Photo- and radiation-chemistry of halide anions in ionic liquids.

One- and two- photon excitation of halide anions (X(-)) in polar molecular solvents results in electron detachment from the dissociative charge-transfer-to-solvent state; this reaction yields a solvated halide atom and a solvated electron. How do such photoreactions proceed in ionic liquid (IL) solvents? Matrix isolation electron paramagnetic resonance (EPR) spectroscopy has been used to answer this question for photoreactions of bromide in aliphatic (1-butyl-1-methylpyrrolidinium) and aromatic (1-alkyl-3-methyl-imidazolium) ionic liquids. In both classes of ILs, the photoreaction (both 1- and 2-photon) yields bromine atoms that promptly abstract hydrogen from the alkyl chains of the IL cation; only in concentrated bromide solutions (containing >5-10 mol % bromide) does Br2(-•) formation compete with this reaction. In two-photon excitation, the 2-imidazolyl radical generated via the charge transfer promptly eliminates the alkyl arm. These photolytic reactions can be contrasted with radiolysis of the same ILs, in which large yield of BrA(-•) radicals was observed (where A(-) is a matrix anion), suggesting that solvated Br(•) atoms do not occur in the ILs, as such a species would form three-electron σ(2)σ(*1) bonds with anions present in the IL. It is suggested that chlorine and bromine atoms abstract hydrogen faster than they form such radicals, even at cryogenic temperatures, whereas iodine mainly forms such bound radicals. These XA(-•) radicals convert to X2(•-) radicals in a reaction with the parent halide anion. Ramifications of these observations for photodegradation of ionic liquids are discussed.

Femtosecond mid-infrared optical parametric oscillator based on CsTiOAsO(4).

A high-repetition-rate Ti:sapphire laser is used to synchronously pump a type-II angle-tuned CsTiOAsO(4) (CTA) optical parametric oscillator. When pumped at 809 nm, the optical parametric oscillator is tunable from 1.007 to 1.180 microm in the signal branch and from 2.590 to 4.120 microm in the idler branch. Powers as high as 235 mW are obtained in the signal branch. Pulse widths as short as 56 fs are generated at 1.115 microm. CTA is shown to have a unique combination of low walk-off and low dispersion that contributes to its high gain and conversion efficiency.