Communication: collective dynamics of room-temperature ionic liquids and their Li ion solutions studied by high-resolution inelastic X-ray scattering.

High-resolution inelastic X-ray scattering (IXS) measurements were performed for room-temperature ionic liquids (ILs) of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide and bis(fluorosulfonyl)amide, [C2mIm(+)][TFSA(-)] and [C2mIm(+)][FSA(-)], respectively, at ambient temperature. The observed spectra as a function of Q of 1.4-6 nm(-1) can be ascribed to quasi-elastic and inelastic scatterings, so that they are well represented with the fitting by using the Lorentz and the damped harmonic oscillator model functions to yield the dynamic structure factors. It was found in the intermediate scattering function, F(Q, t) that both ILs show the relaxation at t < 10 ps. The IXS measurements were also made on [C2mIm(+)][TFSA(-)] and [C2mIm(+)][FSA(-)] solutions dissolving Li salt. It is suggested that the adding of Li salt to IL significantly prolongs the relaxation time.