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  • Electronic Resource  (65)
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  • 1
    Electronic Resource
    Electronic Resource
    Stabilization of transient negative ions by vibrational energy transfer: A cluster and thin film study on SF6 and C6F6 (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 9046-9051 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Resonant low energy electron attachment to SF6 and C6F6 in argon clusters and on a multilayer krypton film is presented. The energy dependence of the absolute attachment cross sections for the condensed molecules is determined in the range 0–8 eV by measuring charge localization on the film. We find a cross section maximum of (3.0±0.8)×10−15 cm2 at 60 meV for SF6− formation, which is close to the gas phase value. For C6F6, we obtain a maximum value (4.8±1.2)×10−15 cm2 at 75 meV. In contrast to isolated gas phase molecules, where both ions are only observed in a very narrow energy range around 0 eV, SF6− resulting from electron attachment to clusters is observed up to 1.5 eV. The same is true for the condensed submonolayers, where charge localization is observed up to 1.5 eV for SF6. The extension of stable SF6− formation to much higher energies in condensed media is attributed to the transfer of intramolecular vibrational energy from SF6− to phonon vibrational modes of the Ar cluster or the Kr multilayer film. C6F6− formation is also observed up to 1.7 eV from clusters, but on the film, charge localization is only detected up to 0.7 eV. This difference is explained by the orientation of the C6F6 molecules on the rare gas film, which diminishes vibrational energy transfer to the phonon bath of the Kr lattice. Similar results are also obtained for the dimeric configurations of these molecules seeded into Ar clusters. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481516
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  • 2
    Electronic Resource
    Electronic Resource
    Reactive scattering of O− in organic films at subionization collision energies (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 8635-8640 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Anion desorption stimulated by the impact of 0–20 eV electrons on O2/hydrocarbon mixed films is reported. It is shown that part of the H−, OH−, CH−, and CH2〈sup ARRANGE="STAGGER"〉− desorption yields from O2/hydrocarbon films is the result of reactive scattering of O− fragments produced via dissociative electron attachment (DEA) in the alkane and alkene thin films. These results support the interpretation that the DEA O− react with the hydrocarbon molecules to form a transient molecular anion complex which, in addition to autodetachment, may decay by dissociation into various anion and neutral fragments, and thus cause chemical modification of the solid. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477530
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  • 3
    Electronic Resource
    Electronic Resource
    Low energy electron total scattering cross section for the production of CO within condensed methanol (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 3478-3484 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We propose a method based on high-resolution electron-energy-loss spectroscopy to measure in situ the neutral fragmentation products arising from the impact of low energy electrons on thin solid films at low temperature. We show more particularly that the detection of electronic states from a dissociation product is a good alternative when the corresponding vibrational levels are obscured by those of the deposited film. In the case of thin methanol film condensed at 18 K, we find that low energy electrons can dissociate the CH3OH molecules into CO fragments that remain within the film. The production of CO fragments, clearly identified from its lowest electronic state a 3Π, is studied as a function of the electron dose, electron energy, and film thickness. The energy dependence of the CO production rate, which is also calibrated in terms of an electron total scattering cross section σp, is characterized by an energy threshold at 8 eV, a shoulder at about 11.5 eV, a broad maximum centered around 14 eV, and a rise above 19 eV. A value of σp(approximate)4.2×10−18 cm2 is obtained at 14 eV. The shoulder and the broad maximum are specifically attributed to the (centered ellipsis)(6a′)1(3sa′)2,2A′, (centered ellipsis)(1a″)1(3sa′)2,2A″, and (centered ellipsis)(5a′)1(3sa′)2,2A′ core excited electron resonances, which decay into their parent repulsive states. The rise above 19 eV is correlated to the lowest dissociative photoionization processes known to produce neutral CO in the gas phase. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474485
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  • 4
    Electronic Resource
    Electronic Resource
