# Observing bands which I shouldn't be!

I performed the iodine spectrum experiment to observe vibronic transitions. I heated the iodine crystals to produce vapors (at a little above room temperature) and observed absorption spectrum with a diffraction grating and a spectrometer. But I am getting some quirky results as indicated following.

1. In Ref 1 and Ref 2, it is stated that near room temperatures, $$I_2$$ is in the ground electronic state and in the lowest vibrational state too ($$X, v''=0$$ state), so transitions are observed only from thsat state.
2. Ref 2, Ref 3, Ref 4 and Ref 5 state that from $$X, v''=0$$ state, Franck-Condon principle tells that transitions can be observed only to $$B, v'$$ states with $$v'$$ ranging around $$20-50$$, and they also cite the standard values of energies corresponding to particular $$v'$$ values so as to numerate the observed bands accordingly.

Now, at near room temperatures, I after numerating my bands, observed that I was seeing dark bands for $$v'=0$$ onwards! (I after assigning the standard band, numerated the lower energy bands backwards.) So, one of the above points have to be false!

Questions: Please tell what can possibly go wrong? If I'm getting some contributions from $$X, v''>0$$ states, how to identify them (since Ref 4 clearly states that the bands with transitions from $$x, v''>0$$ states "should not be included in this numeration" in for calculating the parameters from Birge-Sponer plot)?

• There are lots of things that can potentially go wrong in an experiment so it would help if you posted a spectrum. The ground state vibrational frequency is $\approx 214$ cm$^{-1}$ and $k_BT \approx 210$ cm$^{-1}$ at room temperature so you expect to see absorption from $v \gt 0$. It helps in the assignment if you look for the wavenumber where dissociation occurs. Feb 2, 2020 at 9:20
• @porphyrin Thanks! I hadn’t thought about that. But can you please help then how to assign $v’$s to my observed wave numbers? Or point me to a good resource?
– Atom
Feb 2, 2020 at 9:31
• It would be much preferable, if you would provide the references in human readable form and link through the DOI to them. Feb 5, 2020 at 15:49