Penning Ionization of Acene Molecules by Helium Nanodroplets. 
Acene molecules (anthracene, tetracene, pentacene) and fullerene (C60) are embedded in He nanodroplets (HeN) and probed by EUV synchrotron radiation.  When resonantly exciting the He nanodroplets, the embedded molecules M are efficiently ionized by the Penning reaction HeN* + M → HeN + M+ + e-.  However, the Penning electron spectra are all broad and structureless, largely differing from those measured by binary Penning collisions, as well as from those measured for dopants bound to the He droplet surface.  Simulations based on elastic binary electron-He collisions qualitatively reproduce the measured spectra only when assuming unexpectedly large He droplets, indicating that electron spectra of molecules embedded in helium nanodroplets are severely affected by collective electron-helium interactions.