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  • 1990-1994  (11)
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Year
  • 1
    Electronic Resource
    Electronic Resource
    Molecular orientation control for ||JKMJ〉 —selected CH3I via influence of electric field on nuclear hyperfine coupling (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4024-4032 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using the polarized laser photofragment photoionization technique, measurements have been made of the degree of orientation of CH3I in ||JKMJ〉=||111〉 and ||222〉 parent rotational states under the influence of homogeneous electric fields (E) from 0 to 1.0 kV cm−1. From a series of experiments on hexapole-oriented molecules in weak fields, it has been found that the original degree of orientation of the symmetric-top molecules can be recovered after they pass through a homogeneous weak-field region provided that the field strength therein exceeds some minimum value (E(approximately-greater-than)0.3 V cm−1), sufficient to maintain an orientation axis. Even though the ||JKMJ〉 parent states have "relaxed'' via hyperfine coupling to an ensemble of ||FJKIMF〉 states, when the molecules later pass into a strong E field, the ensemble is restored to its original degree of orientation. Another set of experiments in moderate to strong fields provided "saturation curves,'' i.e., the dependence of the degree of orientation upon E. The results show that quite high field strengths (〉0.5 kV cm−1) are required to ensure total decoupling of J from I and thus recovery of the full orientation. From these experiments it is now clear that one can control the degree of molecular orientation by placing the state-selected molecules in a know E field.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458734
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  • 2
    Electronic Resource
    Electronic Resource
    Surface temperature dependence of the steric effect in the scattering of oriented tert-butyl chloride and fluoroform molecules by graphite(0001) (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 7406-7415 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The scattering of focused beams of hexapole-oriented t-BuCl and CHF3 molecules from a graphite (0001) surface has been studied over the surface temperature range 165 〈 Ts〈730 K. The detected number densities of the surface-scattered beams are compared for parallel vs antiparallel incident orientation of the molecular dipole with respect to the surface normal. From the fractional difference in scattered signals, i.e., the so-called steric effect, and from the scattering angular distributions of the unoriented molecules, the relative difference in scattering probabilities for opposite molecular orientations is obtained. For both molecules the magnitude of the relative difference in scattering (the steric effect) increases as Ts decreases. At the lowest surface temperature of 165 K the relative difference in scattering probability reaches +1.5 for t-BuCl (and −0.8 for CHF3), approaching the largest possible magnitude of 2.0, as compared to +0.3 (and −0.4 for CHF3) at the highest temperatures. At all temperatures the steric effect for directly (near-specularly) scattered molecules depends linearly on the degree of molecular orientation. From the angular distribution measurements, it is found that the inelastic translational energy loss in the "direct'' scattering process increases as Ts decreases. Finally, the temperature dependence of the scattering probability of unoriented beams of t-BuCl and CHF3 has been obtained.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459415
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  • 3
    Electronic Resource
    Electronic Resource
    Steric effect in the scattering of hexapole-oriented beams of symmetric-top molecules by graphite(0001) (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 7387-7405 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The orientational dependence of the scattering of hexapole-focused, oriented molecule beams of six symmetric-top molecules (CH3Cl, CH3F, CHCl3, CHF3, t-BuCl, CH3CN) and CH3OH from the (0001) surface of a graphite crystal is reported. Experimental angular distribution data are well represented by a two-component model, consisting of a "trapped/desorbed'' and a "direct'' scattered contribution. The steric effect, defined as the difference ("heads'' vs "tails'') divided by the average of the scattered signals, has been measured as a function of the scattering angle and the degree of orientation of the molecules. There is considerable diversity among the different molecules with respect to the direction and magnitude of the steric effect of the scattering (and trapping). In all cases, however, the magnitude of the steric effect is essentially a linear function of the degree of orientation. Limited data on the incident energy dependence of the angular distributions and the steric effect are also presented. A model which deconvolutes the steric effect for the trapped/desorbed and directly scattered components is introduced. It also provides an independent estimate of the trapping probability of the incident molecules.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459414
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  • 4
    Electronic Resource
    Electronic Resource
