SECONDARY IONS PRODUCED BY LOW-ENERGY IMPACT OF HE-DER-WAALS THIN-FILMS( AND NE+ IONS ON VAN)

Secondary ions produced by low-energy He+ and Ne+ ion impact on van de r Waals solid thin films deposited on a silicon substrate were measure d as a function of film thickness using a reflectron-type time-of-flig ht mass spectrometer. The intensities of secondary ions N+ and N3+ pro duced by 400 eV He+ impact on an N-2 film were found to reach a platea u with deposition of a 5 monolayer (ML)-thick sample. In contrast, the Ar+ ion formed by 400 eV He+ impact on an Ar film showed a gradual in crease with increase of the film thickness from 1 to 20 ML. This obser ved marked difference is mainly due to the fact that a considerable am ount of energy of the incident He+ ions is dissipated by the bond diss ociation of N-2 in the solid N-2 target whereas it is mainly dissipate d via electronic excitation and momentum transfer in the impact on sol id Ar. For a sample consisting of an Ar film deposited on a 30 ML-thic k N-2 film, the intensity of the N-2(+) ion originating from the under lying N-2 film was found to be nearly independent of the Ar film thick ness between 1 ML and 20 ML. It seems likely that the hot spike produc ed by He+ impact in the Ar film vaporizes the N-2 molecules in the und erlying N-2 film and that vaporized N-2 molecules are ionized by charg e-transfer reaction with the pre-formed Ar+ in the spike. (C) 1997 by John Wiley & Sons, Ltd.