We report homogeneous ice nucleation rates between 202 K and 215 K, thereby reducing the measurement gap that previously existed between 203 K and 228 K. These temperatures are significantly below the homogenous freezing limit, T(H)≈ 235 K for bulk water, and well within no-man's land. The ice nucleation rates are determined by characterizing nanodroplets with radii between 3.2 and 5.8 nm produced in a supersonic nozzle using three techniques: (1) pressure trace measurements to determine the properties of the flow as well as the temperature and velocity of the droplets, (2) small angle X-ray scattering (SAXS) to measure the size and number density of the droplets, and (3) Fourier Transform Infrared (FTIR) spectroscopy to follow the liquid to solid phase transition. Assuming that nucleation occurs throughout the droplet volume, the measured ice nucleation rates J(ice,V) are on the order of 10(23) cm(-3) s(-1), and agree well with published values near 203 K.

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We used a tunable diode laser absorption spectrometer and a static-pressure probe to follow changes in temperature, vapor-phase concentration of D2O, and static pressure during condensation in a supersonic nozzle. Using the measured static-pressure ratio p/p0 and the mass fraction of the condensate g as inputs to the diabatic flow equations, we determined the area ratio (A/A*)wet and the corresponding centerline temperature of the flow during condensation. From (A/A*)wet we determined the boundary-layer displacement thickness during condensation (delta#)wet. We found that (delta#)wet first increases relative to the value of delta# in a dry expansion (delta#)Dry before becoming distinctly smaller than (delta#)Dry downstream of the condensation region. After correcting the temperature gradient across the boundary layers, the temperature determined from p/p0 and g agreed with the temperature determined by the laser-absorption measurements within our experimental error (+/-2 K), except when condensation occurred too close to the throat. The agreement between the two temperature measurements let us draw the following two conclusions. First, the differences in the temperature and mole fraction of D2O determined by the two experimental techniques, first observed in our previous study [P. Paci, Y. Zvinevich, S. Tanimura, B. E. Wyslouzil, M. Zahniser, J. Shorter, D. Nelson, and B. McManus, J. Chem. Phys. 121, 9964 (2004)], can be explained sufficiently by changes in delta# caused by the condensation of D2O, except when the phase transition occurs too close to the throat. Second, the extrapolation of the equation, which expresses the temperature dependence of the heat of vaporization of bulk D2O liquid, is a good estimate of the heat of condensation of supercooled D2O down to 210 K.

We have combined static pressure, spectroscopic temperature, Fourier transform infrared spectroscopy (FTIR), and small angle X-ray scattering (SAXS) measurements to develop a detailed picture of methanol condensing from a dilute vapor-carrier gas mixture under the highly supersaturated conditions present in a supersonic nozzle. In our experiments, methanol condensation can be divided into three stages as the gas mixture expands in the nozzle. In the first stage, as the temperature decreases rapidly, small methanol n-mers (clusters) form, increase in concentration, and evolve in size. In the second stage, the temperature decreases more slowly, and the n-mer concentrations continue to rise. Thermodynamic and FTIR experiments cannot, however, definitively establish if the average cluster size is constant or if it continues to increase. Finally, when the vapor becomes supersaturated enough, liquid droplets form via nucleation and growth, consuming more monomer and reducing the concentration of clusters. At the point where liquid first appears, cluster formation has already consumed up to 30% of the monomer. This is significantly more than is predicted by a model that describes the vapor phase as an equilibrium mixture of methanol monomer, dimer, and tetramer. An energy balance suggests that a significant fraction of the cluster population is larger than the tetramer, while preliminary SAXS measurements suggest that these clusters contain, on average, 6 monomers.