Sizes of large He droplets.

Area of Science:

• Atomic and Molecular Physics
• Quantum Fluids
• Nanoscale Science

Background:

• Helium droplets are crucial for studying quantum phenomena and as nanoscale environments.
• Characterizing droplet size distribution is essential for understanding their properties and applications.
• Previous studies focused on smaller droplet sizes, leaving larger sizes less understood.

Purpose of the Study:

• To precisely measure the average sizes of helium droplets across an unprecedented size range (10^3-10^10 atoms).
• To investigate the influence of nozzle temperature on droplet formation and size.
• To explore the potential of mass spectrometry for helium droplet size characterization.

Main Methods:

• Continuous nozzle beam expansion of helium at varying temperatures and 20 bar stagnation pressure.
• Beam attenuation measurements using collisions with argon and helium gases at room temperature.
• Electron impact ionization mass spectrometry of the helium droplet beam.

Main Results:

• Average droplet sizes were accurately determined for N(He) from 10^3 to 10^10 atoms, extending previous measurements.
• A rapid increase in droplet size and beam flux was observed below 6 K, indicating droplet formation within the nozzle.
• The He(4)+ signal intensity in mass spectra correlates with droplet size, offering a potential secondary standard.

Conclusions:

• The study successfully characterized helium droplet sizes in a wide, previously unmeasured range.
• Nozzle temperature significantly impacts droplet formation, with sub-6K temperatures favoring larger droplets.
• Mass spectral analysis provides a viable method for secondary helium droplet size determination.