J. Phys. Chem. B 2001, 105, 2779-2784
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Factors Influencing Cl- and F- Enhancements in Electron-Stimulated Desorption of CF2Cl2 Coadsorbed with Other Gases Q.-B. Lu† and Theodore E. Madey* Laboratory for Surface Modification and Department of Physics and Astronomy, Rutgers UniVersity, 136 Frelinghuysen Road, Piscataway, New Jersey 08854-8019 ReceiVed: August 31, 2000; In Final Form: January 17, 2001
We have investigated the possible influence of various factors on the giant enhancement of Cl- and F- yields observed for the electron stimulated desorption (ESD) of CF2Cl2 coadsorbed with polar molecules. We have measured the variations of the secondary-electron energy spectrum and the work function of a fractionalmonolayer CF2Cl2 precovered Ru(0001) coadsorbed with polar H2O/NH3 molecules or nonpolar Xe atoms. The changes of Cl-, F-, and F+ yields under bombardment by 250 eV electrons have also been measured for the coadsorption systems. It is observed that the anionic enhancement is much smaller for bombardment by 30 eV electrons. The results indicate that the variations in secondary-electron yield and energy distribution due to coadsorption have a negligible effect on the negative-ion enhancements observed. The work-function decrease caused by coadsorption may have some effect on the anionic enhancements, but it is not the dominant factor. The monotonic attenuation of F+ with increasing coadsorbate coverage indicates that coadsorption does not induce exchange in the adsorption sites of CF2Cl2 and coadsorbates. The observation of giant Cland F- enhancements only for coadsorption of CF2Cl2 with polar CH3OH, but not for coadsorption with Kr, further supports the proposed mechanism that giant negative-ion enhancements are due to dissociation of CF2Cl2 by capture of electrons self-trapped by polar molecules.
I. Introduction Secondary electrons are invariably emitted upon exposure of matter to high-energy radiation, including electron-, photon-, and ion-radiation. Dissociative electron attachment (DEA) is an important process in interactions of low-energy electrons with molecules, in which an electron attaches to a molecule to form a temporary negative ion (TNI) that then dissociates into a neutral and an anionic fragment. This process was first identified in gas-phase experiments,1 and has more recently been studied in the condensed phase.2 DEA cross sections in the condensed phase can be orders of magnitude larger than those in the gas phase.3 This enhancement has been described by R matrix calculations: the polarization interaction between the TNI and the medium lowers the potential curve of the TNI relative to the ground state of the molecule, and thus increases the survival probability of the resonance against autodetachment.3 Even for molecules in the condensed phase, negative ion yields in electron-stimulated desorption (ESD) may be strongly affected by the change in the surrounding environment. For instance, it was observed that the F- yield from ESD of ∼1 monolayer (ML) of PF3 adsorbed on Ru(0001) is enhanced by 1.5-4 when a ∼1 ML rare gas (Xe, Kr) or water film is deposited on the PF3 monolayer (bombardment by 200 eV electrons).4 This enhancement was attributed to a dielectric screening effect:5 the dielectric layer on the surface induces a potential barrier that increases the survival probability of desorbing ions. In ESD of fractional-monolayer molecules (ABs) adsorbed on top of thick rare gas (RG) films (tens of mono* Author for correspondence. Email:
[email protected]. † Current address: Group of the Medical Research Council of Canada in the Radiation Sciences, Faculty of Medicine, University of Sherbrooke, Sherbrooke, Canada, J1H 5N4.