    Sample morphology and porosity in electron stimulated desorption: N2* from N2 adsorbed onto glassy and crystalline n-hexane (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 8112-8118 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The electron-stimulated desorption of metastable molecular nitrogen (N2*) from N2 condensed onto n-hexane spacer films deposited on Pt(111) is investigated as a function of electron-impact energy (5–25 eV), and thickness (1–20 monolayers) and deposition temperature (T=20–80 K) of the n-hexane layer. At low T, the N2* yield is found to be inversely proportional to the thickness of the amorphous n-hexane layer. This behavior is explained by the existence of a porous structure in the n-hexane film which efficiently absorbs N2. The metastable signal is also strongly dependent on the temperature of n-hexane deposition, which controls the morphology of the spacer from amorphous at low T to crystalline at T〉70 K. Increasing the deposition temperature of n-hexane from 20 to 70 K results in an increase of the N2* signal due to the decrease of the pore volume. Formation of the n-hexane layers in the crystalline state at higher temperatures further inhibits N2 migration into the spacer which results in further increases in the magnitude of the N2* desorption signal. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.478714
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  • 5
    Electronic Resource
    Electronic Resource
    Low-energy electron-energy-loss spectroscopy of condensed acetone: Electronic transitions and resonance-enhanced vibrational excitations (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 6707-6715 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report electron-energy-loss spectroscopy, within the incident electron energy range 1 to 19 eV, of solid films of acetone condensed at 18 K. The strong Rydberg progressions, which usually dominate the spectra in the gas phase, are found to completely disappear in the solid phase. In the absence of these transitions, the remaining broad bands centered at 4.3, 4.5, 6.2, 8.7, and 9.8 eV energy loss can be assigned to the 1 3A2(n→π*), 1 1A2(n→π*), 1 3A1(π→π*), 1 3B1(σ→π*), and 2 3A2(σ→π*) valence electronic transition of acetone, respectively. A broad feature ranging from 11 to 16 eV and having a maximum around 13.8 eV is ascribed to several overlapping autoionizing excited states. From a comparison with infrared and Raman spectra, the energy-loss peaks observed below 1 eV are found to be due to excitation of the fundamental, overtone, and combination vibrational modes of the molecule. Their incident energy dependence is showing broad vibrational enhancement maxima at 4, 7, and 9 eV, which are attributed to the formation of single-particle or shape resonances of 2B1, 2A1, and 2A2 (or 2B2) symmetries, respectively. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481245
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  • 6
    Electronic Resource
    Electronic Resource
    Absolute cross sections for dissociative electron attachment to condensed CH3Cl and CH3Br: Effects of potential energy curve crossing and capture probability (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 749-760 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report cross sections for the trapping of 0–10 eV electrons by CH3Cl and CH3Br physisorbed onto a Kr covered Pt substrate, measured as a function of Kr film thickness and methyl halide concentration. The molecules stabilize electrons incident at the surface by the dissociation of transient CH3Cl− and CH3Br− ions into an atomic anion and a neutral fragment [dissociative electron attachment DEA]. For CH3Cl, the condensed phase absolute DEA cross section at (approximate)0.5 eV, reaches 13×10−18 cm2±50%, which is 104–106 times larger than the gas phase cross section. At higher energies (5–10 eV) for CH3Cl, our measurements provide a lower limit for the DEA cross section. For CH3Br, the maximum DEA cross section occurs below the vacuum level; we measure an absolute magnitude of 3.0×10−16 cm2±50% near 0 eV, which is 100 times larger than the corresponding gas phase value. These enhancements in cross section arise from the lowering of the potential energy surfaces of intermediate anions due to polarization induced in the Kr layer and metal substrate. An increase in DEA cross section with a reduction in the distance of transient anions from the metal surface, is explained by the effect of image charges on the energy at which anion and neutral ground state potential energy curves cross. Below thicknesses of 5 ML of Kr, a decrease in DEA cross section is observed and attributed to a reduction in the electron capture probability of the halide due to competition with transfer to the metal substrate. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473163
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  • 7
    Electronic Resource
    Electronic Resource
    Charge stabilization by chloromethane molecules on multilayer Kr films (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 4258-4266 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have measured the charge trapping cross section for the chloromethane molecules CCl4, CHCl3, and CH2Cl2 physisorbed onto Kr films. Within the 0–10 eV range investigated, charge is stabilized at the surface as atomic anions which are formed by dissociative electron attachment (DEA) to the molecules. Intermediate anion states previously observed in gas phase experiments are apparent. Below ≈4 eV electron impact energy, the charge trapping cross section becomes an absolute cross section for DEA because of the absence of a desorbed ion signal. At higher energies, our results provide a lower limit for this same quantity. Comparisons with gas phase cross sections, show substantial and systematic differences that can be understood in terms of the interaction between the intermediate anion states and electronic polarization field they induce on the Kr surface. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471236
