  Photoionization spectroscopy and mass spectrometry of doped helium (He)
nanodroplets rely on the ability to efficiently detect ions and/or electrons.
Using a commercial quadrupole mass spectrometer and a photoelectron-photoion
coincidence (PEPICO) spectrometer, we systematically measure yields of ions and
electrons created in pure and doped He nanodroplets in a wide size range and in
two ionization regimes -- direct ionization and secondary ionization after
resonant photoexcitation of the droplets. For two different types of dopants
(oxygen molecules, O$_2$, and lithium atoms, Li), we infer the optimal droplet
size to maximize the yield of ejected ions. When dopants are ionized by
charge-transfer to photoionized He nanodroplets, the highest yield of O$_2$ and
Li ions is detected for a mean size of $\sim5\times10^4$ He atoms per
nanodroplet. When dopants are Penning ionized via photoexcitation of the He
droplets, the highest yield of O$_2$ and Li ions is detected for $\sim10^3$ and
$\sim10^5$ He atoms per droplet, respectively. At optimum droplet sizes, the
detection efficiency of dopant ions in proportion to the number of primary
photoabsorption events is up to 20\,\% for charge-transfer ionization of O$_2$
and 2\,\% for Li, whereas for Penning ionization it is 1\,\% for O$_2$ and
4\,\% for Li. Our results are instrumental in determining optimal conditions
for mass spectrometric studies and photoionization spectroscopy of molecules
and complexes isolated in He nanodroplets.
