MCsn+ cluster formation on organic surfaces: A novel way to depth-profile organics?

In this work, we investigated the bombardment of polymer surfaces (PC, PMMA) with very low energy (250 eV) Cs+ ions. In the positive mode, we observed many cations combining a molecular fragment of the polymer (M) with two Cs atoms, similar to the well-known MCs2+ clusters commonly observed in inorganic depth profiling with Cs. For example, Cs2COOH+ or Cs2C6H5O+ were measured on PC and Cs2CH3O+ or Cs2C4H5O2+ were measured on PMMA. In fact, most of the polymer characteristic fragments measured in negative spectra also appear in the positive spectra, combined with two Cs atoms. It is remarkable that stable negative ions tend to combine not with one Cs, but with two Cs atoms to create the MCs2+ cluster. This effect is, to some extent, similar to the well-known cationisation of polymers (with Ag or Au) although here few clusters with only one Cs atom are detected (MCs+ clusters), like in classical cationisation. The most abundant clusters are the MCs2+ clusters, but heavier clusters (MCs3+, MCs4+ and above) are also measured with high yields in the spectrum. Surprisingly, some of those MCs2+ clusters were still detected even after a very long sputtering fluence (above 1017 ions/cm2), proving that some molecular depth profiling is also possible in this “Cs2-cationisation” mode. In other words, this work could open the way to an extension of the MCsn+ cluster analysis, commonly used in inorganic depth profiling, to the in-depth molecular analysis of organic layers.