I have measured some fluorescence emission from excited iodine and am now trying to do a Birge-Sponer extrapolation. I'm just wondering if the vibrational quantum number increases or decreases with increasing wavelength? In my instructions there's a plot showing how it might look, the linear curve has a negative slope. If I use increasing quantum numbers with decreasing wavelength then I get a positive slope. The way I understand it is that as the iodine drops from the excited state to the ground state it emitts radiation, the photon energy is inversely proportional to the wavelength and quantum number increases with energy and therefor quantum number should decrease with increasing wavelength. Any ideas?