Molecule Types Symmetric Tops <Prev Next>

Making a Data File for a Symmetric Top Molecule

A simple symmetric top molecule file can be generated from File, New, Symmetric Top This initializes the program for a parallel band of a molecule of C3v symmetry with a band origin of 1500 cm-1and with rotational constants in both states of A = 5 cm-1, B = 0.5 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. At the Molecule level (Click on "SymmetricTop") the PointGroup and the statistical weights are the most important. A symmetric top must have equivalent nuclei and the defaults of 1 are only appropriate for 3 equivalent spin ½ nuclei in C3v. 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. Adjust the upper state by clicking on the "v=1" entry under "Excited". The key things to set here are:
  4. The transition moment "<v=1|T(1)|v=0>" may also need adjusting, as the default is along the a axis. The axis is likely to change if you change the symmetry and for low symmetry asymmetric tops more than one transition moment may be needed. (Right click on the transition moment in the tree view to show the copy and paste menu items that can be used for duplicating objects). See here for Raman and multiphoton transitions.
For details of the Hamiltonian used, see the symmetric top state section.

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.