The Dissociative Excitation and Dissociative Ionization of Simple Aliphatic Hydrocarbons by Controlled Electron Impact

Abstract Photoemissions from the following excited species were observed from simple aliphatic hydrocarbons (CH4, C2H2, C2H4, 1,3-C4H6) under controlled electron-impact excitation (0–400 eV) at wavelengths from 300 to 600 nm: H(Balmer), CH(A–X, B–X, C–X), CH+(A–X, B–A, b–a), C2(C–A, d–a; not from CH4), and C4H2+(A–X; not from CH4). All the excited species except for C4H2+ from C2H2 and C2H4 were confirmed to be primarily produced. The excitation function (0–400 eV) and the appearance potential of H, CH, and CH+ produced from CH4 and C2H2 were measured, and the corresponding processes of the fragmentation near the threshold were determined. The CH4+e→CH+(B)+H2(X)+H(n=1) and C2H2+e→CH+(B)+CH(X) processes were concluded to be responsible for the formation of CH+(B). The formation of ionic species, H+ and CH+, can be considered as the limiting case of the dissociative excitation for producing H and CH. A triplet excitation process may contribute to the formation of CH(A) from C2H2. The observed threshold energies indicated that superexcited states were involved in the formation of excited neutral fragment species.