The adsorption of monolayer and multilayer benzene on the Ag(111) surface was characterized using temperature programmed desorption (TPD). TPD spectra revealed two broad peaks at approximately 205 and approximately 150 K at submonolayer coverage and a sharper, multilayer peak at 140 K. Analysis of the coverage-dependent shape and shift of the two submonolayer peaks has resulted in their assignment to desorption from two different binding geometries on threefold-hollow sites with symmetries C(3v)(sigma d) and C(3v)(sigma v). The TPD peak analysis incorporated inter-adsorbate repulsive interaction that resulted from the local dipole moment at the adsorption site induced by the adsorbate-surface charge transfer bonding. The analysis has yielded desorption energies of 54.9 +/- 0.8 and 50.4 +/- 0.4 kJ/mol for the C(3v)(sigma d) and C(3v)(sigma v) configurations, respectively. The interface dipole and polarizability of the benzene-silver complex have been determined to be 5.4 +/- 1.8 D and 14 +/- 10 A3, respectively. Repulsive interactions in the monolayer were found to lower the desorption energy from the zero-coverage value by 14.8 kJ/mol. Leading edge analysis of the multilayer peak yielded a desorption energy of 40.9 +/- 0.7 kJ/mol.

Temperature programmed desorption (TPD) is used for examining surface binding, intermolecular interaction, and morphology of mono-and multilayer films of tetracene on Ag(111). TPD of monolayer tetracene revealed strong inter-adsorbate repulsion caused by interaction among interface dipoles resulted from charge-transfer bonding. A modified Albano model, in which a point interface dipole is assigned to each of the aromatic rings of tetracene, is proposed to account for the interfacial dipole interaction at short range. It is found that desorption energy at the zero-coverage limit is 142 ( 7 kJ/mol. The interface dipole is determined as 8.2 ( 2.1 D, which corresponds to a partial charge transfer of 0.4 e per tetracene molecule to the Ag substrate. At full monolayer coverage, the strong inter-adsorbate repulsion reduces the desorption energy to 105 ( 14 kJ/mol. Annealing at elevated temperature (350-400 K) but below desorption temperature, on minute time scale followed by cooling, appears to produce a more stable structure. Multilayer TPD spectra show three separate half-order desorption peaks that merge into one bulk peak at higher coverage. The half-order kinetics agrees with the previously reported Stranski-Krastanov growth mode in which islands with high height-towidth ratio are formed. The desorption energies for these peaks are 100 ( 7, 110 ( 10, and 116 ( 4 kJ/mol respectively. Upon annealing, the lower energy structure transform into the higher energy ones. † This article was originally intended to be a contribution to the "Giacinto Scoles Festschrift".

Ultrathin (1−10 layers) aniline and pyridine films deposited on a Ag(111) surface at 90 K have been examined using thermal programmed desorption (TPD). Multiple desorption peaks have been observed for both systems showing film growth from the first chemisorbed layer to the bulk structure through multiple intermediate layers. The growth mechanisms for the two systems are, however, apparently different. Aniline films are formed one layer at a time with five thermodynamically stable layered structures. In contrast, pyridine films grow through metastable phases, similar to benzene film deposition on metal. In particular, a skin layer over the bulk phase is proposed to account for the TPD spectra. The difference in film growth mechanisms is attributed to the effect of intermolecular interactions.

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