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-1
' = B
= 1 cm-1
. To adjust the constants (and set the type of
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
- 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:
(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.
- 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, specify g or u
symmetry for the state with 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 (spherical) transition
moment may also need adjusting; the default is for a
standard one photon electric dipole transition. See here for Raman or multiphoton
- The Origin
- All the settings and values listed above under the ground
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
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
, 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:
- 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.)
- The upper state can be deleted - right click on "Excited" and select
- The default frequency range for the plot will still be in the
infra-red; press the button and
then the button to see a spectrum.