Thermal desorption induced by kiloelectronvolt ion bombardment of thiol-bound self-assembled monolayers on gold

Time distributions of neutral molecules desorbed from a chemisorbed self-assembled monolayer of phenylethanethiol on gold have been measured subsequent to 8 keV Ar⁺ and H₂⁺ ion bombardment. These distributions show that, regardless of the projectile used, most of the ejected molecules leave the surface with thermal kinetic energies (~0.03 eV). The shapes of the distributions have a strong surface temperature dependence over the range 240-300 K. This behavior is well described by a convolution of the Maxwell-Boltzmann distribution and the rate equation for first-order desorption. The results imply that kiloelectronvolt ion bombardment initiates a process which breaks the adsorbate-surface bond, leaving the resulting physisorbed molecules to evaporate after attaining thermal equilibrium with the substrate. A mechanism for this gentle cleavage of the adsorbate-substrate bond is proposed.