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 (N-n(+)) up to n = 30 were desorbed by keV He+ n ion bombardments. The mass spectral pattern of the desorbed cluster ions exhibited that the N-n(+) 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 N-2(+) 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. (c) 2005 Elsevier B.V. All rights reserved.