Obtain the wavelengths of all the observed fluorescence lines from your calibrated spectrum. Convert these line positions to vacuum wavenumbers (frequency in units of wavenumbers [cm-1]). Make a table of the vibrational first differences defined as:
Plot 
G" versus (v"+ 1/2) (this is called a Birge Sponer plot).
A Birge-Sponer plot for H2+ is shown above.
Your plot will contain more points on a smaller
vertical axis.
From your data, obtain values for the ground state vibrational frequency,
e,
and the anharmonicity, exe, in
wavenumbers, as defined for a Morse potential. Extrapolate your Birge Sponer plot to
G"=0 to
obtain the number of bound vibrational levels in the ground state potential
(for H2+ this is v=16,
your value for I2 will be much larger). The dissociation energy, D0,
may be obtained from the
area under the line in the plot of G" versus v"+1/2.
The dissociation energy, De, may be obtained
from the relation (exact for a Morse potential)
