Knowledge Bank

Some years ago we reported an analysis of a high resolution absorption spectrum for the $A3\Pi 1u-X1\Sigma g+$ system of $I2$ consisting of approximately 9700 lines from 79 bands spanning the vibrational range $\nu \prime =0-35$ and $\nu \prime \prime =3-17.a$ Results were presented in the forms both of empirical Dunham-type expansions and of near-dissociation expansion (NDE) expressions for the band constants of the A-state. Because of the use of NDE's, the resulting expressions were expected to be fairly reliable for extrapolation to dissociation $(vD=55.57(\pm 0.01))$. Shortly thereafter, $Yukiyaetal.b$ reported additional high resolution absorption data spanning the range $\nu \prime (A)=16-45$, and showed that the Appadoo et al. NDE expression for $B\nu$ did not, in fact, extrapolate reliably. However, they analyzed their results only in terms of a set of band constants for levels $\nu \prime (A)=16-45$. The present paper describes a comprehensive analysis of both data sets (which together span 99.2% of the potential well) which critically compares the utility of Dunham expansions vs. NDE expressions vs. direct-potential-fit techniques, and yields an optimum description of this system.