Absorption, Ionization, and Ion-Fragmentation Cross Sections of Hydrocarbon Vapors under Vacuum-Ultraviolet Radiation

Abstract

Absorption and ionization cross sections of the hydrocarbon vapors ethane, propane, n-butane, n-pentane, cyclopropane, ethylene, propylene, cis-butene-2, and 1,3-butadiene were measured in the spectral range 9 to 25 eV. With the exceptions of ethylene and cyclopropane, the absorption cross sections rose smoothly to peaks near 16 eV and decreased smoothly toward higher energy. The maximum absorption cross section of an alkane could be estimated by multiplying the number of bonding electrons by 6 Mb. Similarly, a smooth rise and fall of the ionization cross sections was observed with the exceptions noted. The maximum ionization cross section could be estimated by multiplying the number of bonding electrons (π electrons excluded) by 5 Mb. The peaks of the ionization cross-section curves of the alkanes were near 16 eV and those of the olefins near 17.5 eV. Beyond the peaks the ionization efficiency was almost 100%. By combining the Seya-type monochromator with a mass spectrometer, cross sections for the production of various ion fragments of ethane, propane, cyclopropane, ethylene, and acetylene were obtained. Almost all ion-fragment cross-section curves rose for more than 2 eV above the threshold. No evidence was found for a sharp cutoff of the parent mass caused by fragmentation.