We demonstrate a
proof-of-concept of a new analytical technique
to measure relative F atom exposure at the surfaces of fluorinated
materials. The method is based on reactive-atom scattering (RAS) of
Al atoms, produced by pulsed laser ablation of solid Al at 532 nm.
The properties of the incident ground-state Al were characterized
by laser-induced fluorescence (LIF); at typical ablation fluences,
the speed distribution is approximately Maxwellian at ∼45000
K, with a most-probable kinetic energy of 187 kJ mol–1and a mean of 560 kJ mol–1When these Al atoms
impact the surfaces of perfluorinated solids (poly(tetrafluorethylene),
PTFE) or liquids (perfluoropolyether, PFPE), gas-phase AlF products
are clearly detectable by LIF on the AlF A–X band. Quantitative
AlF yields were compared for a small representative set of a widely
studied family of ionic liquids based on the common 1-alkyl-3-methylimidazolium
([Cnmim]+) cation. Yields of
(1.9 ± 0.2):1 were found from [C2mim][Tf2N] and [C8mim][Tf2N], containing the common
fluorinated bis(trifluoromethylsulfonyl)imide anion ([Tf2N]−). This is in quantitative agreement
with previous independent low-energy ion scattering (LEIS) measurements
and consistent with other independent results indicating that the
longer cationic alkyl chains cover a larger fraction of the liquid
surface and hence reduce anion exposure. The expected null result
was obtained for the ionic liquid [C2mim][EtSO4] which contains no fluorine. These results open the way for further
characterization and the potential application of this new variant
of the RAS-LIF method.