Electron Diffraction Studies of the Kinetics of Phase Changes in Clusters. 4. Freezing of Ammonia

Abstract

Clusters of liquid ammonia produced by the condensation of vapor in supersonic flow were monitored by electron diffraction, in flight, while they froze. Freezing began when clusters had cooled to 120 K by evaporation and froze to single crystals of the normal cubic phase, space group P2 1 3. Even though initially undercooled by 75 deg, they would have warmed to the melting point well before the completion of freezing if the process in the clusters had been strictly adiabatic. Yet they froze completely, and their temperature was 128 K within a dozen microseconds after nucleation. The kinetics of evaporative cooling of the liquid and of the freezing clusters are treated, and the amount of ablation suffered during solidification is calculated