Matrix effects on secondary ion emission from a room-temperature ionicliquid, 1-ethyl-3-methylimidazolium bis[trifluoromethanesulfonyl]imide

摘要

The ionization mechanism of room-temperature ionic liquids has been investigated usingtime-of-flight secondary ion mass spectrometry in the temperature range of 15-300 K. Analyses of1-ethyl-3-methylimidazolium bis[trifluoromethanesulfonyl]imide ([emim][Tf_2N]) deposited on aNi(111) substrate revealed that the [emim]~+and [Tf_2N]~- yields increase together with the Ni~+ yieldat monolayer coverage; no such increase was observed for the films deposited on a D_2O spacerlayer. Results indicated that the [emim][Tf_2N] molecule is not perfectly ionized; the Ni(111) surfaceaccepts (for [emim]~+) or donates (for [Tf_2N]~-) an electron with higher efficiency than the counterionbecause of the metal band effect. This phenomenon might be induced by electrostatic interactionsbetween the separated cation and anion during sputtering. It is also suggested that the sputtered Niatom can be ionized nonadiabatically by the formation of a quasimolecule with adspecies. Themultilayer of [emim][Tf_2N] deposited at 15 K has a porous structure, resembling that of polarmolecules, because of nonionic intermolecular interactions. The phase transition is identifiable,together with the morphological change in the crystalline film, from temperature evolutions of thesecondary ion yields.