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  • 1
    Electronic Resource
    Electronic Resource
    Backward mode detection of laser-induced wide-band ultrasonic transients with optoacoustic transducer (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 87 (2000), S. 2003-2014 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Time-resolved piezoelectric detection of wide-band ultrasonic transients induced by laser pulses in absorbing medium was studied. An optoacoustic transducer was developed for measuring the profiles of ultrasonic transients propagating in backward direction out of the laser-irradiated medium. For this purpose, an optical fiber for delivery of laser pulses to the surface of absorbing medium and a wide-band lithium niobate acoustic transducer were incorporated in one compact system, optoacoustic front surface transducer (OAFST). The transducer possesses temporal resolution (rise time) of 3.5 ns, low effective thermal noise pressure (10 Pa), and high sensitivity of piezoelectric detection (10 μV/Pa) over the ultrasonic frequency range from 1 to 100 MHz. Nd:YAG laser pulses at 355 nm were employed to generate distribution of acoustic sources in water solutions of potassium chromate with various concentrations. A temporal course of ultrasonic transients launched into an optically and acoustically transparent medium, coupled to the absorbing medium, was studied. Ultrasonic profiles experimentally measured at the site of laser irradiation were compared with profiles calculated using theoretical model. Experimental curves were in a good agreement with theoretical profiles. The backward detection mode permitted accurate reconstruction of the axial distribution of heat deposition in the laser-irradiated homogeneous or layered medium from the measured profiles. OAFST may become a useful modality for optoacoustic imaging in biological tissues and nondestructive evaluation of industrial materials. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.372127
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  • 2
    Electronic Resource
    Electronic Resource
    Restricted TCR Vβ gene expression and enterovirus infection in Type I diabetes: a pilot study (2000)
    Springer
    Diabetologia 43 (2000), S. 1484-1497 
    Details
    ISSN: 1432-0428
    Keywords: Keywords Type I diabetes ; etiology ; autoimmunity ; Coxsackievirus ; T cell receptor.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract Aims/hypothesis. High frequencies of T-cell receptor (TCR) V β7+ T cells were detected among the lymphocytes isolated from pancreatic islets of children at the onset of Type I (insulin-dependent) diabetes mellitus. We assessed whether a preferential expression of certain TCR V β gene families could also be detected among the peripheral blood mononuclear cells from diabetic patients. Methods. T-cell receptor repertoires were evaluated by using a semi-quantitative RT-PCR-based technique and confirmed by FACS analysis in peripheral blood mononuclear cells from diabetic patients before, at and after onset of the disease. These patients were also tested for exposure to enteroviruses by RT-PCR and by measuring titres of enterovirus-specific antibodies of the IgA, IgG, and IgM classes. Results. T-cell receptor V β7 gene family values were higher in recently-diagnosed diabetic patients (10.5 % ± 3.7) than in age-matched non-diabetic control subjects (5.1 % ± 1.6) (p 〈 0.001). In a time-course analysis of people who developed diabetes during clinical monitoring (i. e., converters), T-cell receptor V β7 gene expression showed values consistently above 10 % (p 〈 0.0005). Long-standing diabetic patients showed lower percentage of V β7 expression compared to values measured at disease onset. In the longitudinal study of the converters, multiple acute enterovirus infections were also detected. These infections appeared to be temporally related to increased percentage of V β7 gene transcripts. Conclusion/interpretation. The deviation in the T-cell receptor V β repertoire among circulating T cells from diabetic patients seems to re-emphasize the importance of enterovirus infections in accelerating the progression of Type I diabetes. [Diabetologia (2000) 43: 1484–1497]
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s001250051559
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  • 3
    Electronic Resource
    Electronic Resource
    Electronic coupling between Watson–Crick pairs for hole transfer and transport in desoxyribonucleic acid (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 5614-5620 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Electronic matrix elements for hole transfer between Watson–Crick pairs in desoxyribonucleic acid (DNA) of regular structure, calculated at the Hartree–Fock level, are compared with the corresponding intrastrand and interstrand matrix elements estimated for models comprised of just two nucleobases. The hole transfer matrix element of the GAG trimer duplex is calculated to be larger than that of the GTG duplex. "Through-space" interaction between two guanines in the trimer duplexes is comparable with the coupling through an intervening Watson–Crick pair. The gross features of bridge specificity and directional asymmetry of the electronic matrix elements for hole transfer between purine nucleobases in superstructures of dimer and trimer duplexes have been discussed on the basis of the quantum chemical calculations. These results have also been analyzed with a semiempirical superexchange model for the electronic coupling in DNA duplexes of donor (nuclobases)–acceptor, which incorporates adjacent base–base electronic couplings and empirical energy gaps corrected for solvation effects; this perturbation-theory-based model interpretation allows a theoretical evaluation of experimental observables, i.e., the absolute values of donor–acceptor electronic couplings, their distance dependence, and the reduction factors for the intrastrand hole hopping or trapping rates upon increasing the size of the nucleobases bridge. The quantum chemical results point towards some limitations of the perturbation-theory-based modeling. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1352035
