Structure of water and carbon dioxide clusters formed via homogeneous nucleation in nozzle beams

Abstract

A nozzle molecular beam is operated so as to optimize the molecular clustering in the free jet expansion. Continuous beams of carbon dioxide and water are sampled with an ionization gauge detector. The increased beam intensity under some operating conditions can only be attributed to clustering. A high energy electron beam of 39.5 keV is used to obtain Debye-Scherrer diffraction patterns from carbon dioxide and water clusters. The largest size clusters produced are solid and have the structure of the bulk material which is simple cubic for CO2 and diamond cubic for H2O. The average diameters for CO2 and H2O are 52±5 and 54±5 Å, respectively, corresponding to 1600 and 2600 molecules per cluster. The CO2 data are in agreement with other beam results and, to the authors' knowledge, these are the first data published on the structure of water clusters formed from the vapor phase via homogeneous nucleation. Based on the diffraction data the structure of clusters down to average sizes in the range of 300–450 molecules per cluster can be treated as bulk phase.