Spectroscopy of PTCDA attached to rare gas samples: clusters vs. bulk   matrices. I. Absorption spectroscopy
  The interaction between PTCDA (3,4,9,10-perylene-tetracarboxylic-dianhydride) and rare gas or para-hydrogen samples is studied by means of laser-induced fluorescence excitation spectroscopy. The comparison between spectra of PTCDA embedded in a neon matrix and spectra attached to large neon clusters shows that these large organic molecules reside on the surface of the clusters when doped by the pick-up technique. PTCDA molecules can adopt different conformations when attached to argon, neon and para-hydrogen clusters which implies that the surface of such clusters has a well-defined structure and has not liquid or fluxional properties. Moreover, a precise analysis of the doping process of these clusters reveals that the mobility of large molecules on the cluster surface is quenched, preventing agglomeration and complex formation. 
M. Dvorak, M. M\"uller, T. Knoblauch, O. B\"unermann, A. Rydlo, S.   Minniberger, W. Harbich and F. Stienkemeier
physics.atm-clus