Collision-induced desorption of hydrocarbons physisorbed on Au(111)

We have studied the collision-induced desorption (CID) of hydrocarbons phys isorbed on Au(111). Adsorbed layers of benzene, toluene, deuterated toluene , and n-alkanes of various chainlengths (CnH2n+2, n=5,7,10,12) were exposed to a beam of hyperthermal Xe generated in a supersonic expansion. Adsorbat e coverages and their evolution were detected using He atom reflectivity. T he CID cross-sections and threshold energies were determined for Xe transla tional energies in the range of 1.6-5.8 eV. According to their desorption b ehavior the physisorbed hydrocarbons can be assigned to two categories. "Ri gid" aromatic molecules, such as benzene and toluene, show large CID cross sections. The ratios between the CID threshold energies and the desorption energies as determined from TPD (temperature programmed desorption) experim ents can be understood within the simple hard sphere-cube (HSC) collision m odel. For the "flexible" n-alkanes a strong chain-length-dependent reductio n of the CID cross sections is found, whereas the desorption thresholds are shifted to higher energies and appear to be directly proportional to the c orresponding desorption energies. These effects are tentatively assigned to the excitation of internal degrees of freedom for the latter group of adso rbates, for which only a fraction of the atoms composing the molecule is in volved in the initial collisional energy transfer. (C) 2000 American Instit ute of Physics. [S0021-9606(00)70701-X].