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In an experiment, I gently heated iodine crystals to create vapors and then observed it absorption spectrum with a spectrometer. The temperatures throughout were moderate (maximum $40^\circ\text{C}$). I observed a series of dark bands from red region to blue-green region with spacing between consecutive bands decreasing from red end towards to blue end.

I calculated the wavelengths corresponding to these bands beginning from the first resolvable band in the red region and ending at the $30^{th}$ reading (without missing a single dark band in between). The data is as follows:

$$\begin{array}{c|c} \text{Colour Region} & \lambda/\text{nm} \\ \hline \text{Red} & 624.884 \\ & 624.884 \\ & 621.831 \\ & 617.552 \\ \text{Orange} & 612.581 \\ & 609.518 \\ & 606.070 \\ & 601.798 \\ & 597.629 \\ & 594.171 \\ & 591.095 \\ \text{Yellow} & 586.862 \\ & 584.243 \\ & 581.853 \\ & 577.225 \\ \text{Yellow-Green} & 575.218 \\ & 571.819 \\ & 567.954 \\ & 565.246 \\ \text{Green} & 562.924 \\ & 560.291 \\ & 557.579 \\ & 554.399 \\ & 551.684 \\ & 550.053 \\ & 548.422 \\ & 547.490 \\ & 544.693 \\ & 541.271 \\ & 540.027 \\ & 538.548 \\ \text{Blue-Green} \end{array}$$

Now, in order to obtain information about the parameters like bond dissociation energy, convergence limit, etc., I need to know the quantum numbers which correspond to each observed transition wavelength which are the initial and final electronic states and the vibrational states.

Question: How to find these? I know that generally, sources characterize some standard states, like this says that $541.2\text{ nm}$ corresponds to $v''=0\rightarrow v'=27$. But what am I supposed to do with the larger wavelengths? Cuz the sources say that Franck-Condon prohibits $v''=0$ to low $v'$ transitions. But nevertheless, I do am observing larger wavelengths. Are these from $v''>0$?
How can I even be sure that these are $X\rightarrow B$ transitions? Cuz of operating temperatures?

Please provide some proper approach!


The difficulties that I am facing in this assignment of states are also elucidated in this post of mine.

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    $\begingroup$ If you transform the wavelength into energy, you will find that the peaks are equidistant. $\endgroup$
    – Maurice
    Commented Feb 2, 2020 at 14:25

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