  Helium nanodroplets, commonly regarded as the "nearly ideal spectroscopic
matrix", are being actively studied for more than two decades now. While they
mostly serve as cold, weakly perturbing and transparent medium for
high-resolution spectroscopy of embedded molecules, their intrinsic quantum
properties such as microscopic superfluidity still are subject-matter of
current research. This article reviews recent work on pure and doped He
nanodroplets using PI spectroscopy, an approach which has greatly advanced in
the past years. While the notion of the ideal spectroscopic matrix mostly no
longer holds in this context, photoionization techniques provide detailed
insights into the photo-physical properties of pure and doped He nanodroplets
and their relaxation dynamics following electronic excitation. Exploiting
nowadays available high laser fields, even highly ionized states of matter on
the nanoscale can be formed. Our particular focus lies on recent experimental
progress including fs time-resolved spectroscopy, photoion and electron
imaging, and novel sources of highly energetic radiation.
