Since 1940 many spectroscopic techniques have been discovered and refined to further investigations into the electronic structure of atoms and molecules. Most of these, however, measure differences in absorption of electromagnetic energy of various wavelengths which are in turn related to atomic or molecular phenomena. In photoelectron spectroscopy and related techniques (Auger, Electron Impact, and Penning Ionization) a liberated ejected electron is examined. Therefore, the technique is sensitive to ionization processes and the parameters governing electron-molecule interaction. It thus provides a separate and distinctly unique approach to probing electronic structure.