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  • 4
    Electronic Resource
    Electronic Resource
    Triple excitation effects in coupled-cluster calculations of indirect spin–spin coupling constants (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 1619-1622 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The effect of triple excitations in coupled-cluster calculations of indirect spin-spin coupling constants is investigated in coupled-cluster singles and doubles (CCSD) calculations augmented by a perturbative treatment of triples [CCSD(T)], in calculations based on the CC3 model as well as in coupled-cluster singles, doubles, and triples (CCSDT) calculations. Though triple excitation effects are in most cases not particularly pronounced, it is demonstrated that among the approximate schemes for handling triples only the CC3 model with no orbital relaxation included (unrelaxed CC3) provides an adequate description. The otherwise successful CCSD(T) aproach appears to either significantly overestimate triple excitation effects or to yield corrections with the wrong sign in comparison to CCSDT. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1386698
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  • 5
    Electronic Resource
    Electronic Resource
    Dipolar relaxation in a many-body system of spins of 1/2 (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 1425-1443 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The method utilized in Paper I [J. Chem. Phys. 112, 1413 (2000)] for treating the density matrix equation for a two-spin system in the presence of the dipolar interaction that is randomly modulated by translational diffusion, is extended to a many-body system of identical spins of 1/2. Generalized cumulant expansions are used, which allow one to take full advantage of the statistical independence of the motions of spins. In the high-temperature approximation (appropriate for dilute solutions), for a single nonselective pulse, the symmetry of the problem allows one to obtain a compact ordered binomial expression for the free-induction decay signal that is related to the two-particle solution, and it still contains the two spin-isochromat components. The latter are evaluated by solving the corresponding stochastic Liouville equation, which allows one to recover in a unified way the two limiting cases including Anderson's result for statistical broadening in a rigid lattice and the classical Torrey–Bloembergen–Redfield expression for the motional narrowing, as corrected by Hwang and Freed. The line shape expression in the thermodynamic limit, i.e., for large numbers of particles in a macroscopic volume, is obtained. It is found that the many-body dipolar line shapes are very close to Lorentzians over the entire motional range studied, with the linewidths proportional to the spin concentration, as predicted earlier for the limiting cases. Linewidths plotted versus the values of the translational diffusion coefficient clearly show the solid-state limit, the motional-narrowing limit, and the intermediate region. The method is extended to describe the behavior of the many-body system in a solid-echo sequence. This enables one to obtain the homogeneous T2's over the whole range of motions. A minimum in T2 is found at approximately the same value of translational diffusion coefficient as was found for the two-spin case in Paper I. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480709
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  • 6
    Electronic Resource
    Electronic Resource
    A many-body analysis of the effects of the matrix protons and their diffusional motion on electron spin resonance line shapes and electron spin echoes (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 2416-2429 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The method for treating the evolution of the density matrix developed in the accompanying paper for many-spin systems is applied here for calculating magnetic resonance signals of a spin A interacting with a bath of N identical spins B. Spins B are assumed to have much smaller gyromagnetic ratios than the spin A (e.g., the former are nuclear spins, I and the latter is an electron spin, S). The experimentally observed quadratic dependence of the spin-echo envelope decay on concentration and time is explained from considering the dipolar coupling of spin A to all the B spins in the presence of B–B dipolar interactions. It is shown that the spin-echo envelope decay in the rigid limit is due to the interaction of the A spin with the coherent many-body states of the coupled spins B via the nuclear flip-flop terms I±I(minus-plus), which becomes a dissipative mechanism in the thermodynamic limit. This represents a more rigorous analysis than simplified models based on an incoherent version of "spin diffusion," and it leads to good quantitative agreement with experiment. Moreover, this analysis represents a unified description of both the modulation and decay of the A-spin echoes. Spin echoes and line shapes for the A–BN systems are also calculated for finite motions which randomize the B spins. Even for very slow motions (modeled as translational diffusion) an effective mechanism for spin-echo envelope decay is generated, which readily overtakes the coherent mechanism in importance. The intensity distribution for the forbidden components in the A-spin line shape resulting from multiquantum transitions of the B spins caused by the pseudosecular interaction terms SzI±, is calculated. In the rigid limit it is found to behave like a Poisson distribution. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1382817
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  • 7
    Electronic Resource
    Electronic Resource
    Theoretical investigation of the initial reaction of the NO decomposition on the Si (100) (2×1) reconstructed surface (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 8237-8248 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have investigated theoretically the initial reaction of nitric oxide (NO) with the Si(001)(2×1) surface, followed by N and O insertion into the silicon film during the initial growth of the oxynitride film. We use quantum chemical [ab initio and density functional theory (DFT) cluster approach] and solid state physics (DFT with periodic boundary conditions) computational methods. Our study suggests a low barrier reaction path for NO decomposition on the Si(100)(2×1) reconstructed silicon surface. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1316040
