GROWTH AND CHARACTERIZATION OF SULFURIC-ACID UNDER ULTRAHIGH-VACUUM

The growth and characterization of sulfuric acid (H2SO4) films in ultrahigh vacuum (UHV) is described. The films were synthesized in situ by co-condensing SO3 and H2O onto a single crystal metal surface at 100 K and carefully annealing the mixtures above 160 K. Films were typically 30-50 monolayers thick and were characterized using Fourier transform infrared reflection absorption spectroscopy (FTIRAS), temperature-programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). The FTIRAS, TPD, and XPS spectra show that solid homogeneous mixtures of varying water content can be prepared, from approximately 10 mol % K2SO4 to nearly pure H2SO4. The nearly pure films can be prepared in crystalline and amorphous forms, depending on the annealing procedure employed. Some surface chemical properties of the pure H2SO4 films were investigated. 2-Propanol is readily adsorbed and absorbed by H(2)S(O)4, with extensive dehydration to water and propene occurring at relatively low temperatures (<320 K). Pure solid H2SO4 does not adsorb or absorb HCl at 100 K, in contrast to ice, which readily adsorbs HCl at 100 K, but in striking similarity to concentrated, liquid H2SO4 at room temperature.