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  • 1
    Electronic Resource
    Electronic Resource
    Substrate mediated photochemistry on a molecular surface: OCS on Ag clusters (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 2474-2479 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photochemistry of carbonyl sulfide (OCS) adsorbed to small silver clusters is shown to exhibit a striking odd–even dependence on the number of Ag atoms in the cluster. OCS is found to desorb nondissociatively from even numbered silver clusters. In contrast, on odd silver clusters, a new product channel corresponding to AgnS is observed. Parallels are found with the photochemistry of adsorbates on extended surfaces. Cross-section measurements for OCS desorption from Ag10 and wavelength-dependent measurements of OCS dissociation on Ag9 both indicate that electronic excitation of the cluster initiates chemistry, analogous to substrate mediated surface photochemistry. The size dependence is reasonably explained in terms of a charge-transfer mechanism involving an ion-pair state of the AgnOCS complex. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476818
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  • 2
    Electronic Resource
    Electronic Resource
    Reactivity of niobium clusters with nitrogen and deuterium (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 5476-5490 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Absolute rate coefficients are reported for reactions of Nbn clusters (n=2–20) with D2 and N2 at 280, 300, and 370 K. Most clusters are highly reactive but there are conspicuous exceptions at n=8, 10, and 16 for both D2 and N2. The origin of this trend in reactivity with cluster size and the reason why D2 and N2 show similar trends are discussed. Density functional theory (DFT) electronic structure calculations have been used to investigate the details of the reactions for the smallest clusters Nb2 and Nb8 with H2 and N2. The steric and electronic requirements for dissociation of H2 and N2 are described in terms of frontier orbital interactions. The main conclusion from the DFT calculations is that complete dissociation of H2 or N2 requires charge transfer by transit of an avoided crossing between neutral and ionic potentials. This idea is extended to larger clusters by using a simple charge transfer model that predicts an inverse correlation between reactivity and an appropriately defined effective ionization potential. Such a correlation is observed and indicates that the effective ionization potential is the dominant influence on reactivity. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475936
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  • 3
    Electronic Resource
    Electronic Resource
    Reaction of tungsten clusters with molecular nitrogen (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 4012-4018 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Reactions of tungsten clusters with molecular nitrogen have been investigated by using a fast-flow reactor equipped with a laser vaporization cluster source and time-of-flight mass spectrometer detector. Absolute rate coefficients are reported for reaction of Wn clusters in the range n=4–26, at temperatures 277, 300, and 370 K in He buffer gas at 1 and 2 Torr pressure. For smaller clusters with n〈15, complexes with N2 are formed with binding energies near 16 kcal mol−1, and act as precursors to dissociation of N2 on the clusters. A sharp jump in the binding energy occurs at a cluster size of 15 metal atoms, and may signal the onset of atomic as opposed to molecular binding of N2 on the cluster. It is suggested that the change in reactivity at n=15 is correlated with a structural transition of the clusters, from relatively close-packed to more open structures. The reactivity of tungsten clusters with N2 is compared with that of molybdenum clusters and tungsten metal surfaces. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471213
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  • 4
    Electronic Resource
    Electronic Resource
    Reaction of molybdenum clusters with molecular nitrogen (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 5539-5547 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Reactions of molybdenum clusters with molecular nitrogen have been investigated by using a fast-flow reactor equipped with a laser vaporization cluster source and laser ionization time-of-flight mass spectrometer for detection of clusters. Absolute rate coefficients are reported for reaction of clusters in the range 3–35 metal atoms, at temperatures 279, 300, and 372 K in He buffer gas at 1 and 2 Torr pressure. Rate coefficients generally decrease with increasing temperature, as expected for a mechanism involving formation of a weakly bound precursor complex. By comparison with surface chemistry studies of nitrogen adsorption on molybdenum and tungsten, the reactivity trend with cluster size is interpreted as reflecting the geometric configurations of metal atoms on the surfaces of the clusters. It is suggested that structural transitions occur in molybdenum clusters in the size range 13–20 metal atoms. Evidence is described that suggests that cluster reaction products have nitrogen bound in both atomic and molecular forms. Bulk surfacelike reactivity of the clusters appears to be established in the range 25–30 metal atoms.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470538
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  • 5
    Electronic Resource
    Electronic Resource
    Optical absorption spectra of Au7, Au9, Au11, and Au13, and their cations: Gold clusters with 6, 7, 8, 9, 10, 11, 12, and 13 s-electrons (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 3506-3513 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The optical absorption spectra of a series of small gold clusters and their cations have been measured, between 1.9 and 5.6 eV, using a method based upon the photodepletion of a molecular beam of their van der Waals complexes containing one and two xenon atoms. This method provides size-specific information even though the molecular beam contains a wide range of cluster sizes. There is little difference between the spectra of complexes containing one or two xenon atoms. However there is a pronounced odd–even alternation in the spectra of gold clusters with differing numbers of valence s electrons. This alternation is described in terms of a simple electron pairing scheme. The spectrum for Au13 is in reasonable agreement with Dirac scattered-wave molecular orbital considerations for icosahedral Au13 [A. F. Ramos, R. Arratia-Perez, and G. L. Malli, Phys. Rev. B 35, 3790 (1987)]. This description of the molecular and electronic structure of small gold clusters in terms of localized molecular orbitals is contrasted with other models based upon jellium potentials and delocalized excitations that have been used to describe small clusters of alkali metals and silver. The bonding in gold clusters is influenced by relativistic effects that increase the degree of sd hybridization in the molecular orbitals. Even though gold clusters can be described in this way, some evidence for electron shells is also presented. Thus, it is concluded that structural motifs other than jellium potentials can lead to shell structure in cluster properties.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467535
