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  • 1
    Electronic Resource
    Electronic Resource
    Measurement of the state-specific differential cross section for the H+D2→HD(v′=4, J′=3)+D reaction at a collision energy of 2.2 eV (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 5157-5160 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: HI and D2 are coexpanded into a vacuum chamber. The photolysis of HI at 212.8 nm initiates the H+D2 reaction. The HD(v=4,J′=3) velocity distribution is determined by analyzing the time-of-flight profile of HD+ ions produced by delayed pulsed field ionization of long-lived Rydberg states. The angular distribution is deduced using the law of cosines (photoloc technique). © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470604
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  • 2
    Electronic Resource
    Electronic Resource
    Resonance enhanced multiphoton ionization spectroscopy of the NF molecule: 1,3Φ 3d and 4d Rydberg states (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 1515-1527 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The (3dδ)1,3Φ and (4dδ)1,3Φ Rydberg states of NF have been investigated by multiphoton ionization (MPI) spectroscopy. These states were observed as two-photon resonances in an overall (2+1) MPI process from NF a1Δ produced from the F+N3 reaction. Ab initio calculations performed at the multireference double excitation configuration interaction level showed that the excited Φ states were of Rydberg character with configurations of ...1π45σ22π1(3dδ)1 and ...1π45σ22π1(4dδ)1. The 3Φ←a1Δ two-photon transitions were found to derive their intensities from spin–orbit and spin-uncoupling interactions in the 1Φ3, 3Φ4, 3Φ3, and 3Φ2 upper states. Analysis of the rotationally resolved bands, using a model which includes these factors, allowed the spin–orbit constant, aπ, for the 2π valence orbital to be derived as (159.0±1.0) cm−1. Rotationally resolved envelopes recorded for the (3dδ)1Φ, v'=0, 1←a1Δ, v‘=0 and (3pπ)1Σ+, v'=0←a1Δ, v‘=0 (2+1) NF MPI bands, could be simulated reasonably well assuming a Boltzmann rotational distribution for the a 1Δ state at ≈180 K. Experiments showed, however, that this was not a true measure of the initial state distribution because of predissociation or perturbation effects in the resonant intermediate state. Assignments for other two-photon resonant NF MPI bands observed in this work are also suggested. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468884
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  • 3
    Electronic Resource
    Electronic Resource
    An instrument for the collection of simultaneous small and wide angle x-ray scattering and stress–strain data during deformation of polymers at high strain rates using synchrotron radiation sources (1999)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 70 (1999), S. 4051-4054 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A fully integrated system has been developed which allows the study of the deformation of synthetic polymers by simultaneous small- and wide-angle x-ray scattering (SAXS/WAXS) and stress–strain techniques at synchrotron radiation sources. Two-dimensional x-ray data collection is achieved via two charge coupled device based area detectors which provide video signal outputs. A video extensometer provides sample strain and cross-section data during deformation. All three video signals are processed by a powerful Synoptics i860 processor based video framegrabber, with no loss of data. With this data collection strategy a temporal resolution of 40 ms is possible. In order to study the mechanical yield of the sample, a bridge-type strain gauge is used which reveals the sample loading. An electronic trigger mechanism provides accurate synchronization of the x-ray data, sample video data, sample loading information and controls the onset of deformation. Two experiments are highlighted showing the drawing of polyethylene at an overall rate of (approximate)10 s−1. By analysis of the sample video data it is possible to correlate the SAXS and WAXS data with specific points upon the true stress–strain curve. This novel system is shown to be a useful tool for the investigation of the deformation of polymers at rates that are relevant to industrial processing. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1150034
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  • 4
    Electronic Resource
    Electronic Resource
    Molecular dynamics algorithms for path integrals at constant pressure (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 3275-3290 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Extended system path integral molecular dynamics algorithms have been developed that can generate efficiently averages in the quantum mechanical canonical ensemble [M. E. Tuckerman, B. J. Berne, G. J. Martyna, and M. L. Klein, J. Chem. Phys. 99, 2796 (1993)]. Here, the corresponding extended system path integral molecular dynamics algorithms appropriate to the quantum mechanical isothermal–isobaric ensembles with isotropic-only and full system cell fluctuations are presented. The former ensemble is employed to study fluid systems which do not support shear modes while the latter is employed to study solid systems. The algorithms are constructed by deriving appropriate dynamical equations of motions and developing reversible multiple time step algorithms to integrate the equations numerically. Effective parallelization schemes for distributed memory computers are presented. The new numerical methods are tested on model (a particle in a periodic potential) and realistic (liquid and solid para-hydrogen and liquid butane) systems. In addition, the methodology is extended to treat the path integral centroid dynamics scheme, [J. Cao and G. A. Voth, J. Chem. Phys. 99, 10070 (1993)], a novel method which is capable of generating semiclassical approximations to quantum mechanical time correlation functions. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.478193
