Calorimeter for adsorption energies of larger molecules on single crystal surfaces

A calorimeter for measuring heats of adsorption of large molecules on single crystal surfaces is described. It extends previous instrumentation for single crystal adsorption calorimetry by adding the capability for measuring larger (lower vapor pressure) molecules. This is achieved using a chopped and collimated (similar to4 mm diameter) molecular beam capable of stable 100 ms pulses of low vapor pressure substances, and a line-of-sight modification of the King and Wells method for measuring their sticking probabilities at the single crystal's surface. The heat input to the single crystal due to adsorption is detected using a pyroelectric polymer ribbon pressed against the back of the single crystal, following our previous calorimeter design. Measurements of benzene adsorption on Pt(111) prove the capability to produce a highly stable beam of flux similar to2x10(14) molecules/(cm(2) s) and measure adsorption energies with an absolute accuracy of similar to5% and a pulse-to-pulse standard deviation of 2 kJ/mol. (C) 2004 American Institute of Physics.