Formation of CnHx(n≥2) molecules from adsorbed methane by low-energy ion irradiation

We have investigated the formation of molecular ions and neutral molecules from adsorbed CH4, CD4 and N2 following 1 keV He+ ion irradiation. The thickness of the adsorbed layer was precisely controlled. For mono-layered methane, only monomer ions (CHx+) were desorbed, while a large number of heavy ions (CnHx+) up to n=20 as well as heavy neutral molecules (CnHx) were desorbed from multi-layered film. Among the desorbed ions and neutral molecules, molecules with C-C covalent bonds such as acetylene and ethylene were found, which indicates that chemical bonds are newly formed by ion irradiation. Based on the results for thickness dependences of the mass spectra and calculation of He+ energy loss process from TRIM-Code, it was elucidated that the monomer ions are desorbed from the top surface layer through single electron excitation. On the other hand, the cluster ions are formed mainly in the inside of the layers along the nuclear track due to phonon excitation, which is produced by nuclear collision between incident He+ ions and adsorbed molecules. (author)