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  • 5
    Electronic Resource
    Electronic Resource
    Spectroscopy and dynamics of rare gas–spherical top complexes. II. The infrared spectrum of the ν3 band of Ne–SiH4 ( j=1←0 and j=0←1 transitions) (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 5391-5405 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The infrared spectrum of the rare gas–spherical top complex Ne–SiH4 has been recorded in a supersonic jet in the region of the SiH4 ν3 triply degenerate stretching vibration at ∼2189 cm−1. In contrast to the previously measured Ar–SiH4 spectrum which showed almost equal rotational spacings within each band (corresponding to transitions between different internal rotor states of SiH4 within the complex), the Ne–SiH4 spectrum is complex with no obvious regular band structure. However, by analogy with the Ar–SiH4 spectrum, four bands of the Ne–SiH4 have been assigned and analyzed in terms of Hamiltonians incorporating Coriolis interaction between the angular momentum of the SiH4 monomer unit and the overall end over end rotation of the complex. These bands correlate with the SiH4 R(0) (K=0←0, K=1←0) and P(1) (K=0←0, K=0←1) transitions. Derived rotational constants demonstrate that the neon–silane separation (∼4.13 A(ring) in the ground vibrational state) is larger than expected by analogy with Ar–SiH4, indicative of nearly free internal rotation by the silane monomer unit in Ne–SiH4. The smaller anisotropy of Ne–SiH4 compared with Ar–SiH4 results in a new angular momentum coupling scheme. Transitions arising from 22Ne–SiH4 correlating to SiH4 R(0) have also been observed and fitted; the higher than anticipated intensities demonstrate a novel isotope enrichment effect in the supersonic jet which is discussed. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471779
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  • 6
    Electronic Resource
    Electronic Resource
    Spectroscopy and dynamics of rare gas-spherical top complexes. III. The infrared spectrum of the ν3 band of Ne–SiH4 ( j=1←1 and j=2←1 transitions) (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 2738-2751 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The infrared spectrum of the rare gas-spherical top complex Ne–SiH4 has been recorded in a supersonic jet in the region of the SiH4 ν3 triply degenerate stretching vibration at ∼2189 cm−1. The Ne–SiH4 spectrum is complex with no obvious regular band structure; a previous paper has reported the assignment and analysis of bands correlating with the SiH4 R(0) (K=0←0, K=1←0) and P(1) (K=0←0, K=0←1) transitions. This paper concludes the study of Ne–SiH4, with the assignment and analysis of bands correlating with the SiH4 Q(1) (K=0←0, K=1←0, K=0←1, K=1←1) and R(1) (K=0←0, K=1←0, K=1←1, K=2←1) transitions. This was facilitated by a twofold approach: The bands were identified by the use of a theoretical intermolecular potential, with potential parameters determined by reference to the two bands previously fitted; and the relative magnitudes of the Coriolis interactions between the K sublevels within the bands determined by explicit calculation of the Coriolis matrix elements. Having fitted the bands to appropriate energy-level expressions, an effective anisotropic potential for the internal rotation of the SiH4 molecule within the complex was fitted to the total of 15 observed band origins. With a fixed R approximation, the V3 anisotropic term was found to be ∼30 cm−1, approximately one-third that of Ar–SiH4, reflecting the freer rotation of the SiH4 molecule when complexed with the less polarizable Ne atom. However, the overall quality of the fit was an order of magnitude worse than that for Ar–SiH4, indicating increased importance of the radial dependence of the Ne–SiH4 potential compared to that of Ar–SiH4. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474700
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  • 7
    Electronic Resource
    Electronic Resource
    NMR excitation of quadrupolar order using adiabatic demagnetization in the rotating frame (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 876-889 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Adiabatic demagnetization and remagnetization in the rotating frame (ADRF and ARRF) are shown to be practical and efficient techniques for exciting and observing quadrupolar order, T2,0, in NMR of quadrupolar nuclei such as 2H or 23Na. A detailed theoretical description of ADRF and ARRF of spin I=1 and 3/2 nuclei, based on the well-known vector model of NMR, is presented and used to derive analytically a variety of pulse shapes for ADRF and ARRF. This theory is also used to calculate the state of the spin I=1 and 3/2 density operators following an ADRF or ARRF pulse and it is shown that the desired coherence transfer processes have the maximum amplitudes allowed by the well-known "universal bound" theorem. In principle, therefore, ADRF is shown to be superior as a method of exciting quadrupolar order to the Jeener-Broekaert experiment since the latter fails to excite the maximum T2,0 amplitude for spin I=3/2 nuclei. The performance of ADRF is investigated using computer calculations and simulations and the conditions under which it yields broadband (i.e., non-oscillatory) excitation of quadrupolar order are derived. Using both 2H (I=1) and 23Na (I=3/2) NMR of liquid crystalline and biological samples, ADRF and ARRF are demonstrated experimentally. The predicted broadband excitation behavior is observed for both ADRF and ARRF, whereas both the Jeener-Broekaert and double-quantum filtration experiments show excitation profiles that oscillate sinusoidally as a function of either the quadrupolar splitting parameter, ωQ, or the duration of the pulse sequence, τ. Finally, a more general discussion of ADRF and ARRF of quadrupolar nuclei is presented and it is shown that the maximum coherence transfer amplitudes are achieved for nuclei of any spin quantum number I. