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 collision-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 the chemisorbed state. We employ He atomic beam reflectivity in order to detect adsorbate coverages and their evolution in the low coverage regime. Combining this method with temperature-programmed desorption spectroscopy (TPD) we are able to independently determine cross sections and threshold energies for all three processes and compare those to the activation energies of the corresponding thermal processes. Large differences for both the cross sections and the ratios of threshold (E-th) and activation energies (E-a) are found. The ratio of the threshold energy (E-th,E-c) for CID and the desorption 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) closely 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 chemisorbed 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 small cross sections. These dissimilarities can be rationalized in terms of the differences in collisional energy transfer to the particular reaction coordinate.