  Helium nanodroplets irradiated by intense near-infrared laser pulses ignite
and form highly ionized nanoplasmas even at laser intensities where helium is
not directly ionized by the optical field, provided the droplets contain a few
dopant atoms. We present a combined theoretical and experimental study of the
He nanoplasma ignition dynamics for various dopant species. We find that the
efficiency of dopants to ignite a nanoplasma in helium droplets strongly varies
and mostly depends on (i) the pick-up process, (ii) the number of free
electrons each dopant donates upon ionization, and remarkably, (iii) by the
hitherto unexplored effect of the dopant location in or on the droplet.
