Examining molecular iodine through absorption and fluorescence spectroscopy has long been a typical procedure in the undergraduate physical chemistry lab.  It is a choice diatomic system for students to study as it is both experimentally straightforward while able to produce a rich amount of useful data with modest laboratory instrumentation.  Numerous versions of the experiment have been published through the years which predominately describe slight variations of recording and analyzing absorption and fluorescence spectral data.  Traditionally, Birge-Sponer extrapolation of the absorption spectral data is used to obtain the bond dissociation energy and vibrational constants associated with the excited state.  Unfortunately, this extrapolation falls short when the diatomic molecule is not well described by a Morse potential, which is common for numerous diatomic molecules near the dissociation limit of a bound state.  As a companion exercise, students are asked to determine the bond dissociation energies of diatomic molecules using both the traditional extrapolation and the method of LeRoy and Bernstein.  This method improves upon the Birge-Sponer extrapolation by allowing for curvature near the dissociation limit.  Through this exercise, students gain an improved understanding of the shape and nature of the potential energy curves of diatomic molecules.  A criticism of the original iodine procedure is that the extrapolation is viewed by students as a tedious analysis where they input values and are returned spectroscopic data.  By spending more time on the analysis, students gain a better appreciation of how the interactions of the elements in the diatomic molecule lead to unique potential energy curves, and how those curves are associated with the spectrum recorded.

Updating a classic:  Using absorption spectroscopy of diatomic molecules to highlight the similarities and differences of diatomic potential energy curves

255th National Meeting (2018)

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