The adsorption and decomposition of cyanogen halides, XCN (X ) Br, Cl), on Si(100) is investigated utilizing X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). For submonolayer exposures, XPS indicates that the CN triple bond of XCN remains intact upon adsorption at 100 K. The UPS spectrum contains two peaks assigned to the π-electrons in the CN triple bond. The splitting indicates that some fraction of the XCN molecules adsorbs molecularly at low temperature. XPS analyses of the C 1s photoelectron peak following submonolayer exposure at low temperature suggest a greater fraction of BrCN (60%) adsorbs molecularly than ClCN (40%). XPS and UPS measurements at room temperature show that the X-CN bond breaks, while the CN bond remains intact during room-temperature adsorption on Si(100). Thus, the UPS spectrum of XCN adsorbed at room temperature on Si(100) contains a peak at 6.0 eV due to the unperturbed π electrons of the CN species. Upon annealing a CN-saturated Si(100) surface to higher temperatures, the UPS spectra indicate that the CN bond remains intact until approximately 700 K. Simultaneous changes in the C 1s and N 1s photoelectron peaks are consistent with the idea that CN bond cleavage is correlated with silicon carbide and nitride formation. These results are compared with a previous study of ICN adsorption on Si(100).
The adsorption kinetics and dynamics of typical combustion gases have been studied on CaO(100) by means
of temperature programmed desorption and molecular beam scattering. In addition, the sample has been
characterized by Auger electron spectroscopy. Whereas CO interacts rather weakly with CaO and adsorbs
molecularly on pristine and oxygen vacancy sites, CO2 and NO interact strongly with CaO, leading to a
variety of structures in TPD. CO2 adsorption leads to the formation of carbonates. NO adsorption results in
the desorption of N2 and N2O species presumably by the formation of NO dimers stabilized at defect sites,
which is consistent with prior reports. In assisting the assignment of the TPD structures that have been seen,
density functional cluster calculations have been performed and are compared with results from prior studies.
Trends seen for the initial adsorption probabilities, S
0, are consistent with a simple mass-match model, S
0−CO > S
0−CO2. The coverage dependence of the adsorption dynamics is dominated by precursor effects for
CO2 but shows direct Langmuirian adsorption dynamics of CO.
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