  Helium (He) nanodroplets irradiated by intense near-infrared laser pulses
form a nanoplasma by avalanche-like electron impact ionizations even at lower
laser intensities where He is not directly field ionized, provided that the
droplets contain a few dopant atoms which provide seed electrons for the
electron impact ionization avalanche. In this theoretical paper on calcium and
xenon doped He droplets we elucidate the mechanism which induces ionization
avalanches, termed ignition. We find that the partial loss of seed electrons
from the activated droplets starkly assists ignition, as the Coulomb barrier
for ionization of helium is lowered by the electric field of the dopant
cations, and this deshielding of the cation charges enhances their electric
field. In addition, the dopant ions assist the acceleration of the seed
electrons (slingshot effect) by the laser field, supporting electron impact
ionizations of He and also causing electron loss by catapulting electrons away.
The dopants' ability to lower the Coulomb barriers at He as well as the
slingshot effect decrease with the spatial expansion of the dopant, causing a
dependence of the dopants' ignition capability on the dopant mass. Here, we
develop criteria (impact count functions) to assess the ignition capability of
dopants, based on (i) the spatial overlap of the seed electron cloud with the
He atoms and (ii) the overlap of their kinetic energy distribution with the
distribution of Coulomb barrier heights at He. The relatively long time delays
between the instants of dopant ionization and ignition (incubation times) for
calcium doped droplets are determined to a large extent by the time it takes to
deshield the dopant ions.
