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

We have studied the collision-induced desorption (CID) of hydrocarbons physisorbed 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. Adsorbate coverages and their evolution were detected using He atom reflectivity. The CID cross-sections and threshold energies were determined for Xe translational energies in the range of 1.6-5.8 eV. According to their desorption behavior the physisorbed hydrocarbons can be assigned to two categories. "Rigid" 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) experiments can be understood within the simple hard sphere-cube (HSC) collision model. For the "flexible" n-alkanes a strong chain-length-dependent reduction of the CID cross sections is found, whereas the desorption thresholds are shifted to higher energies and appear to be directly proportional to the corresponding desorption energies. These effects are tentatively assigned to the excitation of internal degrees of freedom for the latter group of adsorbates, for which only a fraction of the atoms composing the molecule is involved in the initial collisional energy transfer. (C) 2000 American Institute of Physics. [S0021-9606(00)70701-X].