We have investigated ion desorption from adsorbed methane following keV He+ ion irradiation. The thickness of the adsorbed layer was precisely controlled. For mono-layered methane, only monomer ions (CHx+) were desorbed by 1 keV He+ ion irradiation. On the other hand, a large number of cluster ions (CnHx+) up to n = 20 were desorbed from multi-layered film. Among cluster ions, molecular ions with C-C bonds were found, which indicates that chemical bonds are newly formed by ion irradiation. Based on the results for 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 cluster 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. (c) 2008 Elsevier B.V. All rights reserved.