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A lowenergy electron diffraction data acquisition system for very low electron doses based upon a slow scan charge coupled device camera Rev.Irradiating molecularly adsorbed NH 3 with controlled fluences of 50 eV electrons on Pt͑111͒ at 100 K leads to NH 2 , NH, and H species. Temperature programmed desorption shows dihydrogen desorption, confirming electron induced decomposition of ammonia, and a new NH 3 desorption peak ͑210 K͒ attributed to hydrogenation of NH 2 . High resolution electron energy loss spectroscopy identifies NH 2 through two vibrational features at 830 and 1392 cm Ϫ1 that are assigned to rocking and wagging modes, and NH through a single strong stretching feature at 3280 cm Ϫ1 . NH was stable up to 400 K where it dehydrogenated, leaving N on the surface, which was stable up to 700 K. In x-ray photoelectron spectroscopy a N(1s) peak, shifted to lower binding energy, appears after electron irradiation.

Using temperature programmed desorption (TPD), predosed oxygen TPD (POTPD), high-resolution electron energy loss spectroscopy (HREELS), and Auger electron and X-ray photoelectron spectroscopy (AES and XPS), we have investigated the chemistry of chloroiodomethane (ClCH21) dosed onto clean, D-covered and 0-covered Pt( 11 1). At or below 100 K, ClCH2I adsorbs molecularly on all these surfaces. While ClCH2I in physisorbed multilayers desorbs reversibly, a significant portion in the first monolayer dissociates during heating. In the absence of D and 0, dissociation begins with C-I bond cleavage at -150 K. Once the C-I bond breaks, several competitive reactions take place below 260 K: (1) hydrogenation of CH2Cl(a) to form CH3Cl(g) beginning near 150 K, ( 2 ) Cl-CH2(a) bond cleavage to form Cl(a) and CH2(a) above 170 K, (3) dehydrogenation of CH2(a) to CH(a) beginning near 180 K and increasing rapidly above 200 K, (4) hydrogenation of CH2(a) to CH4(g) above 170 K, and ( 5 ) HC1 and H2 formation and desorption above 200 K. At 260 K, the surface species are identified as I(a), CH(a), Cl(a), and a small quantity (-0.02 ML) of CH2(a). The remaining CH2(a) reacts with itself and Cl(a) to form CI&(g), HCl(g), and CH(a) at 360 K. Cl(a) remnants react with CH(a) at 415 K, producing HCl(g) and CCH(a). The residual CH(a) fragments react at 520 K, yielding Hz(g), C,(a), and more CCH(a). Finally, dehydrogenation of CCH(a) occurs between 550 and 700 K, releasing H2 and leaving carbon, presumably clustered. Coadsorbed D atoms weaken the bonding between ClCHJ and the surface, decrease the amount of ClCH2I dissociating, and suppress the complete decomposition to carbon for those ClCHJ molecules that do dissociate. In TPD with coadsorbed D, besides the addition products (Le., CH3D, CH2D2 and CH2DCl), there are also H-D exchange products for methane (i.e., CHD3 and CD4) but not for methyl chloride (Le., no CHDzCl and CD3Cl). Coadsorbed 0 atoms attenuate slightly the dissociation of ClCH21, but strengthen its bonding with the surface. With increasing 0 coverage, the yields of CI&, CH3C1, H2, and HC1 (reaction products found in the absence of O(a)) decrease; other reaction productts, H20, C02, CO, CH20, and CH2C12, appear and increase. To our knowledge, this is the first report of formaldehyde produced by the oxidation of a CH2 precursor on Pt(ll1). Reaction paths are discussed, as are the effects of coadsorbed halogen atoms on hydrogenation, C-C coupling, and oxidation of CH2.

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Iodobenzene (C6H5I) dosed on sapphire(0001) at 110 K adsorbs molecularly and desorbs with a peak near 187 K, depending slightly on coverage. Irradiation with 193 nm photons induces dissociation attributable to photon absorption by C6H5I and C−I bond cleavage. A measurable fraction of the products are ejected during irradiation. Normalized to the initial intensities, the I XPS signal decays 2.5 times faster than the C signal. The cross sections for the loss of C6H5I, I, and C are (2.8 ± 0.3) × 10-18, (6.6 ± 0.7) × 10-19, and (2.7 ± 0.3) × 10-19 cm2, respectively. Benzene (C6H6) and biphenyl (C12H10) appear in TPD at temperatures consistent with desorption-limited processes.