THERMAL-DESORPTION INDUCED BY KILOELECTRONVOLT ION-BOMBARDMENT OF THIOL-BOUND SELF-ASSEMBLED MONOLAYERS ON GOLD

Time distributions of neutral molecules desorbed from a chemisorbed se lf-assembled monolayer of phenylethanethiol on gold have been measured subsequent to 8 keV Ar+ and H-2(+) ion bombardment. These distributio ns show that, regardless of the projectile used, most of the ejected m olecules leave the surface with thermal kinetic energies (similar to 0 .03 eV). The shapes of the distributions have a strong surface tempera ture dependence over the range 240-300 K. This behavior is well descri bed by a convolution of the Maxwell-Boltzmann distribution and the rat e equation for first-order desorption. The results imply that kiloelec tronvolt ion bombardment initiates a process which breaks the adsorbat e-surface bond, leaving the resulting physisorbed molecules to evapora te after attaining thermal equilibrium with the substrate. A mechanism for this gentle cleavage of the adsorbate-substrate bond is proposed.