layers), negative-ion enhancements due to formation of anionic excitons (RG*-) were observed.6 A mechanism involving resonant coupling between a core-excited negative-ion state (AB*-) of the molecule and an anionic exciton (RG*-) has been proposed:6 RG*- + AB f AB*- + RG, followed by dissociation of AB*-. It is also observed that the electron attachment cross section is increased by a factor of ∼2.5 when 0.1 ML CF3Cl is sandwiched between two 13 ML Kr layers relative to adsorption on a 13 ML Kr surface; this is attributed to a lowering of the CF3Cl- potential energy surface with respect to groundstate CF3Cl due to the additional polarization of the added Kr layer.7 More recently, Nagesha and Sanche8 have observed much larger cross sections for charge trapping in molecules adsorbed on a glassy n-hexane (nHg) film than adsorbed on a Kr film, which was attributed to electron trapping in image states of the substrate; the nHg film has a negative electron affinity (NEA) (i.e., the vacuum level is below the conduction-band minimum), which enhances the lifetime of electrons trapped in image states. We have recently observed that, with an incident electron beam of hundreds of eV, F- and Cl- yields in ESD of a fractional monolayer of CF2Cl2 adsorbed on Ru(0001) are enhanced by several orders of magnitude up to ∼104 when CF2Cl2 is coadsorbed with some polar molecules, such as H2O and NH3.9,10 In contrast, coadsorption of CF2Cl2 with nonpolar atoms (Xe) or nonpolar molecules (CH4) leads to only small enhancements. As pointed out in refs 9 and 10, the mechanisms mentioned above cannot be the dominant factors contributing to the observed giant enhancements of F- and Cl- yields. Instead, we have proposed another mechanism for the giant anionic enhancements: upon electron bombardment low-energy secondary electrons from the metal substrate are injected into and trapped by polar molecules, and are subsequently transferred to CF2Cl2 molecules that then dissociate via DEA.9,10 Excess
10.1021/jp003161y CCC: $20.00 © 2001 American Chemical Society Published on Web 03/13/2001
2780 J. Phys. Chem. B, Vol. 105, No. 14, 2001 electrons in nonpolar media remain quasi-free and thus have short lifetimes, decaying quickly into the metallic substrate; and therefore the anionic enhancements are much smaller. On the other hand, there are factors that were not considered in detail in refs 9 and 10, which could influence the giant anionic enhancement. Specifically, coadsorbates may modify the secondary-electron spectrum, the work function and the adsorption sites of the CF2Cl2-precovered surface. We describe here the relative contributions of these effects, and show that they do not play a dominant role in the giant anionic enhancement. Transfer of trapped electrons is one of the key reactions in diverse fields ranging from radiation chemistry and biology to disposal of environmentally hazardous waste.11 In particular, chlorofluorocarbons (CFCs) (e.g., CF2Cl2, the so-called Freon12), once widely used as refrigerants and propellants in industries, have been blamed for causing depletion of the ozone layer in the Earth’s atmosphere.12 The presence of the ozone hole in the spring Antarctic stratosphere is suggested to be related to the existence of polar stratospheric clouds (PSCs) that consist of water/nitric acid ice.13 The negative-ion chemistry due to dissociation of CFCs by capture of electrons trapped in polar molecular ices may have far-reaching implications for the ozone hole creation in the polar stratosphere.14 The resultant Cl- ions can be directly or indirectly converted to Cl atoms, which then destroy ozone. It is, therefore, of great importance to further clarify the mechanism for giant negative-ion enhancements. Here, we report experimental results on the variations of the secondary electron spectrum of the CF2Cl2-precovered Ru(0001) with the coadsorption of polar molecules (H2O, NH3) or nonpolar Xe atoms and on the variations of the positive F+ yield with coadsorption, for bombardment by 250 eV electrons; the anionic changes for coadsorption of CF2Cl2 with H2O at an electron beam energy of 30 eV are shown as well. These results are used to evaluate the effects mentioned above on the negativeion enhancement. Moreover, results