Universal Equation for Argon Cluster Size-Dependence of Secondary Ion Spectra in SIMS of Organic Materials

A study has been made of the fragmentation of three organic molecules, fluorenylmethyloxycarbonyl-L-pentafluorophenylalanine, tris(8-hydroxyquinolinato)aluminum, and Irganox 1010 under bombardment by argon cluster ions with sizes, n, between 500 and 10000 and for 10 and 20 keV energies, E. It is shown that the intensities of the fragments are governed by the same relation as that for the total volume sputtering yield but that, importantly, the parameter A(M) that governs the transition in behavior between low and high values of E/n is thought to be related to the energy required to remove that particular fragment from the ensemble of molecules. This causes a great reduction in intensity for the more strongly bound fragments as n is increased, such that, at n = 10000, the intensities of the molecular group peaks, which are weakly bound to the substrate, then dominate the spectrum. For studies involving weakly bound molecules, the low E/n values give the most intense molecular group peaks, but high E/n values give a greater number of small, characteristic fragments. For molecules more strongly bound to the bulk or substrate, it is not as likely that the low E/n data will be so helpful.