Changes of the near-surface chemical composition of the 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide room temperature ionic liquid under the influence of irradiation

Abstract

Radiation induced degradation effects are studied for a model ionic liquid (IL) – [EMIm]Tf₂N – in order to distinguish in which way the results of X-ray based material analysis methods can be falsified by the radiation supplied by typical X-ray sources itself. Photoelectron spectroscopy is commonly used for determining the electronic structure of ionic liquids. Degradation effects, which often occur e.g. in organic materials during X-ray or electron irradiation, are potentially critical for the interpretation of data obtained from ionic liquids. The changes of the chemical composition as well as the radiation-induced desorption of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm]Tf₂N) fragments are analysed by X-ray photoelectron spectroscopy (XPS) as well as quadrupole mass spectroscopy (QMS) upon exposure to monochromated or non-monochromated AlKα X-rays from typical laboratory sources. During the irradiation of [EMIm]Tf₂N, an increasing carbon concentration is observed in both cases and especially the [Tf₂N]⁻ ion is strongly altered. This observation is supported by the results from the QMS analysis which revealed a variety of different IL fragments that are desorbed during X-ray irradiation. It is shown that the decomposition rate is directly linked to the photon flux on the sample and hence has to be considered when planning an XPS experiment. However, for typical experiments on this particular IL the measurements suggest that the changes are on a larger time scale as typically required for spectra acquisition, in particular if monochromated X-ray sources are used.