An adsorption calorimeter for studies on well-defined single crystal surfaces under ultrahigh vacuum conditions is now available, based on supersonic molecular beam dosing onto ultrathin metal single crystals. Here we discuss the relationship between the calorimetric heat of adsorption as measured in this system and the related parameters: the differential heat of adsorption, the isosteric heat, and the Arrhenius desorption energy. Coverage-dependent calorimetric heats of adsorption and sticking probabilities for CO on Ni{III}, {11O}, and {100} are presented, and comparisons made with literature values for isosteric heats and Arrhenius desorption energies. At intermediate coverages some significant discrepancies occur which are attributed to a temperature-dependent adlayer structure. By combining sticking probability with heat measurements at high coverage, at 300 K, where significant desorption occurs, the desorption preexponential has been accurately determined; differential entropies of adsorption are also obtained. Differences in initial heats of adsorption and in the coverage dependencies for the three crystal planes are discussed, particularly in relation to surface stoichiometry, and to CO-CO interactions.

Using single-crystal adsorption calorimetry, heat data have been measured for the adsorption of oxygen on the three low-index planes of Ni at 300 K along with corresponding sticking probabilities. New data are presented with coadsorbed potassium on each plane, and temperature-dependent data for O2/Ni{100}. The initial heats of adsorption of oxygen on Ni{100}, {110}, and {111} are 550, 475, and 440 kJ (mol O2)−1, respectively, at 300 K, and the heat is found to drop rapidly with coverage in the chemisorption regime, indicating strong interadsorbate interactions. However, this rapid decline is not seen with coadsorbed potassium, a difference discussed both in terms of electron availability and coadsorbate attractions. The integral heats of adsorption for oxide film formation are 220, 290, and 320 kJ mol−1, respectively. Corresponding sticking probability measurements show initial values, all less than unity, of 0.63, 0.78, and just 0.23, again for the {100}, {110}, and {111} surfaces in that order. The coverage dependence of the sticking probability is consistent in each case with a passivating oxide film four layers thick. Comparable data for Ni{100} obtained using a pyroelectric detector gave good agreement with the conventional results at 300 K. At 410 K, however, the heat-coverage curve was flat up to 0.25 monolayers. Data were also obtained at 90 K. Analysis and Monte Carlo simulation of the temperature-dependent adsorption heat curves indicates that the large drop in adsorption heat with coverage seen at room temperature is consistent with a local second-nearest neighbor adatom–adatom repulsion rather than a long-range electronic effect.

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