M. Farnik et al., COMPARISON BETWEEN POSITIVE AND NEGATIVE CHARGING OF HELIUM DROPLETS, Zeitschrift fur Physik. D, Atoms, molecules and clusters, 40(1-4), 1997, pp. 93-98
Helium droplets of approximately 10(4)-10(8) atoms have been produced
in free jet expansions of liquid helium through a 5 mu m nozzle into v
acuum. 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 a
nd 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 Nth approximate to 2
x 10(5) the positively charged droplets were found to be doubly ionize
d. These observations are in good agreement with the assumption, that
the positively charge carriers are stable ''snowballs'' while the nega
tive 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 clus
ter of about 50 atoms is ejected.