Sizes of large He droplets

Helium droplets spanning a wide size range, N-He = 10(3)-10(10), were formed in a continuous-nozzle beam expansion at different nozzle temperatures and a constant stagnation pressure of 20 bars. The average sizes of the droplets have been obtained by attenuation of the droplet beam through collisions with argon and helium gases at room temperature. The results obtained are in good agreement with previous measurements in the size range N-He = 10(5)-10(7). Moreover, the measurements give the average sizes in the previously uncharacterized range of very large droplets of 10(7)-10(10) atoms. The droplet sizes and beam flux increase rapidly at nozzle temperatures below 6 K, which is ascribed to the formation of droplets within the nozzle interior. The mass spectra of the droplet beam upon electron impact ionization have also been obtained. The spectra show a large increase in the intensity of the He-4(+) signal upon increase of the droplet size, an effect which can be used as a secondary size standard in the droplet size range N-He = 10(4)-10(9) atoms. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3650235]