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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)?

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    $\begingroup$ 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. $\endgroup$
    – porphyrin
    Feb 2, 2020 at 9:20
  • $\begingroup$ @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? $\endgroup$
    – Atom
    Feb 2, 2020 at 9:31
  • $\begingroup$ It would be much preferable, if you would provide the references in human readable form and link through the DOI to them. $\endgroup$ Feb 5, 2020 at 15:49

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