Surface structure of [XMIm]Tf2N ultrathin ionic liquid films probed by metastable He atoms and photoelectron spectroscopies (UPS and XPS)

The metastable induced electron spectroscopy (MIES), in combination with photoelectron spectroscopies, is a powerful tool to analyze the molecular arrangement at surfaces due to its high surface sensitivity. In this contribution MIES is applied to analyze structural changes at the surface of [EMIm]Tf2N and [OMIm]Tf2N films on two different substrates, Highly Ordered Pyrolytic Graphite (HOPG) and Au(111), as a function of the film thickness, starting from submonolayer coverage up to multilayers. The discussion is based on previous MIES studies of the vacuum–liquid interface of ionic liquids with imidazolium cations and on available DFT calculations. The presented data indicate that – in contrast to multilayers where the surface is dominated by the alkyl chains of the imidazolium cations – the structure of the first adlayer of [XMIm]Tf2N molecules on solid surfaces depends on the strength of their interaction with the surface resulting in alkyl chains aligned parallel or oblique to the substrate surface.