Superthermal Al Atoms as a Reactive-Atom Probe of Fluorinated Surfaces

We demonstrate aproof-of-concept of a new analytical techniqueto measure relative F atom exposure at the surfaces of fluorinatedmaterials. The method is based on reactive-atom scattering (RAS) ofAl atoms, produced by pulsed laser ablation of solid Al at 532 nm.The properties of the incident ground-state Al were characterizedby laser-induced fluorescence (LIF); at typical ablation fluences,the speed distribution is approximately Maxwellian at & SIM;45000K, with a most-probable kinetic energy of 187 kJ mol(-1) and a mean of 560 kJ mol(-1) When these Al atomsimpact the surfaces of perfluorinated solids (poly(tetrafluorethylene),PTFE) or liquids (perfluoropolyether, PFPE), gas-phase AlF productsare clearly detectable by LIF on the AlF A-X band. QuantitativeAlF yields were compared for a small representative set of a widelystudied family of ionic liquids based on the common 1-alkyl-3-methylimidazolium([C( n )mim](+)) cation. Yields of(1.9 & PLUSMN; 0.2):1 were found from [C(2)mim][Tf2N] and [C(8)mim][Tf2N], containing the commonfluorinated bis(trifluoro-methylsulfonyl)imide anion ([Tf2N](-)). This is in quantitative agreementwith previous independent low-energy ion scattering (LEIS) measurementsand consistent with other independent results indicating that thelonger cationic alkyl chains cover a larger fraction of the liquidsurface and hence reduce anion exposure. The expected null resultwas obtained for the ionic liquid [C(2)mim][EtSO4] which contains no fluorine. These results open the way for furthercharacterization and the potential application of this new variantof the RAS-LIF method.