on coadsorption of CF2Cl2 with polar CH3OH molecules and nonpolar Kr atoms are also shown to further support the proposed mechanism mentioned above. II. Experimental Section The experimental details have been described previously.9,10,14,15 The base pressure of the ultrahigh vacuum (UHV) chamber is 4 × 10-11 Torr. The chamber is equipped with apparatus for Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and thermal desorption spectroscopy (TDS), as well as an electron stimulated desorption ion angular distribution (ESDIAD) detector with time-of-flight (TOF) capability for mass- and angle-resolved ion detection. The ESDIAD/TOF detector is composed of four grids, five microchannel plates, and a position-sensitive resistive-anode encode (RAE). The RAE is connected to a position-analyzing computer to obtain a direct acquisition of two-dimensional (2D) digital data. This detector permits a direct measurement of the yield and the angular distribution of a specific positive or negative ion species. A Ru(0001) crystal can be cooled to 25 K with a closed-cycle helium refrigerator and heated to 1600 K by electron bombardment. The surface is cleaned by sputtering using 1 keV Ar+ and annealing in oxygen; its cleanliness is checked by AES and work-function measurements. The purity of CF2Cl2 and coadsorbate (H2O, NH3, CH3OH, Xe) gases is checked by mass spectra obtained with a quadrupole mass spectrometer (QMS) as each of the gases is introduced into the chamber. CF2Cl2 and coadsorbate gases are dosed normally onto
Lu and Madey
Figure 1. Secondary electron spectra for incidence of 250 eV electrons onto clean Ru(0001), the 0.3 ML CF2Cl2-adsorbed Ru(0001) and the surface subsequently coadsorbed with 1ML Xe, 1ML H2O and 1ML NH3, respectively. Note that the spectra are obtained with a bias voltage of ˜ -10 eV applied to the sample for detecting the low-energy threshold. The shift in the vacuum edge defined as zero eV in the electron spectrum reflects the change of the work function.
the surface in sequence at 25 K with two separate directional dosers, and their relative coverages are determined using TDS. Here, one monolayer (ML) is defined as the coverage corresponding to the saturation of the monolayer peak(s) in thermal desorption spectra, i.e., the onset of the multilayer peak.15 In measurements of negative (positive) ions, a bias voltage of -(+)100 eV applied to the sample for increasing the detection efficiency. The electron current is adjustable between 0.01 and 20 nA with a beam spot ∼1 mm2 and the collection time for each data point is 5 s, to avoid detector saturation and to minimize beam damage. Secondary electron energy spectra were recorded with a concentric hemispherical electrostatic analyzer (50 mm radius), and a bias voltage of -10 V was applied to the sample in order to measure the low-energy threshold. III. Results A. Variations in Secondary Electron Energy Spectra Caused by Adsorption and Coadsorption. Secondary electron spectra for clean Ru(0001), the 0.3 ML CF2Cl2 precovered surface and the subsequent coadsorption of 1 ML H2O (1 ML NH3 or 1 ML Xe) on the CF2Cl2 precovered surface are shown in Figure 1, respectively. First, it is clear that the low-energy threshold (the “vacuum” edge) shifts with the adsorption of CF2Cl2 and the subsequent coadsorption of other species. This is because of the variation in the surface work function. The low-energy threshold is given by ET ) ΦS - ΦA + eVb, where ΦS and ΦA are the work functions of the sample and the analyzer, respectively, and Vb the bias potential applied to the sample.16 From the shifts in ET at constant ΦA and Vb, it is found that adsorption of 0.3 ML CF2Cl2 on the Ru substrate leads to a work function reduction of ∼0.2 eV, and the subsequent coadsorption with H2O, NH3 and Xe lowers the surface work function further by ∼0.8, ∼1.8, and 0.2 eV, respectively. Second, the adsorption of 0.3 ML CF2Cl2 on the clean substrate slightly increases the secondary electron yield in the low-energy range of 1-3 eV, and the subsequent
