Supercooling of H2 clusters produced in free-jet expansions from supercritical states

Abstract

A combined theoretical and experimental investigation has been undertaken to determine optimum conditions for achieving rapid cooling of H2 clusters in nozzle-beam expansions with the goal of producing superfluid H2 clusters. Theory predicts that a temperature less than 6.6 K, well below the 13.8 K triple-point temperature of p-H2, is required. Terminal specific enthalpies of clusters are determined experimentally from terminal velocities of clusters measured using the time-of-flight technique. The results are interpreted in the context of isothermal and adiabatic spinodals for p-H2 constructed using thermodynamic methods and a van der Waals equation-of-state model. The lowest terminal enthalpies are achieved in expansions starting from supercritical source conditions and crossing the binodal curve of the phase diagram with densities far to the liquid side of the critical point. In this case the clusters are formed via relatively late fragmentation of metastable liquid H2. These clusters are expected to be liquid and, at the point in the expansion at which collisions cease, to have temperatures of about 9 K. Before arriving at the detector they are cooled further by evaporation to temperatures between 4 and 5 K. Further experiments are needed to determine if these clusters are superfluid.

Why are para-hydrogen clusters superfluid? A quantum theorem of corresponding states study

“…Time-of-flight distributions and mass spectra of pure clusters with N Ϸ͑2 ϫ 10 3 ͒ -͑1.4ϫ 10 4 ͒ molecules indicate that these clusters have temperatures of about 3.1 K and are probably liquid. 28,29 Indeed evidence suggests that after heterogeneous nucleation, large clusters solidify internally but appear to have a liquid surface. 29 The liquid nature of large clusters was also confirmed by time resolved VUV fluorescence spectroscopy of embedded Xe atoms.…”

Spectroscopy of 3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA) attached to rare gas samples: Clusters vs. bulk matrices. I. Absorption spectroscopy

“…6 To our knowledge no such studies have been performed involving large doped hydrogen clusters. Large neat hydrogen clusters obtained by supersonic expansion have a temperature of 4.2 to 4.5 K. 11 For smaller clusters, Monte-Carlo simulation postulated a superfluid phase for clusters sizes less than 26 molecules and for temperatures below 1.5 K. 12 These numbers have been obtained for para-hydrogen which correspond to molecules with their two proton spins forming a singlet state. Experiments on large hydrogen clusters involving Raman spectroscopy revealed a solid structure when formed in a supersonic expansion.…”

High density cluster jet target for storage ring experiments

“…They produce small nanoparticles from cooled gases or liquids via expansion in Laval-type nozzles, by either condensation of the gas or by breaking up the liquid into a spray of tiny droplets. These clusters typically consist of 10 3 − 10 6 molecules [8,9]. Different to pellet targets, the random nature of the cluster production introduces a homogeneous spatial distribution and no distinct time structure.…”