Interlayer cohesive energy of graphite from thermal desorption of polyaromatic hydrocarbons

We have studied the interaction of polyaromatic hydrocarbons (PAHs) with the basal plane of graphite using thermal desorption spectroscopy. Desorption kinetics of benzene, naphthalene, coronene, and ovalene at submonolayer coverages yield activation energies of 0. 50 eV, 0. 85 eV, 1. 40 eV, and 2. 1 eV, respectively. Benzene and naphthalene follow simple first order desorption kinetics while coronene and ovalene exhibit fractional order kinetics owing to the stability of two-dimensional adsorbate islands up to the desorption temperature. Preexponential frequency factors are found to be in the range 10^14--10^21s^-1 as obtained from both Falconer-Madix (isothermal desorption) analysis and Antoine's fit to vapor pressure data. The resulting binding energy per carbon atom of the PAH is 525 meV and can be identified with the interlayer cohesive energy of graphite. The resulting cleavage energy of graphite is 615 meV/atom, which is considerably larger than previously reported experimental values.