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  • 8
    Electronic Resource
    Electronic Resource
    Cl+HD (v=1; J=1,2) reaction dynamics: Comparison between theory and experiment (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 670-685 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Vibrationally state-resolved differential cross sections (DCS) and product rotational distributions have been measured for the Cl+HD(v=1, J=1)→HCl(DCl)+D(H) reaction at a mean collision energy of 0.065 eV using a photoinitiated reaction ("photoloc") technique. The effect of HD reagent rotational alignment in the Cl+HD(v=1, J=2) reaction has also been investigated. The experimental results have been compared with exact quantum mechanical and quasiclassical trajectory calculations performed on the G3 potential energy surface of Allison et al. [J. Phys. Chem. 100, 13575 (1996)]. The experimental measurements reveal that the products are predominantly backward and sideways scattered for HCl(v′=0) and HCl(v′=1), with no forward scattering at the collision energies studied, in quantitative agreement with theoretical predictions. The experimental product rotational distribution for HCl(v′=1) also shows excellent agreement with quantum-mechanical calculations, but the measured DCl+H to HCl+D branching ratio is near unity, which is at variance with the theoretical calculations that predict about 3 times larger yield of HCl+D at these collision energies. The reactivity shows a marked dependence on the direction of the HD(v=1, J=2) rotational angular momentum, and experimental measurements of this reagent alignment effect are in good agreement with theoretical predictions. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480602
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  • 9
    Electronic Resource
    Electronic Resource
    Direct-product formalism for calculating magnetic resonance signals in many-body systems of interacting spins (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 2401-2415 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Modern applications of nuclear magnetic resonance and electron spin resonance, and especially quantum computing problems call for more effective formalisms to describe relaxation and evolution of various orders of coherence in the presence of motions of the interacting spins. Here we suggest a formulation for the description of multiquantum spin states based on direct-product structures that take into account the inherent permutation symmetries and quantum coherences of a multispin system. Convenient recursion relations are obtained for the matrix representations of the N-body Hamiltonian superoperator and pulse propagators. This allows one to obtain compact expressions for the evolution of magnetic resonance signals when the dipolar interactions amongst spin-bearing molecules are modulated by their motions. These expressions include the free-induction decay and solid echoes, as well as the decay of higher-order coherences under the assumption of statistical independence of the motions of the spin-bearing molecules. Exact results, not requiring this assumption, are obtained for the case of the truncated dipolar Hamiltonian. Important phenomena that arise in multispin systems, such as instantaneous diffusion and spectral diffusion arising from motions, are studied more rigorously by solving the equation for the time evolution of the spin-density states. The many-body magnetic-resonance signals in the presence of motions are obtained by solving the appropriate stochastic Liouville equations. These solutions may be compared to solid-echo experiments to extract translational diffusion coefficients even in the slow motional regime. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1382816
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  • 10
    Electronic Resource
    Electronic Resource
    Spin relaxation by dipolar coupling: From motional narrowing to the rigid limit (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 1413-1424 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A coupled system of two molecules bearing spins of 1/2, which are allowed to diffuse relative to each other, is considered. By using a symmetry-adapted basis operator set, the overall density matrix equation is decoupled into two equations for the time-resolved isochromat components, the sum of which yields the observed signal. The appropriate stochastic Liouville equation is solved by a combination of eigenfunction expansion and finite-differences for the angular and radial relative motions, respectively. A full range of spectra from classic Pake patterns in the rigid limit to motionally narrowed Lorentzians is recovered. As an extension of the above approach, the solid-echo experiment is described in terms of the ensemble-averaged isochromats. Homogeneous transverse relaxation times (T2) as a function of the translational diffusion coefficient (DT) are obtained from simulating SECSY (spin-echo correlation spectroscopy) signals, which show distinct T2 minima vs DT. The present method of separating the quantum and stochastic classical variables constitutes a useful approach for treating multiquantum statistical systems, and it can be generalized to an arbitrary number of spins as shown in a companion paper. In the present study we obtained the usual linear dependence of T2 on DT in the motional narrowing (or Redfield) limit, whereas in the slow motional regime a DT−1/2 dependence is observed, consistent with studies of rotational diffusion. Varying the distance of maximum separation between the two spins (rmax) has virtually no effect on the location of the T2 minimum with respect to DT, implying that the onset of slow motion is essentially independent of rmax. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480695
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