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.
- Overlay the experimental spectrum over
the PGOPHER simulation,
and drag
the simulation so that there is roughly no offset between the two
spectra.
- Right click on a peak that can be assigned
to a peak in the experimental spectrum. A Line
List Window will appear:
- 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
.)
- 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.)
- Repeat steps 2 to 4 to assign more peaks.
- 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”.
- 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.
- If the results look reasonable, then press "Fit" repeatedly
until the error converges.
- 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.
- 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.
- Press the Edit Button
. There will be a prompt saying
that the file does not exist. Say “yes” to create it.
- Copy (use the copy button
in the line list window) and paste
the assignments into your observations (.lin) file, and save it.
- 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.
- The final stage is to add more constants and lines into the fit
as required.
Notes:
- It is easy to misassign peaks, and wrongly assigned peaks can
distort the fit quite badly. Look at the obs-calc values in the log
window for problem peaks.
- If you want to only use part of a .lin file, a "-1" or "stop" on
a line on
its own will force the program to ignore the rest of the file.
- If there is not enough information to determine a constant then
the
constants and their errors will become very large or you may get a
"Singular Matrix" error message. Use the Undo Fit
button to go back and fix one or
more of the constants affected in this way.