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

Abstract

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.

Electron-driven chemistry of halogenated compounds in condensed phases: Effects of solvent matrices on reaction dynamics and kinetics

“…Thus, Lu and Madey discovered that co-adsorption of CF 2 Cl 2 with H 2 O on a Ru(0001) substrate leads to a $100 Â enhancement of Cl À ion yields in electron-stimulated desorption, compared to the yield from CF 2 Cl 2 adsorbed alone [14,30]. Moreover, Lu and Sanche have also reported a large increase in the charge trapping coefficient [30,32,70] of CF 2 Cl 2 co-adsorbed with H 2 O [46]. This was ascribed to an increase in the DEA cross-section of CF 2 Cl 2 : ¼ 1.3 Â 10 À14 cm 2 vs. ¼ 1.4 Â 10 À15 cm 2 in the absence of co-adsorbed H 2 O.…”

“…For example, there have been a number of studies detailing negative ion desorption from condensed-phase CF 2 Cl 2 [68,69]. In contrast to gas-phase studies on CF 2 Cl 2 , there is general agreement that the total yield of ejected F À ions during low-energy electron excitation of condensed CF 2 Cl 2 considerably exceeds the Cl À yield [70]. This difference is attributed to the low kinetic energies of Cl À ions, which results in the majority of the Cl À ions remaining trapped on the surface as they have insufficient kinetic energies to overcome the polarization barrier.…”

“…NH 3 , H 2 O) has been observed to result in an increase in the anion ESD yield, an effect ascribed to an increased charge trapping coefficient [30,32,70]. Thus, Lu and Madey discovered that co-adsorption of CF 2 Cl 2 with H 2 O on a Ru(0001) substrate leads to a $100 Â enhancement of Cl À ion yields in electron-stimulated desorption, compared to the yield from CF 2 Cl 2 adsorbed alone [14,30].…”

“…Figure 6 shows a comparison of the gas-phase negative ions produced during electron irradiation of neat CF 2 Cl 2 and CF 2 Cl 2 co-adsorbed with water. The spectra measured for films containing Freon (top three spectra) show strong F À ion signals, indicating that electron-induced desorption of the relatively low mass F À ion from CF 2 Cl 2 dominates [70]. The Cl À fragment is the second most abundant ion fraction for thick (!…”

Revisiting the electron stimulated desorption of anions from thin films of CF2Cl2

“…The other desorbed anions have been observed previously only when CF 2 Cl 2 films of varying preparation (i.e., deposited with Xe and with NH 3 on Ru(0 0 0 1)) were exposed to ∼170 eV electrons [14]. The exact mechanisms underlying their appearance in these experiments are unclear.…”