Knowledge Bank

The $B-X$ transition in I$2$ stands as one of the most precisely characterized diatomic electronic systems, with thousands of lines having been measured with precision sufficient to serve as frequency standards across the 500-700-nm spectral region. Accordingly the molecular constants of the $B-X$ system permit recalculation of those line positions with similar precision (~0.001 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$). Yet the $B-X$ electronic transition strength remains a ~10-percent quantity, in part from the difficulty of dealing with two overlapping continuous 1u-$X$ transitions, $A-X$ and $C-X$. In this work, oscillator strengths are estimated from a long-known but little used method -- integration of absorption cross sections over single rotational lines -- using measurements obtained for lines near 650 nm with a diode laser. The results are combined with new, precise low-resolution (1 nm) absorption data to obtain a refined assessment of electronic transition strengths in the I$2$ visible absorption spectrum.