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  • 8
    Electronic Resource
    Electronic Resource
    Electron stimulated desorption from PF3 adsorbed on Pt. I. Positive ions (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 6032-6042 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have studied electron stimulated desorption (ESD) of positive ions from PF3 molecules adsorbed on a Pt substrate over a wide electron energy range (0–175 eV). Electron bombardment of 1 ML PF3 adsorbed on the Pt surface gives rise mainly to an F+ signal, whereas ESD from 6 ML thick PF3 film (thick PF3 layer) leads to P+, PF+, and PF+2 signals, in addition to F+. We find that the onset for F+ desorption from the 1-ML PF3/Pt is at ∼26.5 eV, while the F+ threshold from the thick PF3 layer is ∼28.5 eV. The P+ appearance potential from the thick PF3 layer is ∼23 eV. The ESD F+ ion energy distribution has a peak energy of ∼4 eV for all electron impact energies and a full width at half maximum (FWHM) of ∼3 eV. The P+ ions desorb with a peak energy of ∼2 eV under 55 eV electron impact; the FWHM of the P+ energy distribution is ∼2 eV. We suggest that the near threshold P+ formation from PF3 corresponds to the excitations of the 6a1 level, while the F+ threshold for adsorbed PF3 on the Pt surface is due to the excitation of the F 2s level. Our results suggest that beyond near threshold ((approximately-greater-than)32 eV), the excitation of the F 2s level also contributes significantly to the formation of P+ and PF+ ions from adsorbed PF3. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472439
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  • 9
    Electronic Resource
    Electronic Resource
    Electron stimulated desorption from PF3 adsorbed on Pt. II. Negative ions (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 6043-6051 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have studied electron stimulated desorption (ESD) of negative ions from PF3 molecules adsorbed on a Pt substrate over a wide electron energy range (0–175 eV). ESD from adsorbed PF3 gives rise to several negative ion fragments: F− (predominantly), F−2, P−, and PF−. The F− yield produced in the electron energy range 0–15 eV proceeds via dissociative electron attachment (DEA); the F− yield exhibits a peak around 11.5 eV with an onset around 7.5 eV. At electron energies above ∼15 eV, F− ions are produced via dipolar dissociation (DD). We have found that the F− ions produced from a 1 ML PF3/Pt surface via a DEA process with 11.5 eV electron impact desorb with a peak kinetic energy of ∼0.7 eV, while the F− ions generated via DD by 175 eV electron impact desorb with a peak kinetic energy of ∼1.2 eV. The F−2 yield curve also shows a peak at ∼11.5 eV; the onset of the F−2 yield from adsorbed PF3 is ∼9 eV. The F−2 yield in the electron range 9–15 eV is initiated via DEA. The P− signal from PF3 adsorbed on Pt has an onset at ∼16 eV. We have identified some possible DEA and DD processes leading to desorption of negative ions from adsorbed PF3. We suggest that Rydberg core-excited (1-hole, 2-electron) transient anion states of PF3, formed by capture of low energy electrons, dissociate to produce the F− and F−2 ions for E(e)〈15 eV. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472440
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  • 10
    Electronic Resource
    Electronic Resource
    Electron stimulated desorption of O− and metastable CO* from physisorbed CO2 (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 6775-6782 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report electron stimulated desorption (ESD) measurements of O− yields produced by dissociative electron attachment (DEA) to physisorbed CO2. The molecules are condensed at about 17–20 K on polycrystalline Pt, either as pure multilayer films, or in submonolayer (ML) quantities onto thick rare gas substrates. For the pure disordered multilayer solids, we observe four peaks in the O− yield function at incident electron energies, E(e), of about 4.1, 8.5, 11.2, and 15 eV. The lowest two are assigned, respectively, to the 2Πu and 2Πg resonance states of CO−2, which dissociate into O−(2P)+CO(X 1Σ+), and are known to dominate the gas phase DEA O− production cross section for E(e)≤20 eV. Measurements of ESD CO* metastable yields from similar CO2 multilayer solids on Pt(111), also presented here, suggest that the 11.2 and 15 eV O− peaks are associated with the manifold of close-lying CO2*− states which dissociate into O−(2P)+CO* (a 3Π, a′ 3Σ+, or d 3Δ). For 0.15 ML of CO2 physisorbed on 20 ML thick rare gas substrate films significant sharp enhancements (fwhm ≤0.5 eV) are observed in the ESD O− yields at about 0.3–0.4 eV below the lowest substrate exciton energy. These enhancements are attributed to a coupling of the (electron plus exciton) core-excited anion resonances of the rare gas atoms to the dissociative Rydberg anion states of the coadsorbed CO2 at the solid's surface. This is followed by a transfer of the charge and excitation energy to the coadsorbate. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470356
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