Comparison between positive and negative charging of helium droplets

Helium droplets of approximately 10⁴–10⁸ atoms have been produced in free jet expansions of liquid helium through a 5 μm nozzle into vacuum. The size distributions of the positively and negatively charged droplets were measured as a function of the electron emission current. A simple model has been developed to describe the charging process and formulas for production of singly and doubly charged droplets were derived. The ratio of the ionization cross section to the geometrical cross section and its dependence on N was obtained. In the experiment single negatively and positively charged droplets were observed. Only for sizes N larger than a certain threshold size N _th ≈ 2 × 10⁵ the positively charged droplets were found to be doubly ionized. These observations are in good agreement with the assumption, that the positively charge carriers are stable “snowballs” while the negative droplets contain an excess electron located in the inside within a metastably bound “bubble”. The threshold size N _th corresponds to a simple model in which for smaller droplets a positively charged cluster of about 50 atoms is ejected.