  2. Adjust the ground state by clicking on the "v=0" entry under "Ground". The key things to set are:
  3. Now switch to the upper state (click on the "v=1" entry under "Excited") . The key things to set here are:
  4. The (spherical) transition moment may also need adjusting; the default is for a standard one photon electric dipole transition. See here for Raman or multiphoton transitions.
For details of the Hamiltonian used, see the section on the Linear Molecule Hamiltonian.

Pure Rotational (Microwave) Spectra

For a pure rotational transition only do steps 1 and 2 as above and then:
  1. Right click on <Excited|mu|Ground> (the transition moments object) and select rename. Change the "Bra" entry in the rename dialog so that both entries read "Ground". (This converts the transition moment to act within the ground state, rather than connecting two different states.)
  2. The upper state can be deleted - right click on "Excited" and select delete.
  3. The default frequency range for the plot will still be in the infra-red; press the simulate button and then the All button to see a spectrum.