Entropic stabilization of large adsorbates on weakly binding substrates-a thermal desorption and scanning tunneling...

Area of Science:

• Surface science
• Physical chemistry

Background:

• Oligopyridine adsorption on highly oriented pyrolytic graphite (HOPG) is crucial for understanding molecular behavior on surfaces.
• Thermal desorption (TD) is a key technique for studying surface interactions and molecular stability.

Purpose of the Study:

• To investigate the thermal desorption behavior of oligopyridine on HOPG across different coverage regimes.
• To elucidate the molecular mechanisms governing desorption kinetics and stability.
• To correlate desorption parameters with molecular orientation and mobility.

Main Methods:

• Thermal desorption spectroscopy (TD) was employed to measure desorption temperatures and kinetics.
• Transition State Theory (TST) was applied to model the desorption process.
• Time-resolved scanning tunneling microscopy (TR-STM) was used to visualize molecular arrangements and mobility at room temperature.

Main Results:

• Desorption temperature decreased from ~700 K for submonolayers to ~500 K for multilayers.
• The frequency factor (nu) increased significantly from 10^15 s^-1 (submonolayers) to 10^24 s^-1 (multilayers), while the desorption barrier remained constant.
• TR-STM confirmed the existence of a mobile phase of flat-lying molecules at room temperature for low coverages.

Conclusions:

• The observed shift in desorption temperature is attributed to changes in molecular mobility and orientation, not the desorption barrier.
• Submonolayers consist of mobile, flat-lying molecules, while multilayers feature immobile, upright molecules.
• The findings provide insights into the fundamental surface interactions and phase transitions of adsorbed molecules.