This problem illustrates both the structure of a diatomic molecule and the large disparity between changes in rotational, vibrational, and electronic energy levels. It also illustrates the formation of a band head, the abrupt end of a spectroscopic band. The following data for I2 are available from the NIST website:
(a) Construct an energy-level diagram for the ground electronic state. Consider only the first 10 rotational levels within the first two vibrational levels. Label the ordinate in cm−1 and indicate the total degeneracy of each individual rotational energy level on the diagram. 242
(b) Determine the energies (in cm−1 ) of the 10 purely rotational transitions for the v = 0 energy level within the ground electronic state and plot them on a second energy-level diagram. Draw a vertical line representing v = 0, J = 10 → 9 on the diagram from part (a).
The eigenstate-to-eigenstate transitions (e.g., 1→2) possible are numerous and have absorption lines at
and for simplification, we refer to constants associated with these states as | ′⟩|′⟩ and ||″⟩, respectively.
Also important to note that typically vibronic transitions are usually the result of the vibrational v
′=0 vibrational state. Within this assumption and excluding the rotational contributions (due to their low energies)