The present theoretical study explores the interaction of various energetic molecular projectiles and clusters with a model polymeric surface, with direct implications for surface analysis by mass spectrometry. The projectile sizes (up to 23 kDa) are intermediate between the polyatomic ions (SF(5), C(60)) used in secondary ion mass spectrometry and the large organic microdroplets generated, for example, in desorption electrospray ionization. The target is a model of amorphous polyethylene, already used in a previous study [Delcorte, A.; Garrison, B. J. J. Phys. Chem. C 2007, 111, 15312]. The chosen method relies on classical molecular dynamics (MD) simulations, using a coarse-grained description of polymeric samples for high energy or long time calculations (20-50 ps) and a full atomistic description for low energy or short time calculations (<1 ps). Two regions of sputtering or desorption are observed depending on the projectile energy per nucleon (i.e., effectively the velocity). The transition, occurring around 1 eV/nucleon, is identified by a change of slope in the curve of the sputtering yield per nucleon vs energy per nucleon. Beyond 1 eV/nucleon, the sputtering yield depends only on the total projectile energy and not on the projectile nuclearity. Below 1 eV/nucleon, i.e., around the sputtering threshold for small projectiles, yields are influenced by both the projectile energy and nuclearity. Deposition of intact molecular clusters is also observed at the lowest energies per nucleon. The transition in the sputtering curve is connected to a change of energy deposition mechanisms, from atomistic and mesoscopic processes to hydrodynamic flow. It also corresponds to a change in terms of fragmentation. Below 1 eV/nucleon, the projectiles are not able to induce bond scissions in the sample. This region of molecular emission with minimal fragmentation offers new analytical perspectives, out of reach of smaller molecular clusters such as fullerenes.

We present periodic density functional theory (DFT) calculations combined with thermodynamic analysis to study the structure of isolated molybdenum oxide entities supported on titania (anatase) under ambient and dehydrated conditions. The TiO 2 support is represented by the perfect and hydrated ( 101) and ( 001) surfaces. The calculation of the vibrational wavenumbers of the stable structures under various conditions allows us to access to structural information by comparison with the experimental data obtained in in-situ conditions. The calculation of the most stable model on the (101) surface indicates that in dry conditions molybdenum is in a distorted tetrahedral environment with a single molybdenyl bond whereas dioxo entities are more stable on the (001) surface. Furthermore, it appears that the ModO stretching wavenumber is strongly influenced by the hydration state of the surface through formation of hydrogen bonds with the surface OH or H 2 O groups that induce a shift to lower wavenumber (more than 60 cm -1 ) in agreement with the Raman shift observed during the hydration-dehydration process.