Sum frequency generation vibrational spectroscopy was used to determine the orientation of the cations in a room-temperature ionic liquid at the air-liquid interface. The ionic liquid that was studied was 1-butyl-3- methylimidazolium bis-trifluoromethylsulfonimide, [BMIM]+[imide]-. The orientation of the cation, [BMIM]+, was studied as a function of water pressure in the vapor over the range of 10-5 to 200 Torr. At water pressures below 10-4 Torr, the imidazolium ring was oriented parallel to the surface plane. On increasing the water pressure to greater than 10-4 Torr, the imidazolium ring tipped up from the surface with an angle along the surface normal of 40-55°. No water signal was observed in the spectra obtained at the water pressures studied here. 表面和頻光譜分析系統(tǒng)(SFG)