Sizes of pure and doped helium droplets from single shot x-ray imaging.

Advancements in x-ray free-electron lasers on producing ultrashort, ultrabright, and coherent x-ray pulses enable single-shot imaging of fragile nanostructures, such as superfluid helium droplets. This imaging technique gives unique access to the sizes and shapes of individual droplets. In the past, such droplet characteristics have only been indirectly inferred by ensemble averaging techniques.

Evaporation Dynamics from Ag-Doped He Droplets upon Laser Excitation.

Silver clusters were assembled in helium droplets of different sizes ranging from 10 to 10 atoms. The clusters were heated upon laser irradiation at 355 nm, and evaporation dynamics of He atoms were studied by quadrupole mass spectroscopy using signals from He, He, and He splitter ions. We found that for droplets containing less than 10 atoms the laser irradiation leads to evaporation of He atoms.

Formation of He via electron impact of helium droplets.

Electron impact ionization of superfluid helium droplets containing several thousand atoms produces a broad distribution of He ions that peaks at n = 2 and decreases monotonically toward larger n. In larger droplets (say 10 or more atoms), however, the He signal intensity is anomalously large. We have studied the mechanism for the formation of He ions in large helium droplets by varying the duration of the electron impact excitation pulse.