Multiscale Ice Fluidity in NO_x Photodesorption from Frozen Nitrate Solutions

Abstract

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_2O 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.