    Translational energy dependence of the reaction cross section: Reactions of Sr+CH3I, CD3I, and CH3Br (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 1896-1903 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The excitation functions for the crossed beam reactions of Sr+CH3I, CD3I, and CH3Br have been investigated over the collisional energy (ET) range 0.1 to ∼1.0 eV. A highly sensitive single photon ionization technique has been developed to detect the reaction products (SrI and SrBr). The integrated total reaction cross sections (σR) exhibit very different behavior in reactions with CH3I vs CH3Br. In the reaction of Sr+CH3I, σR decreases sharply with increasing ET to a minimum near 0.35±0.05 eV and then gradually increases with increasing ET. In the isotopically substituted reactant, CH3I, no difference in the excitation function is seen within the resolution of the experiment. However, for the reaction Sr+CH3Br, σR exhibits a threshold around 0.12±0.05 eV and gradually increases with increasing ET. The present data for the Sr reactions with CH3I and CH3Br are compared with analogous alkali metal reactions and with several theoretical models.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462090
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  • 5
    Electronic Resource
    Electronic Resource
    Orientational dependence of the translational energy transfer in the scattering of oriented fluoroform and tert-butyl chloride molecules by a graphite(0001) surface (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 7416-7426 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Time-of-flight distributions of beams of hexapole-oriented CHF3 and t-BuCl molecules scattered from a graphite (0001) surface have been measured for parallel vs antiparallel incident orientations of the molecular dipole with respect to the surface normal, over a range of surface temperatures 170≤Ts≤730 K. The observed difference in arrival times, Δtexp, for opposite initial orientations depends strongly on the degree of orientation of the incident molecules. In the analysis of the Δtexp data, we make use of the two-component model, which assumes that the scattered beams are composed of directly scattered and trapped/desorbed molecules. It is shown that in the common case of short residence times for the trapped molecules, the difference in arrival times for the directly scattered molecules, Δtdir, can be ascertained from the measured Δtexp. The magnitudes of the calculated Δtdir correspond to a strong orientation dependence in the translational energy transfer accompanying the direct scattering of CHF3 and t-BuCl by graphite (0001). The final translational energy of directly scattered molecules E' is found to be smaller for the collision of the H "end'' of fluoroform with the graphite surface; for t-BuCl, E' is smaller for the Cl "end'' collision. These are the orientations that also give rise to higher trapping probability. In the course of the present study, the residence times of t-BuCl on graphite (0001) have been measured over the surface temperature range 170〈Ts〈240 K.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459416
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  • 6
    Electronic Resource
    Electronic Resource
    Strong orientation dependence of the scattering of fluoroform by graphite (0001) (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 801-802 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Measurements are reported of the scattering of hexapole−oriented fluoroform by the (0001) face of graphite. The experiments were conducted on the UCLA oriented−molecule beam machine. (AIP)
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458382
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  • 7
    Electronic Resource
    Electronic Resource
    Hard-cube analysis of the steric effect in molecule–surface scattering (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 1564-1571 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A modified hard-cube model is employed to account for the recent experimental results of the Bernstein group on steric effects in the scattering of polyatomic molecules on a graphite (0001) surface. In the low surface temperature (Ts) limit simple analytical formulas are obtained for Ts dependences of the probability of direct scattering, the average postcollisional translational energy and the average tangent of the scattering angle. The model accounts for the surface temperature dependences of the scattering probability and the angular distributions of direct scattering over the whole surface temperature range. It also gives the experimentally observed functional dependence of the steric effect in scattering probability vs the surface temperature, but fails to give quantitative agreement.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460014
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  • 8
    Electronic Resource
    Electronic Resource
    Tight focusing of beams of polar polyatomic molecules via the electrostatic hexapole lens (1991)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 95 (1991), S. 8129-8136 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100174a022
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  • 9
    Electronic Resource
    Electronic Resource
    Orientation dependence of the induction contribution in molecule-graphite interactions (1991)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 95 (1991), S. 8175-8177 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100174a029
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  • 10
    Electronic Resource
    Electronic Resource
    Dependence of the reaction cross section on the collision energy in reactions of Sr + RX .fwdarw. SrX + R (R = C2H5, n-C3H7, tert-C4H9; X = Br, I): effect of the alkyl group (1991)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 95 (1991), S. 8226-8232 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100174a040
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