Absorption spectrum of iodine vapor—An experiment

The absorption spectrum of iodine was photographed in the visible region using a 3.4-m Ebert plane-grating spectrograph. An evacuated glass envelope containing a small amount of iodine was used as the absorption cell while a tungsten filament lamp served as the continuous source. A number of band heads were identified and these measurements led to the determination of the dissociation limit, the dissociation energy, the force constant, and the fundamental vibrational frequency of the iodine molecule. Such an experiment will provide the physics major with an introduction to molecular spectra and, in light of the discovery of molecular lasers, it is recommended that such an experiment be included in the undergraduate physics curriculum.