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  • American Institute of Physics (AIP)  (15)
  • 1
    Electronic Resource
    Electronic Resource
    Non-Boltzmann rotational and inverted spin–orbit state distributions for laser-induced desorption of NO from Pt(111) (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 5344-5345 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The internal state distributions of NO desorbed from a Pt(111) surface by visible and near-visible laser radiation (355, 532, and 1064 nm) were measured by laser-induced fluorescence. Non-Boltzmann rotational state distributions and inverted spin–orbit populations were observed and both were found to be relatively insensitive to the desorption-laser wavelength. It is suggested that the internal state distributions arise from the charge exchange processes occuring during desorption via a short-lived negative-ion resonance intermediate.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455627
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  • 2
    Electronic Resource
    Electronic Resource
    The populations of bridge and top site CO on Rh(100) vs coverage, temperature, and during reaction with O (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 6710-6721 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The intensities of the stretch modes of CO adsorbed in bridge and top sites on Rh(100) are presented as a function of coverage, temperature, and during reaction with O. Following adsorption at 90 K, the top site is predominantly occupied at low coverages, and at 0.5 atomic monolayers (ML) only the top site is occupied and a c(2×2) low energy electron diffraction (LEED) pattern is observed. With higher coverages the bridge site is increasingly occupied, and at saturation a p(4(2)1/2×(2)1/2)R45° LEED pattern is observed. The occupations of bridge and top sites at a fixed (0.5 ML) total coverage are observed to vary reversibly when the temperature is linearly ramped from 87 to 371 K and back down to 90 K; the difference in the binding enthalpy of bridge and top sites is determined to be 1.10±0.06 kcal mol−1. The enthalpy difference between bridge and top sites is also obtained from temperature programmed EELS (TP-EELS) which follows the site occupations as the temperature is increased past desorption. Surface order is found to have a significant effect on the normalized intensities of CO in conventional EEL spectra. The normalized intensities in angle integrated spectra, obtained by modulating the voltage difference applied to the two halves of the split analyzer input lenses, are insensitive to changes in the degree of surface order. The effect of low frequency frustrated rocking motions of diatomic molecules on the temperature dependence of the intensities observed in electron energy loss spectroscopy (EELS) is discussed. TP-EELS of coadsorbed CO and O allows the direct observation of the changes in the occupation of CO binding sites during reaction, and the results are correlated with LEED and temperature programmed reaction spectroscopy (TPRS) measurements of the reaction. Segregation of the reactants during the reaction is important.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453407
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  • 3
    Electronic Resource
    Electronic Resource
    The influence of adsorbate–adsorbate interactions on surface structure: The coadsorption of CO and H2 on Rh(100) (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 477-490 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The coadsorption of CO and H2 on Rh(100) at 100 K has been studied using temperature programmed desorption (TPD), low energy electron diffraction (LEED), electron energy loss spectroscopy (EELS), and temperature programmed EELS (TP-EELS). The preferred binding sites, long ranged order, and degree of segregation are dependent on the order of adsorption. When H2 is exposed to a CO preexposed surface, segregation of the surface species (atomic H and CO) is observed. The postdosed H2 causes isolated CO molecules to change from the top site to the bridge site, and compresses the c(2×2) CO islands that develop during the CO preexposure. When CO is exposed to a H2 saturated surface (one hydrogen per surface Rh atom) an intimately mixed c(2×2) CO and H structure is formed with all the CO molecules occupying the top site. Strong repulsive CO–H interactions in this mixed adlayer result in two new low temperature H2 TPD states. During the desorption of the lowest temperature H2 TPD peak, the c(2×2) LEED pattern streaks and the CO molecules shift from the top site to the bridge site. It is proposed that the preferred binding site for hydrogen in the c(2×2) bridge bonding CO structure is different from the fourfold hollow site preferred in the c(2×2) top bound CO structure. After the second H2 TPD peak, the remaining adsorbed H and CO segregate, and the CO regions are compressed. The compression is relaxed as the hydrogen desorbs. The development of the surface structures and their influence on the H2 TPD can be qualitatively understood in terms of precursor adsorption of both CO on H covered surfaces and H2 on CO covered surfaces, strong CO–H repulsions, and local poisoning of H2 dissociation by CO.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452587
