Surface sensitivity of secondary ion mass spectroscopy in the electronic sputtering regime

The secondary ions emitted from the surfaces of ionic liquids (ILs), 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([CnMIM][TFSI], n = 2, 6), were measured when 1 MeV H+ ions were incident on the surface at glancing angles of 2-6 mrad. Because the nuclear energy loss is negligible in the present case secondary ions cannot be emitted by simple elastic collisions. Instead, secondary ions were emitted after electronic excitation of the surface induced by the incident H+ ions. The intensities of the observed secondary ions originating from the [TFSI] anion for [C6MIM][TFSI] were much smaller in comparison with [C2MIM][TFSI]. Considering the known surface structures of these ILs, where [TFSI] anions in [C6MIM][TFSI] are located at slightly deeper positions than those in [C2MIM][TFSI], the present result indicates that SIMS can provide excellent surface sensitivity even in the electronic sputtering regime.