Entropic stabilization of large adsorbates on weakly binding substrates—a thermal desorption and scanning tunneling microscopy study

Thermal desorption (TD) of oligopyridine from HOPG shifts from approximately 700 to approximately 500 K when going from (sub-)mono- to multilayers. Stabilization of low coverages results from a continuous shift of the frequency factor nu, from 10(15) s(-1) for submonolayers to 10(24) s(-1) for multilayers, whereas the desorption barrier is virtually constant. Applying transition state theory (TST), we can explain this by a change from rotationally/translationally mobile, flat-lying molecules (submonolayers) to immobile, upright molecules (multilayers). At room temperature, (time resolved) scanning tunneling microscopy (STM) gives evidence for the existence and the stability of the mobile phase.