Collision-induced chemical dynamics in ethanethiol adsorbed on Au(111)

Exposing a layer of ethanethiol adsorbed on Au(lll) to a hyperthermal beam of Xe colliders with translational energies from 1.6 to 5.8 eV, three colli sion-induced processes can be studied: (1) the collision-induced desorption (CID) of physisorbed ethanethiol, (2) the CID of chemisorbed ethanethiol, and (3) the collision-induced conversion of the physisorbed molecules to th e chemisorbed state. We employ He atomic beam reflectivity in order to dete ct adsorbate coverages and their evolution in the low coverage regime. Comb ining this method with temperature-programmed desorption spectroscopy (TPD) we are able to independently determine cross sections and threshold energi es for all three processes and compare those to the activation energies of the corresponding thermal processes. Large differences for both the cross s ections and the ratios of threshold (E-th) and activation energies (E-a) ar e found. The ratio of the threshold energy (E-th,E-c) for CID and the desor ption activation energy (E-a,E-c) for the chemisorbed thiol (E-th,E-c = 4.1 +/- 0.3 eV, E-a,E-c = 1.35 +/- 0.05 eV, E-th,E-c/E-a,E-c = 3.1 +/- 0.3) cl osely resembles the corresponding ratio for the physisorbed molecule (E-th, E-c = 1.7 +/- 0.2 eV, E-a,E-c = 0.61 +/- 0.02 eV, E-th,E-c/E-a,E-c = 2.8 +/ - 0.4). However, the process of converting a physisorbed molecule to the ch emisorbed state, which shows a small thermal activation barrier (E-a,E-pc = 0.42 +/- 0.06 eV), is characterized by a large collision-induced threshold energy (E-th,E-pc = 4.6 +/- 0.4 eV, E-th,E-pc/E-a,E-pc = 11 +/- 3) and sma ll cross sections. These dissimilarities can be rationalized in terms of th e differences in collisional energy transfer to the particular reaction coo rdinate.