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A basic level of a non-Boltzmann population distribution is
available through setting the vibrational temperature, Tvib,
which affects the relative populations of different vibronic
states, or the spin temperature Tspin. For more
complicated situations, such as simulating double resonance
spectra or nascent population distributions following reactions
there are two more general methods of specifying the population
of levels. It is also possible to fit these populations to
measured intensities or band contours, as the populations are
simply parameters that can be floated as any other; see the
sections on Line Intensity
Fitting and Contour Fitting.
It is also possible to simulate and fit time of flight or
kinetic energy release spectra; use IntensityUnits = PopDist to ignore
transition moments and just plot energy levels with populations
derived as described in this section. Plots against velocity (in
units of cm^{-1/2}), rather than energy can be obtained
by Setting PlotUnits to
sqrtcm1.

The populations will show in the parameter area of a manifold; by default no populations will be set so this area will be empty. To set up population distributions, either:

- In the constants window, right click on the relevant
manifold or state in the treeview.

- Select "
`Add All Levels`"; this will add all the levels of that manifold or state (within the J range set up) and initialize them using the current Temperature and Tvib to calculate the population. (This can easily give an unmanageably large number of levels, so set`Jmax`or use the method below.)

- You can get a matching table of energy levels and quantum numbers using the "List" button in the "Levels" window, providing the same manifold or state is selected.

- Use "View","States" to bring up the State Details Window.
- Select J, symmetry
and Manifold such that one of the states of interest is
displayed.

- In the "Eigenvalues" pane (the lower matrix) select highlight
one or more levels with the mouse and then right click on the
selected levels and select "Add to population selection".

- Repeat the above steps for all the required levels.

"View,Constants"
will now show a parameter for each state at the manifold level.
The "Value" column is the relative population, excluding the
degeneracy. The parameter name is the string "*J*
<symmetry number> <eigenvalue number>". If external
fields are present, the parameter name will be "*M*
-1 <eigenvalue number>". (Both types of entries can be present,
but only one set will be active for any given simulation.)

To activate these parameters either:

- Set UsePopParams for the required manifolds
- Set the overall Temperature < 0 to force the use of the numerical populations for all manifolds

To see details of the calculation, right click on a manifold or state as required, and select "

To remove the custom population entries, go to "View,Constants" and right click on the entries in the parameter pane for the manifold. There are items for removing selected or all items.