Formation of molecular ions and neutrals with covalent bond from frozen methane under cryogenic condition by ion irradiation

Narita, Ayumi; Baba, Yuji  ; Sekiguchi, Tetsuhiro  ; Shimoyama, Iwao   ; Honda, Mitsunori   ; Hirao, Norie; Yaita, Tsuyoshi; Hirayama, Takato*; Tachibana, Takayuki*

Chemical reactions induced by ion irradiation in adsorbed molecules is of great importance because these reactions ordinary occur on planetary icy surfaces. In this study, we have investigated the formation of positive ions and neutral molecules from adsorbed methane, deuterated methane, nitrogen and argon following keV He ion irradiation. In the case of mono-layered methane, only monomer ions (CHx) were desorbed by 1 keV He ion irradiation. On the other hand, a large number of acetylene ions(CH) and ethylene ions(CH) as well as cluster ions (CnHx) were desorbed from multi-layered film which indicates that chemical bonds are newly formed by ion irradiation. In order to clarify the mechanism for the formation of molecular ions, we calculated ranges and energy loss processes of He ions in solid methane with Monte-Carlo simulation. Based on the results for the thickness dependences of the mass spectral patterns, it was elucidated that the monomer ions are desorbed from the top surface layer through single electron excitation. While the molecular ions are formed mainly in the inside of the layers along the nuclear track due to the high-density electronic excitation, which is produced by nuclear collision between incident He ions and frozen molecules.