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  • 4
    Electronic Resource
    Electronic Resource
    Photodissociation dynamics of Mo(CO)6 at 266 and 355 nm: CO photofragment kinetic-energy and internal-state distributions (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 7937-7950 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The internal-state and kinetic-energy distributions of the CO photofragments from the 266 and 355 nm photolysis of Mo(CO)6 have been measured under collision-free conditions using vacuum-ultraviolet laser-induced fluorescence. The rotational-state distributions for CO(v‘=0) and (v‘=1) are well represented by Boltzmann distributions with effective rotational "temperatures'' of Tr(v‘=0)=950±70 K and Tr(v‘=1)=935±85 K for 266 nm and Tr(v‘=0)=750±70 K and Tr(v‘=1)=1150±250 K for 355 nm photolysis. The CO(v‘=1/v‘=0) vibrational-state ratios for 266 and 355 nm photolysis are 0.19±0.03 and 0.09±0.02, respectively. The Doppler-broadened CO photofragment line shapes indicate that the translational energy distributions are isotropic and Maxwellian. There is no photolysis-laser wavelength or internal-state dependence to the extracted translational "temperatures.'' The observed energy partitioning and kinetic-energy distributions are inconsistent with an impulsive ejection of a single CO ligand. CO photofragment line shapes for 266 nm photolysis are not consistent with a mechanism involving the repulsive ejection of the first CO ligand, followed by the statistical decomposition of the Mo(CO)5 fragment. While phase-space theories do not predict quantitatively the energy disposal, the photodissociation mechanism appears to be dominated by statistical considerations. The results also suggest that the photodissociation of Mo(CO)6 at 266 and 355 nm involves a common initial "state'' and that similar exit channel effects are operative.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460128
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  • 5
    Electronic Resource
    Electronic Resource
    Reactive adsorption of H2CO on Ni(110) at 95 K (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 2165-2169 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reactions of H2CO on a clean Ni(110) surface have been studied by high resolution electron energy loss spectroscopy (HREELS) and temperature programmed reaction spectroscopy (TPRS). At 95 K, H2CO adsorbs reactively, producing CO(a), CH3O(a), H(a), and small amounts of paraformaldehyde. Upon thermal treatment the CH3O decomposes to CO(a) and hydrogen while the paraformaldehyde reacts to form a surface intermediate which subsequently decomposes to CO2(g) and H2(g) at 350 K. Coadsorbed D2 is not incorporated into the CH3O(a) suggesting the methoxy is formed via a disproportionation reaction. Mixed paraformaldehyde and solid H2CO multilayers are formed at high exposures.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.449308
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  • 6
    Electronic Resource
    Electronic Resource
    Kinetics of unimolecular decomposition on surfaces: Methanol on Ni(110) (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 2569-2582 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The decomposition of methanol on a clean Ni(110) surface has been studied with temperature programmed reaction spectroscopy, low energy electron diffraction and temperature programmed electron energy loss spectroscopy: a new technique for the characterization of surface dynamic properties. Adsorption of methanol at 170 K results in the formation of a methoxy (CH3O) species. Near 270 K the methoxy decomposes into adsorbed hydrogen and carbon monoxide. For initial methoxy coverages greater than 0.20 monolayer (ML) the decomposition is rate limited by the desorption of product hydrogen. For initial methoxy coverages less than 0.20 ML the abstraction of the first methoxy hydrogen is rate limiting. This step is modeled by first order kinetics with an activation energy of 16 kcal mol−1 and a preexponential factor of 1012 s−1. A normal kinetic isotope effect (kH/kD〉1) is observed confirming that C–H bond scission is the rate limiting step at low coverages.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.449251
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  • 7
    Electronic Resource
    Electronic Resource
