Total iron concentrations can exceed 10 micromolar in cloud, fog, and rain droplets in the atmosphere. Photolysis by sunlight of iron(III) species in these droplets is an important source of OH radicals, which then oxidize water-soluble organics in the troposphere. As a high-valent metal ion, iron(III) is extensively hydrolyzed in aqueous solution, but the precise speciation depends sensitively on pH. Previous studies have revealed striking differences in the OH quantum yields of [Fe(H2O)6]3+ vs. [Fe(OH)(H2O)5]2+. The quantum yield for the former ion is low (~0.05) and independent of excitation wavelength, while the quantum yield for the latter ion increases dramatically as wavelength is decreased, attaining a value of ~0.5 at 250 nm. To understand these striking reactivity differences, aqueous solutions of iron(III) perchlorate were studied at low pH values using femtosecond pump-probe spectroscopy. [Fe(H2O)6]3+ behaves as a Lewis photoacid and excitation of its ligand-to-metal charge transfer (LMCT) band at 265 nm hydrolyzes a coordinated water molecule forming [Fe(OH)(H2O)5]2+ and releasing a proton in less than 1 ps. Photoexcitation thus converts the hexaaqua ion into the monohydroxy form without changing the redox state of the iron. This highly efficient channel, which is observed here for the first time, is suggested to greatly suppress the OH radical yield for the hexaaqua iron(III) ion. These experiments also provide insight into the geminate recombination dynamics initiated by the single-electron oxidation of a iron(III)-coordinated water molecule or hydroxide ion.

Ultrafast photolysis of iron(III) aqua ions studied by UV/UV femtosecond pump-probe spectroscopy

251st National Meeting (2016)

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