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Non-Boltzmann Populations

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.

Where many levels are involved, a general function instead of the simple Boltzmann equation can be used, as described under Custom Populations. Alternatively, the populations of each level can be specified individually as described below, and is most straightforward where only a few levels are involved.

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:
  1. In the constants window, right click on the relevant manifold or state in the treeview.
  2. 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.)
  3. 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.
or, to add individual levels or sets of levels:
  1. Use "View","States" to bring up the State Details Window.
  2. Select J, symmetry and Manifold such that one of the states of interest is displayed.
  3. 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".
  4. 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. 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:
If neither of these conditions are met the individual populations will be ignored and the standard Boltzmann distribution will be used.

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 the items.