Multiscale Ice Fluidity in NO x Photodesorption from Frozen Nitrate Solutions

The temperature-programmed desorption of nitric oxide, NO, and nitrogen dioxide, NO 2, during the 302 nm photolysis of KNO 3 -doped, spray-frozen ice layers was investigated using two-photon laser-induced NO x fluorescence detection in the range −35 ≤ T /°C ≤ 0. Upon applying steady illumination and a 0.67 °C min -1 heating ramp, frozen KNO 3 solutions begin to evolve NO 2 at increasing rates, while NO emissions plateau soon after until, at ∼ −8 ° C, both species surge abruptly. Although the primary photoproduct NO 2 avoids geminate recombination by escaping from a permeable molecular cage throughout, NO 2 (g) levels are controlled by desorption from the outermost ice layers rather than by NO 3 - photolysis rates. The NO x accumulated in the deeper layers bursts when the solid undergoes a sintering transition following the onset of surface melting at −10 °C. Since elementary photochemical events occur in a communal fluid phase of molecular dimensions at temperatures far below the KNO 3 /H 2 O eutectic ( T eutectic = − 2.88 °C), we infer that doped polycrystalline ice contains operationally distinguishable fluid phases of low dimensionality over various length scales and temperature ranges.