Acoustic Desorption from a Room Temperature Ionic Liquid.

We report on the acoustic desorption of ions from the surface of a room temperature ionic liquid (RTIL) under vacuum. Our RTIL, 1-butyl-3-methylimidazolium hexafluorophosphate, remains a stable liquid at extremely low pressures ($<$10$^{-9}$ torr). Using the 2$^{nd}$ harmonic of a Nd:YAG laser, 2ns pulse time, we ablate the backside of a metal foil, generating an acoustic pulse that propagates through the foil, desorbing ions off the surface of the RTIL. We determine the m/q of desorbed ions via time of flight (TOF) measurements from an imaging micro-channel plate detector. We detect both positive and negative ion species, depending on the extraction and acceleration voltages. Our TOF spectra demonstrate our acoustic pulses are of short duration ($<$20ns), generating mass spectra with good temporal resolution ($<$40ns FWHM @ t=9000ns), that are stable and reproducible. We discuss the variation of desorbed ion yield as a function of acoustic pulse strength, metal foil properties and extraction fields.