Formation of (CH 4) N van der Waals clusters by seeded supersonic expansion

Abstract

The existence and the size of large methane van der Waals clusters (CH 4) N , have been characterized for the first time. (CH 4) N van der Waals clusters were generated by the supersonic expansion of a gaseous mixture consisting of 90% H 2 and 10% CH 4 at nozzle temperatures of −30 and 27 °C. Based on the buffer gas induced beam-broadening technique, the average sizes of (CH 4) N clusters were determined for all values of expansion at different stagnation pressures. For an identical stagnation pressure, the (CH 4) N cluster size produced by expansion at a nozzle temperature of −30 °C were larger than these produced by expansion at nozzle temperature of 27 °C, and cluster sizes of 120 molecules and 60 molecules have been measured for 10 bar stagnation pressure, respectively. In addition, we found (CH 4) N cluster formation did not occur by supersonic expansion of pure CH 4 gas at nozzle temperature of 27 °C. These results reveal that the presence of H 2 permits the evacuation of the heat of condensation and, thus promotes the (CH 4) N cluster nucleation.