The Absorption Spectra of Acetylene, Ethylene and Ethane in the Far Ultraviolet

Abstract

An absorption spectrum of acetylene in the region 1520A to 1050A has been found. It appears at very low pressures. The analysis was made by obtaining the spectrum of acetylene made from about 50 percent heavy water. The small isotopic shifts of many bands revealed them to be vibrationless electronic transitions. These are arranged into two Rydberg series going to the same limit which corresponds to an ionization potential of 11.35 volts. The upper levels of the two series are designated as $\mathrm{np}\Sigma$ and $\mathrm{np}\Pi$ levels. The excitation and ionization are from the CC bond and only vibrations affecting this bond appear. The character of the vibrations present is deduced from the magnitude of the isotopic shifts. Strong predissociation occurs around 1520A. This yields a value of about 187 Cal/mole for the strength of the triple CC bond. A similar set of bands appears in ethylene in the region 1750A to 1200A. The bands are more diffuse than those in acetylene, and only one strong Rydberg series was discovered. The limit of the series corresponds to an ionization potential of 10.41 volts. The vibrational analysis is similar to that in acetylene. The spectrum of ethane is extremely diffuse and no analysis of it could be made. It is concluded that, as in methane, all the upper states are unstable.