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  • 1985-1989  (18)
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Years
Year
  • 1
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Accounts of chemical research 22 (1989), S. 309-314 
    ISSN: 1520-4898
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 6548-6555 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The competition between direct-inelastic and trapping-desorption scattering of NO from IR(111) has been studied using multiphoton ionization and time-of-flight mass spectrometry. Molecules interacting by each mechanism were observed and characterized by their angular, velocity, and internal state distributions. For Ts〈300 K both channels are observed, while for Ts〉300 K only trapping-desorption is observed. At Ts〈300 K, the trapping fraction was (approximately-equal-to)0.85. The shift in scattering mechanism appears to be caused by a change in the surface composition due to dissociation of chemisorbed NO near room temperature.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 3682-3691 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A technique has been developed for determining the angular correlation between a photofragment's angular momentum vector J, its recoil velocity vector v, and the transition dipole moment of the parent molecule μp . Doppler profile spectroscopy used in conjunction with laser-induced fluorescence probing by polarized light can be used to determine the correlations. The pairwise correlations between these vectors as well as their triple correlation are discussed for limiting cases using a classical approach as well as for the general case using a quantum approach based on density matrices. The current formulations differ in two ways from the recent approach of Dixon, who used a bipolar expansion of the correlated velocity and angular momentum distributions. The physical basis for the influence of the vector correlations on the Doppler profile is somewhat more transparent in the current formulations, and the direct connection between the measured correlations and the t-matrix elements occurring in the theory of Balint-Kurti and Shapiro for the photodissociation of a triatomic molecule is also demonstrated.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 3692-3708 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Photodissociation of OCS in the region from 222–248 nm has been investigated by monitoring the CO and S(1D2) primary photoproducts; as well as the secondary production of S(3P2), S(3P1), and S(3P0) using fluorescence induced by a tunable vacuum ultraviolet laser source based on four-wave mixing in magnesium vapor. The quantum yield of S(3P) was found to be 0.00±0.02 at 222 nm. Thus, in contrast to our preliminary report, the present more detailed investigation shows that the sole sulfur product appears to be S(1D). The CO photofragment is produced almost exclusively in v=0 [CO(v=1)/ CO(v=0)≤0.02], but the rotational distribution is inverted and peaked at very high rotational levels. The peak shifts from J=56 for dissociation at 222 nm to J=31 at 248 nm. Doppler profiles of the CO rotational transitions reveal (1) that all observed levels are produced in coincidence with S(1D), (2) that for 222 nm photolysis the fragment recoil anisotropy shifts from a distribution characterized by β=1.9 at J=67 toward one characterized by β=0 near J=54, (3) that the CO velocity vector is aligned nearly perpendicular to its angular momentum vector, and (4) that the CO angular momentum vector is also aligned parallel to that component of the transition dipole which lies perpendicular to the recoil velocity. These results are interpreted in terms of a model for the dissociation in which excitation takes place to two surfaces of A' and A‘ symmetry derived from a bent 1Δ configuration. Dissociation of OCS clusters was also investigated and was found to produce a photochemistry completely different from that of the monomers. Rotationally cold CO as well as S2 in both the X3Σ−g and a1Δg states was observed.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 82 (1985), S. 2590-2597 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Time-resolved infrared emission from photolytically generated I*(2P1/2) has been studied in a slow flow apparatus. The total rate of deactivation of I*(2P1/2) by Cl2 has been measured to be no more than 8×10−15 cm3 molecule−1 s−1, substantially slower than previous reports. Evidence is presented for a very fast (k(approximately-equal-to)2×10−10 cm3 molecule−1 s−1) relaxation of I* by Cl atoms, which can account for both the earlier and the present observations.