Ultraviolet Photoionization Efficiency of the Vaporized Ionic Liquid 1-Butyl-3-methylimidazolium Tricyanomethanide: Direct Detection of the Intact Ion Pair

We present the first direct measurement of a neutral, intact ion pair photoionization efficiency (PIE) curve for a vaporized ionic liquid, 1-butyl-3-methylimidazolium tricyanomethanide, using tunable vacuum ultraviolet (VUV) photoionization time-of-flight mass spectrometry (PI-TOFMS). The ionization potential (IP) for the ion pair is experimentally determined to be 6.6 +/- 0.5 eV, which matches reasonably well with the adiabatic IP of 7.3 +/- 0.2 eV calculated at the M06/6-31+G(d,p) level of theory. The lifetime to dissociation of the cation-radical complex formed upon ionization of the ion pair is highly dependent upon entropic contributions. Thermal gravimetric analyses (TGA) determined the enthalpy of vaporization to be Delta H-vap(298 K) = 143.5 +/- 6.2 kJ/mol and that vaporization of BMIM+TCM- as ion pairs is the dominant mechanism for mass loss under the experimental conditions for VUV PI-TOFMS (T = 433 K).