Desorption of cluster ions from frozen gases following high-density electronic excitation

Desorption of cluster ions from frozen nitrogen as well as argon and methane following keV He+ ion bombardments has been investigated. For frozen nitrogen, a large number of cluster ions (Nn+) up to n = 30 were desorbed by keV He+ ion bombardments. The mass spectral pattern of the desorbed cluster ions exhibited that the Nn+ ions where n ≦ 4 are major species. For n ≧ 5 clusters, the even-numbered cluster ions were more intense than the adjacent odd-numbered cluster ions. The He+-energy dependencies of desorption yields revealed that the desorption of fragmented N+ ions is interpreted by the sputtering through classical nuclear collision. On the other hand, the desorption of N2+ and cluster ions does not follow the theoretical curve based on the collision cascade model. On the basis of these results and simple calculations for the time-dependent local temperature in ion bombarded region, it is concluded that desorption of cluster ions is initiated by the rapid evaporation as a primary step due to the high-density excitation by He+ ion bombardments.