Line Position Fitting

Note: All quantum numbers (spin, J, K...) are input and displayed at double their actual values to allow for half integral values.

 This is the preferred method of determining constants from an experimental spectrum, and can be used when lines are reasonably well resolved. Where this is not the case it may be possible to do contour fitting, but note that this may not give good results.
  1. Overlay the experimental spectrum over the PGOPHER simulation, and drag the simulation so that there is roughly no offset between the two spectra.
  2. Right click on a peak that can be assigned to a peak in the experimental spectrum. A Line List Window will appear:

  3. The contributions of more than one rotational transition will probably be shown, as above. The transition required will presumably be the most intense one (look in the “Strength” column). Select the rows containing the transitions to be assigned with the mouse. (You can delete the unwanted transitions by selecting the required rows with the mouse and pressing the delete button .)
  4. Find the corresponding peak in the experimental spectrum. Right-click and drag over the peak. The peak position will be determined from the spectrum and a marker indicating the position will be displayed. Provided Auto is selected in the line list window, the "Frequency" column in the line window of the selected rows will change to that of the experimental peak and the “Weight” will be set to 1. (If Auto is off press the Assign button to do this manually.)
  5. Repeat steps 2 to 4 to assign more peaks.
  6. It is now possible to perform a fit. It is best not to float too many parameters at once, so go to the constants window (View, Constants) and select, for example, the excited state origin and the most significant constant, such as A or B for fitting, by doubling-clicking in the “Float” column so it reads “yes”.
  7. Then, click Fit in the line list window. The average error, new values for the constants and a correlation matrix will be displayed in the log window(View, Log). The main window will show an updated simulation (unless you have AutoReplot turned off, in which case you will need to press the "Simulate" button). Note that if you used an offset on the simulated spectrum you will probably need to reset the offset to zero after the first fit.
  8. If the results look reasonable, then press "Fit" repeatedly until the error converges.
  9. If the fit does not look good, the Undo Fit button in the log form will step constants back one fit cycle for each press. If you go too far, the Redo Fit button step the constants forward one fit cycle. You may want to delete peaks from the line window, or try temporarily excluding them by setting the "Weight" to 0.
  10. You will normally want to save the assignments, and it is normally more flexible to work from an independent file for larger fits. Create an empty file by going to the log window(View, Log). The button will allow you to browse to choose a file name - it is normal to give it a .lin or .obs extension. This will put the file name in the observations file name window.
  11. Press the Edit Button . There will be a prompt saying that the file does not exist. Say “yes” to create it.
  12. Copy (use the copy button in the line list window) and paste the assignments into your observations (.lin) file, and save it.
  13. To use this file, rather than lines in the line position window, use the Fit Button in the log window, rather than the line list window.
  14. The final stage is to add more constants and lines into the fit as required.

Notes: