Total neutral H₂ and H production from ethylene by electron-impact ionization

Molecular hydrogen, despite being the most abundant molecule in the universe, cannot be efficiently formed in the gas phase from two hydrogen atoms without the presence of a third body. In this work we show that the electron-impact production of vacancies in the various valence shells of ethylene results in an abundant ejection of neutral H-2 and H. Using the technique of delayed-extraction time-of-flight spectrometry, the cross section of H-2 and H production within the 20- to 800-eV impact energy range is obtained on an absolute scale. This is achieved by combining kinematic and dynamic measurements in conjunction with an association between the cross sections for production of fragment ions and the molecular orbitals where the primary vacancy is produced. A comparison with other common small molecules containing hydrogen shows that ethylene has a greater and outstanding efficiency in the production of H-2 in gaseous media subjected to ionizing radiation.