Knowledge Bank

A weak discharge of $I2$ in the presence of inert foreign gas produces a well-developed system of red-degraded vibrational bands with peak intensity near 3400 \AA. A new vibrational analysis is given for this system. The absolute numbering of the levels in both states is fixed by the observed isotope shifts, and the relative intensities are corroborated by Franck-Condon calculations on model potential curves. Band profile calculations reproduce many of the seemingly anomalous ``spikes” and violet-degraded features in the system. The nature and origin of this transition are discussed in the context of the pronounced pressure dependence of its occurrence.