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 (so g/u symmetry must be specified), 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)
    (You will also find the ratio given as I/(I+1) with the (2I+1) common factor cancelled out; any scaling factor common to both weights does not change the spectrum, though the details of the partition function calculation changes.) 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.
Note that to obtain assignments for the N and F1/F2... labels consistent with the literature, you may need to set LimitSearch to true - see under Ground and Excited for this example. This is now the recommended setting, and versions after 7.1.408 will use this setting for the manifolds set up with File, New, Linear Molecule, but may need setting for older files or manifolds created by other methods. Note this setting does not affect the calculated positions or intensities, but adjusts how PGOPHER assigns quantum numbers not otherwise used in the calculation.

For details of the Hamiltonian used, see the section on the Linear Molecule Hamiltonian.

Pure Rotational (Microwave) Spectra

For this a single state is required, which can be generated with File, New, Microwave, Linear, and then following steps above, omitting step 3. Alternatively you can convert a two state model to a single state by:
  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.