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475451
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  • 8
    Electronic Resource
    Electronic Resource
    A deuterium nuclear magnetic resonance investigation of the symmetry and orientational order of the nematic phase of 4-[3,4,5-tris(4-dodecyloxybenzyloxy)benzoyloxy]-4′- (4-dodecyloxybenzoyloxy)-1,1′-biphenyl. A biaxial nematic? (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 9252-9263 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The first compound for which the previously predicted biaxial nematic phase was claimed is 4-[3,4,5-tris(4-dodecyloxybenzyloxy)benzoyloxy]-4′-(4-dodecyloxybenzoyloxy)-1,1′-biphenyl (I). This assignment was based on the observation of the optical texture and the x-ray diffraction pattern. To confirm this identification of the biaxial nematic phase, we have studied the deuterium nuclear magnetic resonance (NMR) spectra of nonuniformly aligned samples with the deuterons located specifically in the mesogen itself or in the disklike solute, hexamethylbenzene-d18, dissolved in the mesogen. These experiments allow us to determine the biaxiality in the partially averaged quadrupolar tensor. For both systems, the biaxiality parameter is found to be zero within the experimental error which is estimated to be less than ±0.08 for the pure mesogen and ±0.06 for the solute. The orientational order parameters determined from the quadrupolar splitting change discontinuously at the nematic-isotropic transition thus confirming the first order character of the transition and in accord with the uniaxial symmetry of the nematic phase. The order parameters determined for the two groups of equivalent deuterons in the specifically deuteriated oxymethylene links allow us to comment on the conformations of these links. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475318
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  • 9
    Electronic Resource
    Electronic Resource
    Bounds on spin dynamics and the design of multiple-pulse NMR experiments (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 2105-2117 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The "universal bound on spin dynamics'' proposed by Sørensen is examined in detail and shown to be of great assistance in the design of novel multiple-pulse NMR experiments. The efficiency of coherence transfer between all possible states of a spin system, including populations, single-quantum coherences, and multiple-quantum coherences, is investigated. Examples are drawn from coherence transfer processes in quadrupolar coupled spin I=1 and 3/2 nuclei and weakly J coupled systems of two and three spin I=1/2 nuclei. It is found that many of the most commonly used NMR pulse sequences fail to achieve the maximum coherence transfer efficiency when applied to spin I=3/2 or to three spin I=1/2 nuclei. However, it is shown that, with knowledge of the universal bound, novel multiple-pulse NMR experiments that achieve optimal efficiency can be easily derived using computer optimization. The application of the universal bound to two-step coherence transfer experiments presents a number of conceptual difficulties. In particular, examples are presented where the product of the universal bounds on the two individual coherence transfer coefficients is larger than the universal bound on the overall transfer from the initial to the final state. These difficulties are resolved and explained in terms of the presence of a "residue'' that is created together with the intermediate state. The universal bound is used to examine the conditions under which the effect of this residue can be suppressed and the constraints that this places on the design of optimal multi-step coherence transfer NMR experiments. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473783
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  • 10
    Electronic Resource
    Electronic Resource
    The effect of growth interruption on the properties of InGaAs/InAlAs quantum well structures (1998)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 72 (1998), S. 338-340 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The effect of the growth interruption time during the growth of InGaAs/InAlAs quantum well structures is shown to have a significant effect on both the interband transitions, as determined by photoreflectance, and the electrical properties of the as-grown structure. The results show that, for increasing growth interruption time, the quantum well heterointerfaces become more abrupt and the carrier mobility increases, thereby demonstrating that long interruption times are preferable for the growth of high quality rectangular quantum well structures. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.120729
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