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 molecule 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:

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.

Adjust the ground state by clicking on the "v=0" entry under "Ground". The key things to set are:

Lambda (to Sigma+, Sigma-, Pi, Delta, ...)
Overall electronic Spin, S.
If the molecule is symmetric, set gerade (True for g, False for u)
The rotational constant, B
For non Σ states, the spin orbit coupling constant, A.
For S ≥ 1 LambdaSS (the spin-spin coupling constant) may also be important.

Now switch to the upper state (click on the "v=1" entry under "Excited") . The key things to set here are:

The Origin
All the settings and values listed above under the ground state.

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.