Impact of ice structure on ultrafast electron dynamics in D2O clusters on Cu(111).

The structure of D2O clusters on a Cu(111) surface and the femtosecond dynamics of photoexcited excess electrons are investigated by low-temperature scanning tunneling microscopy and two-photon photoemission spectroscopy. Two types of amorphous ice clusters, porous and compact, which exhibit characteristic differences in electron dynamics, are identified. By titration with Xe we show that in both structures solvated electrons preferentially bind on the cluster surface.

Kinetics of electron-induced decomposition of CF2Cl2 coadsorbed with water (ice): a comparison with CCl4.

The kinetics of decomposition and subsequent chemistry of adsorbed CF(2)Cl(2), activated by low-energy electron irradiation, have been examined and compared with CCl(4). These molecules have been adsorbed alone and coadsorbed with water ice films of different thicknesses on metal surfaces (Ru; Au) at low temperatures (25 K; 100 K). The studies have been performed with temperature programmed desorption (TPD), reflection absorption infrared spectroscopy (RAIRS), and x-ray photoelectron spectroscopy (XPS). TPD data reveal the efficient decomposition of both halocarbon molecules under electron bombardment, which proceeds via dissociative electron attachment (DEA) of low-energy secondary electrons. The rates of CF(2)Cl(2) and CCl(4) dissociation increase in an H(2)O (D(2)O) environment (2-3x), but the increase is smaller than that reported in recent literature. The highest initial cross sections for halocarbon decomposition coadsorbed with H(2)O, using 180 eV incident electrons, are measured (using TPD) to be 1.0+/-0.2 x 10(-15) cm(2) for CF(2)Cl(2) and 2.5+/-0.2 x 10(-15) cm(2) for CCl(4). RAIRS and XPS studies confirm the decomposition of halocarbon molecules codeposited with water molecules, and provide insights into the irradiation products. Electron-induced generation of Cl(-) and F(-) anions in the halocarbon/water films and production of H(3)O(+), CO(2), and intermediate compounds COF(2) (for CF(2)Cl(2)) and COCl(2), C(2)Cl(4) (for CCl(4)) under electron irradiation have been detected using XPS, TPD, and RAIRS. The products and the decomposition kinetics are similar to those observed in our recent experiments involving x-ray photons as the source of ionizing irradiation.

Negative ion formation in electron-stimulated desorption of CF2Cl2 coadsorbed with polar NH3 on Ru(0001).

Photon-induced dissociation of CF2Cl2 (freon-12) in the stratosphere contributes substantially to atmospheric ozone depletion. We report recent results on dissociation and negative ion formation in electron-stimulated desorption (ESD) of CF2Cl2 on Ru(0001), when CF2Cl2 is coadsorbed with a polar molecule (NH3), for electron energies ranging from 50 to 300 eV. Two different time-of-flight methods are used in this investigation: (a) an ESD ion angular distribution detector with wide collection angle and (b) a quadrupole mass spectrometer with narrow collection angle and high mass resolution. Many negative ESD fragments are seen (F-, Cl-, FCl-, CF-, F2-, and Cl2-), whose intensities depend on the surface preparation. Using both detectors we observe a giant enhancement of Cl- and F- yields for ESD of CF2Cl2 coadsorbed with approximately 1 ML of NH3; this enhancement (>10(3) for Cl-) is specific to certain ions, and is attributed to an increased probability of dissociative electron attachment due to "trapped" low-energy secondary electrons, i.e., precursor states of the solvated electron in NH3. In further studies, the influence of polar NH3 spacer layers (1-10 ML) on ESD of top-layer CF2Cl2 is determined, and compared with thick films of condensed CF2Cl2. The magnitudes and energy dependences of the Cl- yields are different in these cases, due to several contributing factors.