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The first comparative study of the surface behavior of four small aromatic molecules, benzene, toluene, p-xylene, and o-xylene, adsorbed on graphite at temperatures ≤30 K, is presented. Intermolecular interactions are shown to be important in determining the growth of the molecules on the graphite surface at low (monolayer) exposures. Repulsive intermolecular interactions dominate the behavior of benzene and toluene. By contrast, stronger interactions with the graphite surface are observed for the xylene isomers, with islanding observed for o-xylene. Multilayer desorption temperatures and energies increase with the size of the molecule, ranging from 45.5 to 59.5 kJ mol−1 for benzene and p-xylene, respectively. Reflection absorption infrared spectroscopy gives insight into the effects of thermal processing on the ordering of the molecules. Multilayer benzene, p-xylene, and o-xylene form crystalline structures following annealing of the ice. However, we do not observe an ordered structure for toluene in this study. The ordering of p-xylene shows a complex relationship dependent on both the annealing temperature and exposure. [ABSTRACT FROM AUTHOR]

The rate of the gas-phase N(4S) + C2N(X∼2Πi) reaction has been measured in a continuous supersonic flowreactor down to 54 K through the relative-rate method using the N(4S) + OH(X2Π) → H(2S) +NO(X2Π) reaction as a reference. The microwave dischargetechnique was employed to produce high concentrations of atomic nitrogen.Pulsed laser photolysis of precursor molecules Cl3C2N and H2O2at 212 nm in situ led toC2N and OH radical formation, respectively. The rate constantis shown to be approximately independent of temperature, in contrastto previous studies of atom-radical reactions involving atomic nitrogen.While the reaction rate is faster than previously estimated, astrochemicalsimulations indicate that this reaction is probably only a minor sourceof CN radicals in dense interstellar clouds. [ABSTRACT FROM AUTHOR]