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Room temperature ionic liquids (RTIL) are composed of ions only and are defined as molten salts with a melting point below 100°C1. They have numerous interesting physical properties such as high thermal stability, a wide liquid range and a tuneable viscosity. Besides their low melting points they have also a very low vapour pressure, which enables their analysis in ultra high vacuum devices such as XPS2, 3 and ToF-SIMS2, 4. A systematic study with different ionic liquids was carried out by means of time-of-fight secondary-ion-mass-spectrometry in positive and negative ion mode. Thereby a variation of the anion in 1-butyl-3-methylimidazolium based ionic liquids, a variation of the cation including a variation of the side chain length, in bis(triuoromethylsulfonyl)imide based ionic liquids was taken into account. The compounds were measured under bismuth cluster ion (Bi7+) bombardment, and spectral information and general rules for the fragmentation pattern are presented. Evidence for cation-anion interactions (e.g. hydrogen bonding), due to high molecular secondary cluster ions, could be found. The interaction strength could be estimated by ToF-SIMS via correlation of the secondary anion intensity to secondary cluster ion intensity ratio with donor and acceptor numbers.
Static secondary ion mass spectrometry (SIMS) has become a major analytical technique for the analysis of solid surfaces. In the last years it has also been used for the analysis of liquid/frozen surfaces from ionic liquids1-5. However, the mechanism for the emission of secondary ions has not been investigated so far, especially for the state of the art liquid metal ion guns (LMIG), such as the Bi or the BiMn LMIGs. These sources are not only able to provide monoatomic primary ions, but also singly and doubly charged polyatomic primary ion cluster. This new development has led to the investigation of the primary ion particle to specific parameters in order to determine the capabilities of such LMIGs on different target samples. To further the principle understanding of secondary ion emission, a systematic study using Bi1-7+ and Bi1,3,5,7++ primary ions on ionic liquids with considerably different strength of inter-ionic interactions was performed. As model samples the ionic liquids 1-butyl-3-methylimidazolium acetate and 1-butyl-3-methylimidazolium hexafluorophosphate ([C4C1im][Ac] and [C4C1im][PF6]) were used. To compare spectra or the performance of primary ion particles on substances or substance classes, the SIMS data were evaluated by calculating secondary ion yields, static SIMS limit, disappearing cross section, ion formation efficiency and fragmentation.