The Far Ultraviolet Absorption Spectra and Ionization Potentials of the Alkyl Halides. Part I

The absorption spectra of methyl iodide, bromide and chloride have been examined in the region 2000–1000A. The analysis shows that the bands are due to the excitation of non‐bonding electrons localized on the halogen atoms. They can be fitted into Rydberg series which go to the two components of the doublet state of the molecular ion as limits. The values of the ionization potentials so determined are 9.49 and 10.11 volts for methyl iodide, 10.49 and 10.80 volts for methyl bromide, and 11.17 and 11.25 volts for methyl chloride. The separation of the two ionization potentials is in excellent agreement with those predicted by R. S. Mulliken, though their actual values are somewhat lower. The electronic and vibrational nature of the bands is discussed and some correlation of the spectra of the three molecules is made.