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 1258-1268 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photodissociation of glyoxal has been investigated by monitoring the CO internal energy distribution using tunable vacuum ultraviolet laser-induced fluorescence on the A←X system. Appearance times for the CO are in excellent agreement with the glyoxal fluorescence decay times, indicating that there is no long-lived intermediate in the dissociation. The quantum yield for CO production is independent of the K quantum number describing the glyoxal rotation. The CO is formed almost entirely in v=0 but is spread over a broadly excited rotational distribution peaking at J(approximately-equal-to)42. Analysis of the CO Doppler profiles shows that the velocity of the CO increases with increasing rotational level and that the CO recoil velocity vector is oriented predominantly perpendicular to its angular momentum vector. These observations, which are in agreement with both previous time-of-flight data and molecular orbital calculations, are consistent with a model for the dissociation involving planar intermediates for the two channels leading to CO+H2CO or to 2 CO+H2. It appears that the highest rotational levels of CO are produced in coincidence with the H2CO channel.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 82 (1985), S. 1577-1584 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The probability for deactivation of CO(v=2) and CO2(101) on collision with polycrystalline silver surfaces has been measured. The deactivation probability for CO(v=2) was found to decrease from 0.33 at 300 K to 0.20 at 440 K, while the deactivation probability for CO2 (101) was found to decrease from 0.72 at 300 K to 0.37 at 440 K. Since no population was observed in the CO(v=1) and CO2 (001) intermediate levels, it appears that each deactivation proceeds completely to produce the vibrational ground level. The magnitudes for the deactivation probabilities and the temperature dependencies indicate that a dominant mechanism for relaxation involves trapping and subsequent deactivation by one or more of several mechanisms, including electron–hole pair formation, vibration-to-rotation energy transfer, or perhaps even transfer of energy to the surface phonons. The experiments were performed in a UHV chamber by using a tunable infrared laser source to excite gas-phase molecules vibrationally before their collision with the surface and by measuring the population of vibrationally excited molecules through their time-resolved infrared fluorescence.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 84 (1986), S. 3120-3129 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Photodissociation of ICN in the A˜ continuum has been modeled using classical trajectories assuming that all absorption from the linear ground state is to a single linear diabatic excited state which dissociates to form I*(2P1/2) and CN(2Σ+,v=0). It is also assumed that, in nonlinear excited state configurations, nonadiabatic transitions occur to a bent surface which correlates diabatically to ground state I(2P3/2) and CN(2Σ+,v=0). Empirical potential surfaces with frozen CN bond lengths are employed, while transitions between the surfaces are treated using either the Miller–Meyer classical electron model or a simple diabatic version of the Tully–Preston surface-hopping model. With the above assumptions, the Miller–Meyer method is found to give much better agreement with the experimental results. Theoretical results obtained with the Miller–Meyer method are compared with recent experimental data on the I*/I branching ratio, the average CN rotational energies, and the product CN rotational distributions as a function of photolysis wavelength for λ=248, 266, 280, 290, and 308 nm. Except for the branching ratio at 248 nm, we obtain satisfactory agreement with the experimental results.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 84 (1986), S. 2120-2128 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The alignment of the CN fragment following dissociation of ICN has been measured as a function of wavelength by varying the polarization of the tunable laser used for dissociation relative to that of the tunable laser used for laser-induced fluorescence detection of the CN product. The results show that the angular momentum vector of the CN rotor is aligned predominantly perpendicular to the polarization axis of the dissociation laser, in agreement with the expectation for a predominantly parallel absorption in the ICN. However, the degree of alignment is not perfect. The most plausible explanation appears to be that the absorption occurs via a transition that is a mixture of parallel and perpendicular components. Alternate explanations for the imperfect alignment involving either rotation of the ICN parent molecule prior to dissociation or orbital angular momentum during the dissociation are examined and rejected. Near 263–266 nm the transition moment appears to be a mixture of 85% parallel and 15% perpendicular; it becomes more perpendicular to shorter and longer wavelengths.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 6045-6046 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Angular, velocity and rotational energy distributions are reported for the scattering of NO from IR(111) at surface temperatures from 100–700 °K. (AIP)
    Type of Medium: Electronic Resource
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