Formation of (CH4)N van der Waals clusters by seeded supersonic expansion

The existence and the size of large methane van der Waals clusters (CH4)N, have been characterized for the first time. (CH4)N van der Waals clusters were generated by the supersonic expansion of a gaseous mixture consisting of 90% H2 and 10% CH4 at nozzle temperatures of −30 and 27 °C. Based on the buffer gas induced beam-broadening technique, the average sizes of (CH4)N clusters were determined for all values of expansion at different stagnation pressures. For an identical stagnation pressure, the (CH4)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 (CH4)N cluster formation did not occur by supersonic expansion of pure CH4 gas at nozzle temperature of 27 °C. These results reveal that the presence of H2 permits the evacuation of the heat of condensation and, thus promotes the (CH4)N cluster nucleation.