The Effect of Fluorescence and Dissociation on the Ionizing Potential of Iodine Vapor

Fluorescence and ionization.---When fluorescing under the influence of the mercury green line, iodine vapor shows weak ionization at a potential about 2.6 volts less than the normal ionizing potential. This is attributed to ionization by impact against molecules whose electrons have been displaced by absorption of the exciting radiant energy.Dissociation and ionization.---Another weak ionization always observed at about 1.5 volts less than the normal ionizing potential was attributed to ionization of atomic iodine, present as result of the dissociating action of the hot filament. This was verified by experiments in a pyrex glass apparatus at such high temperatures as to cause a high percentage of dissociation, when it was found that this new type of ionization was very intense.The observed ionizing potentials were: 6.8 volts, due to fluorescing molecules; 8 volts, due to iodine atoms; 9.4 volts due to iodine molecules. The difference between the first and last values corresponds roughly to the quantum of absorbed radiant energy, while the difference between the two latter values agrees accurately with the heat of dissociation of iodine molecules.