    Photodecomposition of Mo(CO)6/Si(111) 7×7: CO state-resolved evidence for excited state relaxation and quenching (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 2929-2939 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: State-resolved detection techniques have been used to characterize the ultraviolet photodecomposition dynamics of Mo(CO)6 on Si(111) 7×7 at 100 K. Details of the excitation/fragmentation mechanism including adsorbate energy transfer were examined by measuring the cross sections and the internal and translational energies of the photoejected CO from submonolayer through multilayer coverage regimes. The CO energy distributions are found to be independent of Mo(CO)6 coverage, and can be characterized by two components with markedly different mean energies. In contrast to the coverage independence of the measured energy disposal, the cross section was found to decrease by a factor of 3 from multilayer coverages to submonolayer coverages.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467605
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  • 8
    Electronic Resource
    Electronic Resource
    Adsorption and photodecomposition of Mo(CO)6 on Si(111) 7×7: An infrared reflection absorption spectroscopy study (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 3187-3200 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The adsorption and photodecomposition of Mo(CO)6 adsorbed on Si(111) 7×7 surfaces has been studied with Auger electron spectroscopy, temperature programmed desorption, low energy electron diffraction and infrared reflection absorption spectroscopy in a single external reflection configuration. The external-reflection technique is demonstrated to have adequate sensitivity to characterize submonolayer coverages of photogenerated Mo(CO)x fragments. It is proposed that the first layer of Mo(CO)6 adsorbs in ordered islands with a Mo(CO)6 atop each adatom of the 7×7 reconstructed Si surface. UV irradiation of these islands produces a carbonyl fragment, identified as chemisorbed Mo(CO)5. The Mo(CO)5 thermally decarbonylates via two subcarbonyl intermediates with little CO dissociation. Photolysis of thicker layers results in the formation of Mox(CO)y dimers/polymers, as evidenced by the appearance of bridging CO, which is attributed to a facile association reaction. The dimer/polymer species correlate with deposition of C and O on the surface.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466410
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  • 9
    Electronic Resource
    Electronic Resource
    State-resolved studies of the laser-induced desorption of NO from Si(111) 7×7: Low coverage results (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 2324-2338 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The results of a quantum-state-resolved study of the laser-induced desorption (LID) of NO from Si(111) 7×7 at a surface temperature of 100 K are reported. All aspects of the LID are found to be sensitive to the initial coverage. The coverage dependence indicates that there are two desorption mechanisms, one operative at low coverages that is quenched with increasing NO exposure, and one operative at high coverage. This report characterizes the low coverage channel. Most of the energy in the desorbed NO occurs as vibration and translation, with the rotations substantially cooler. The desorption is selective for production of the ground spin–orbit state. The energy partitioning shows strikingly little change as the desorption-laser wavelength was varied from 1907 to 355 nm. This, coupled with a quantitative study of the yield over the same photon energy range and selective coadsorption experiments, establishes that the desorption is specifically due to an interaction involving photogenerated holes in the rest-atom localized, intrinsic surface state of the 7×7 reconstructed surface. It is suggested that the surface state hole drives the desorption by neutralization of a NO−δ adsorbate.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462029
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  • 10
    Electronic Resource
    Electronic Resource
    Photodecomposition dynamics of Mo(CO)6/Si(111) 7×7: CO internal state and translational energy distributions (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 7651-7654 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The rotational state and translational energy distributions of CO photodesorption products resulting from the 266 nm photolysis of Mo(CO)6 adsorbed on Si(111) 7×7 with coverages in the multilayer regime are reported. State-resolved measurements show two desorption components with highly disparate energy dispositions. Results for different surface temperatures indicate that the energy content in one component reaches quasi-equilibration with the surface temperature, which is attributed to collisional relaxation of nascent photodecomposition products within the adlayer. The other component exhibits disparate rotational and translational "temperatures'' that are significantly greater than, and independent of, the surface temperature. These nascent photodecomposition products are influenced by both energy quenching effects and dynamical constraints imposed by the existence of the adlayer.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.464705
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