The electronic absorption spectrum of molecular iodine: A new fitting procedure for the physical chemistry laboratory

The electronic absorption spectrum of molecular iodine has become a standard experiment in the undergraduate physical chemistry laboratory course (1-6). Observation of the vibrationally resolved spectrum in the visible region at low resolution (Delta(lambda) - 0.5 nm) affords students a good demonstration of the quantization of molecular energy levels, while the treatment of the spectral data gives them valuable experience in the determination of molecular constants. Students typically observe transitions from the v" = 0, 1, and 2 levels in the ground electronic state to vibrational levels in the electronically excited B-state. Generally, they then analyze the transitions originating from the v" = 0 level using a BirgeSponer plot and thereby determine the various molecular constants and energies.

We use a different approach to data treatment that the students find more satisfying and that we believe has pedagogical advantages over the Birge-Sponer treatment. It involves simply fitting the transition frequencies to a second-order polynomial. This fit directly yields the important...