Monotonic Increase of Nitrite Yields in the Photolysis of Nitrate in Ice and Water between 238 and 294 K

Abstract

The quantum yield, φ, of nitrite formation in the 302 nm band photolysis of fluid or frozen aqueous nitrate solutions increases monotonically with temperature over the range 238−294 K. The presence of formate increases φ 5-fold but does not modify its temperature dependence. Considering that the detection of nitrite as a product is only possible after the initial photofragments (NO_2^- + O) escape the solvent cage and that the diffusivity of ice, D_(ice), is about 6 orders of magnitude smaller than that of supercooled water, D_(aq), at the same temperature, we infer that nitrate photodecomposition takes place in similar liquidlike media at all temperatures. We found that the nitrite dispersed into the bulk is subsequently degraded by OH radicals, another primary photoproduct that can be scavenged by formate. The fact that experimental φ values in ice are actually larger than those derived from linear φ vs D_(aq)T^(1/2) extrapolation of aqueous phase data, as expected for cage processes in homogeneous media, suggests that the photochemically relevant properties of the quasi-liquid layer covering ice below the normal melting point resemble those of bulk supercooled water, but other effects, such as the dissipation of excess photon energy into the medium, may also play a role.

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