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

Author： Koh Christine J. Leone Stephen R.

Publisher： Taylor & Francis Ltd

ISSN： 1362-3028

Source： Molecular Physics, Vol.110, Iss.15-16, 2012-08, pp. : 1705-1712

Disclaimer: Any content in publications that violate the sovereignty, the constitution or regulations of the PRC is not accepted or approved by CNPIEC.

Previous Menu Next

Abstract

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.