Simultaneous ion-pair photodissociation and dissociative ionization of an ionic liquid: velocity map imaging of vacuum-ultraviolet-excited 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

作者：Koh Christine J; Leone Stephen R^*

来源：Molecular Physics, 2012, 110(15-16): 1705-1712.

DOI：10.1080/00268976.2012.673019

• ionic liquid
• photodissociation
• velocity map ion imaging

Isolated gas-phase ionic liquid (IL) molecules, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Emim(+)][Tf2N-]), are excited by laser-produced high-harmonic extreme ultraviolet light in the 17-26 eV range and the ions are probed by velocity map imaging. The velocities of the intact cation, Emim(+), and intact anion, Tf2N-, are recorded in separate velocity map images. The kinetic energy distribution of the intact cation has both slow ions arising from previously reported dissociative ionization and fast ions from ion-pair dissociation of the neutral isolated ionic liquid (positive and negative ion pairs) where the branching ratio is approximately 0.18 +/- 0.02:0.82 +/- 0.02. The intact anion is also detected with a momentum that complements the higher velocity intact cation fragment, indicating that the process arises from two-body dissociation of the neutral ion-pair. Similar angular distributions for the anion and the higher velocity cation indicate that the electronic transition, which is above the ionization threshold and leads to ion-pair dissociation of the neutral, has a dipole moment perpendicular to the dissociating ion-pair bond. Observation of the ion-pair dissociation upon photoexcitation of the IL vapor not only reveals a previously unobserved photodissociation pathway for the ionic liquid, but it also provides direct evidence for the existence of the ion pairs in the isolated gas phase molecules.

• 出版日期2012