Electron-Stimulated Desorption of CF2Cl2 coadsorption of 1 ML H2O (NH3) gives rise to a further increase by a factor of 1.2 (1.3). In comparison, the coadsorption with 1 ML Xe results in a larger increase by a factor of 1.8 in secondary electron yield. Third, among the three coadsorbates, Xe leads to the most significant rise in secondary electron yield at electron energies near the vacuum edge (zero eV). These properties are typical for condensed rare gases (Xe, Kr, Ar), due to their large band gaps and small electron affinities. B. Changes in F+ Yield with Coadsorption. The variations of the F+ yield as a function of Xe/H2O coverage for various CF2Cl2 precoverages are shown in Figures 2a,b, respectively, where the data in each set are normalized to the initial value at zero Xe/H2O coverage. In striking contrast to the giant enhancements in negative-ion yields,9,10 the positive-ion yields show a monotonic attenuation with increasing coadsorbate coverage, which is expected due to elastic and inelastic scattering as the desorbed ions pass through the overlayer.4 This fact indicates that coadsorption results in neither the aggregation of CF2Cl2 molecules (the formation of islands), nor an exchange of adsorption sites of CF2Cl2 with coadsorbates, i.e., CF2Cl2 does not appear to segregate to the outer surface. It is of interest to compare trends for the effects of coadsorption on anion and cation desorption yields. The initial decrease in the F+ yield caused by coadsorption slightly varies with the precoverage of submonolayer of CF2Cl2, which is not surprising due to the difference in availability of bare adsorption sites on the substrate for coadsorbate. Moreover, the difference in the F+ decrease with various CF2Cl2 precoverages is far less than 1 order of magnitude. This is in contrast to the strong CF2Cl2-precoverage dependence observed in giant anionic enhancements, which shows differences by several orders of magnitude.9,10 This indicates that the physical origin for the giant anionic enhancement is significantly different from that for the cationic attenuation. C. Coadsorption of CF2Cl2 with Polar H2O: ElectronStimulated Desorption with an Electron Beam of 30 eV. As mentioned above, the trapped-electron-transfer mechanism for the giant anionic enhancement involves secondary electrons emitted from the metal substrate and the associated DEA process. To clarify this mechanism, we can almost switch off the production of secondary electrons from the substrate. It is well-known that secondary electron yield depends on the energy of the primary electron beam. For metals, there is generally a maximum in secondary electron yield at an beam energy ranging from hundreds of eV to 1 keV, depending on materials.17 With Ru(0001), we have observed that secondary electron yield is negligible for primary electron energies e∼45 eV.18 Moreover, negative-ion formation via dipolar dissociation (DD) occurs generally only for electron energies g15 eV. Thus, we can choose a primary electron energy that is sufficiently high to induce dissociation of molecules via DD, but not high enough to produce a significant yield of secondary electrons that can cause DEA. A beam energy around 30 eV should meet these two requirements. The negative-ion yield variations of CF2Cl2 coadsorbed with polar H2O molecules for incidence of 30 eV electrons are shown in Figure 3a,b. Indeed, the anionic enhancements are only a few times, much smaller than those obtained for incidence of 250 eV electrons,9,10 as we expected. This result indicates the importance of secondary electrons to the giant anion enhancement. D. Coadsorption of CF2Cl2 with Polar CH3OH and Nonpolar Kr. To further examine the mechanism for giant anionic enhancements in electron-stimulated desorption of CF2Cl2, we show the negative-ion variations with coadsorption of
J. Phys. Chem. B, Vol. 105, No. 14, 2001 2781
Figure 2. Relative F+ yields from 250 eV electrons incident onto various amounts of CF2Cl2 covered Ru(0001) as a function of Xe coverage (a) or H2O coverage (b), where the data in each set are normalized to the initial value at 0 ML for Xe or H2O coverage.
polar CH3OH molecules and nonpolar Kr atoms. The variations of F- and Cl- yields as a function of CH3OH coverage for various CF2Cl2 precoverages are shown in Figure 4a,b, respectively, where the data in each set are normalized to the initial value at zero CH3OH coverage. The Cl- data for CF2Cl2 coverages
