Absorption and Fluorescence of Molecular Iodine

The UV-Vis absorption spectrum and LIF emission spectrum of I2 were found for the purpose of gaining information about the energy of the ground X( 1 Σg + ) and excited B( 3 Π0u + ) states. This information was compared to information obtained from the Gaussian program which uses quantum mechanical calculations and numerical approximations to predict electronic energy levels. The iodine molecule was found to be approaching its convergence limit near 500nm. The Gaussian program predicted this to occur at a much higher energy. Gaussian also predicted the term value and the energy gap ΔE(I*) from X( 1 Σg + ) to B( 3 Π0u + ) to be much greater than accepted literature values. This indicates that Gaussian is insufficient at predicting relative energy levels. Gaussian was however able to predict reasonable values for the equilibrium bond lengths indicating that it may be suitable for determining the shape of potential energy curve. The Morse potential energy curves provided a more accurate representation of the iodine molecule than the harmonic oscillator model. The harmonic oscillator model is only valid very near the equilibrium bond length. The most intense transitions occurred from the v”=0 level to the v’=34-36 range as predicted by the Franck-Condon principle. The comparative values of the force constant, equilibrium bond length, and potential energy curve well depth indicate that the bond strength is greater in the ground state than in the excited state. The experimentally determined values of equilibrium vibrational frequency were slightly higher than the literature values.