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  • 6
    Electronic Resource
    Electronic Resource
    Direct measurement of the rate constant for .beta.-scission of the cumyloxyl radical by laser flash photolysis with time-resolved IR detection (1989)
    s.l. : American Chemical Society
    Journal of the American Chemical Society 111 (1989), S. 9269-9270 
    Details
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/ja00208a042
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  • 7
    Electronic Resource
    Electronic Resource
    The first ionization potential of zirconium atoms determined by two laser, field-ionization spectroscopy of high lying Rydberg series (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 3194-3197 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An accurate value for the first ionization potential of zirconium of 53 506.0(3) cm−1 has been obtained from convergence limits of high lying Rydberg series observed using two laser, field- ionization spectroscopy of an atomic beam of zirconium. The new value is consistent with analysis of s-electron binding energies of other second transition series elements.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.450986
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  • 8
    Electronic Resource
    Electronic Resource
    Branching ratios for chemical reaction in collisional quenching of excited 5s1 2S1/2 gallium atoms (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 6852-6861 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report cross sections for fluorescence quenching and branching ratios for chemical reaction in collisional deactivation of Ga(5s1 2S1/2) by added gases at room temperature following resonance 4p1 2P(open circle)1/2 →5s1 2S1/2 excitation at 403.30 nm with a pulsed laser. Total quenching cross sections are obtained from Stern–Volmer analysis of fluorescence lifetimes. Chemical contributions to fluorescence quenching are investigated by a pump and probe technique involving saturation of the resonance transition by a pump laser pulse to produce a known fraction of gallium atoms in the excited state, and measurement of the depletion of the gallium atom concentration due to chemical reaction in the excited state. Relative concentration measurements in the presence and absence of the saturating pump pulse are by resonance fluorescence excitation by a probe laser pulse suitably delayed relative to the pump pulse. An analysis of the experiment in terms of rate equations shows how branching ratios for chemical reaction may be obtained from the depletion measurements. Fluorescence quenching cross sections (A(ring)2) are large for N2O (99±10), CO2 (100±20), CH4 (55±6), C2H6 (77±20), C3H8 (100±20), n-C4H10 (130±20), and C2H4 (75±20), moderately large for CO (11±4) and N2 (8±2), and very small for CF4 (〈0.3) and H2 (〈0.05). Among the efficient quenchers only C2H4 showed no detectable contribution from chemical quenching. Branching ratios for chemical removal of Ga(5 2S1/2) are N2O (0.96+0.04−0.1), CO2 (0.55±0.1), CH4 (0.27±0.07), C2H6 (0.33±0.1), C3H8 (0.26±0.1), and C2H4 (0.0+0.05−0.0). Results for H2 and the alkane hydrocarbons are discussed with reference to simple concepts of orbital interactions in the entrance channel.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452384
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  • 9
    Electronic Resource
    Electronic Resource
    Multiphoton dissociation of Ga(CH3)3 (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 5028-5042 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: UV and visible multiphoton dissociation (MPD) processes of Ga(CH3)3 have been investigated by time-resolved detection of Ga(4 2PJ ) (J=1/2, 3/2) and Ga(5 2S1/2) following pulsed laser photolysis in a low pressure gas cell, and by mass spectrometric detection of ionic photofragments in a molecular beam. The relative abundances of neutral and ionic fragments were monitored as a function of photolysis wavelength and flux. Measurements of the anisotropy of the velocity distribution and the average kinetic energy of nascent Ga(4 2PJ ) are also reported. Under conditions of moderate photolysis flux ((approximately-equal-to)108 W cm−2), MPD of Ga(CH3)3 in the wavelength range 420–520 nm produced mainly neutral fragments, including Ga(4 2PJ ) and a metastable molecular fragment which is identified as GaCH3. Photolysis at 222 nm produced GaCH3 to the exclusion of Ga(4 2PJ ), or primarily ionic fragments at relatively high flux. At 308 nm, ionic Ga(CH3)+n species were produced in relatively high yield even in the limit of low photolysis flux. The mechanisms of MPD and MPI of Ga(CH3)3 are discussed with reference to simple rate equations models.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.449769
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  • 10
    Electronic Resource
    Electronic Resource
    Branching ratios and bond dissociation energies from the excimer laser photolysis of group 6 metal carbonyls (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 5471-5480 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Photolysis of the group 6 (Cr, Mo, W) metal carbonyls in the gas phase, at excimer laser wavelengths, may lead to more than one primary product. Branching ratios between these products have been measured as a function of photolysis wavelength, buffer gas pressure, and temperature using time-resolved infrared spectroscopy. The results are modeled using a sequential dissociation mechanism in which branching ratios are determined by competition between unimolecular dissociation and collisional relaxation. The sensitivity of the results to thermochemical input parameters and assumptions concerning energy disposal mechanisms is discussed. Under qualified assumptions the branching ratio measurements provide estimates for CO bond dissociation energies for coordinatively unsaturated metal carbonyls. For Mo and W the individual bond dissociation energies are close to the average values but for Cr the first three ligands coordinated are significantly less strongly bound than the last three. This finding is discussed in terms of recent ab initio calculations on bonding in metal carbonyls.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460482
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