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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
    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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  • 3
    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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  • 4
    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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  • 5
    Electronic Resource
    Electronic Resource
    Organic Coatings Adaptable to Fuel Storage (1954)
    s.l. : American Chemical Society
    Industrial & engineering chemistry 46 (1954), S. 1977-1985 
    Details
    ISSN: 1520-5045
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/ie50537a058
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  • 6
    Electronic Resource
    Electronic Resource
    Femtosecond intraband modulation spectroscopy (2002)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 73 (2002), S. 136-140 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We demonstrate several femtosecond spectroscopic techniques utilizing intraband carrier reexcitation for studies of relaxation processes in semiconductor materials. By introducing an additional infrared post-pump pulse into a conventional transient absorption experiment, we are able to visualize carrier intraband dynamics and to detect weak conduction-to-valence band transitions unresolved in the traditional transient absorption configuration. Intraband reexcitation spectroscopy also allows us to study dynamics of radiative transitions by monitoring a post-pump-induced modulation of time-integrated photoluminescence. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1427396
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  • 7
    Electronic Resource
    Electronic Resource
    Tandem time-of-flight mass spectrometer (TOF-TOF) with a quadratic-field ion mirror (2002)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 73 (2002), S. 2115-2123 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A tandem time-of-flight (TOF-TOF) mass spectrometer comprised of two ion mirrors is described. The first ion mirror, which is a linear-field, single-stage mirror (MS1) with an intermediate collision cell, has been designed to provide the temporal focus necessary for the second, quadratic-field ion mirror (MS2) to function effectively. Due to the wide energy-range focusing capabilities of the quadratic field employed in the second ion mirror all the fragment ions can be collected in one spectrum without the need to step the reflecting working voltage of the MS2. The size of the active area of the microchannel plate detector used in the preliminary experiments was the limiting factor governing the collection efficiently of fragment ions. The use of the first ion mirror to provide temporal focusing of the precursor ion packet at the first focal point of the quadratic mirror used as the MS2 requires no alteration of the focusing conditions for different masses, in contrast to delayed extraction or postsource pulsed focusing. Precursor ions formed by matrix-assisted laser desorption/ionization were mass-selected with an ion gate located before the collision cell and the fragment ions were mass analyzed using the quadratic-field ion mirror. Experimental results demonstrating effective high-energy collision-induced dissociation of polymer and fullerene molecule-ions are presented. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1470229
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  • 8
    Electronic Resource
    Electronic Resource
    Formation of Ge nanoislands using a scanning tunneling microscope (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 88 (2000), S. 1397-1400 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Germanium islands were grown on a sample surface by accumulating atoms from the surrounding area through directional surface diffusion initiated by the electric field of a scanning tunneling microscope (STM). The Ge islands grew with a constant rate determined by the tip–sample bias voltage. The parameters of tip–sample interaction were estimated from the kinetic data for island growth by using a scaling relationship among the growth rate, the dipole moment of atoms on surfaces, and the tip–sample bias voltage. The results show that continuous atom transfer with a STM occurs with a rate significantly higher for Ge than for Si. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.373829
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  • 9
    Electronic Resource
    Electronic Resource
    Miniband formation in a quantum dot crystal (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 5509-5515 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We analyze the carrier energy band structure in a three-dimensional regimented array of semiconductor quantum dots using an envelope function approximation. The coupling among quantum dots leads to a splitting of the quantized carrier energy levels of single dots and formation of three-dimensional minibands. By changing the size of quantum dots, interdot distances, barrier height, and regimentation, one can control the electronic band structure of this artificial quantum dot crystal. Results of simulations carried out for simple cubic and tetragonal quantum dot crystal show that the carrier density of states, effective mass tensor and other properties are different from those of bulk and quantum well superlattices. It has also been established that the properties of artificial crystal are more sensitive to the dot regimentation rather then to the dot shape. The proposed engineering of three-dimensional mini bands in quantum dot crystals allows one to fine-tune electronic and optical properties of such nanostructures. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1366662
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  • 10
    Electronic Resource
    Electronic Resource
    Thermal and nonthermal regimes of gliding arc discharge in air flow (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 87 (2000), S. 1632-1641 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Gliding discharges comprising both equilibrium and nonequilibrium plasma conditions offer high energy efficiency and selectivity for chemical processes. Prevailing parameters satisfying nonequilibrium plasma conditions at relatively high power levels should be well understood and characterized. In the present work, gliding discharges formed between diverging electrodes in air flow were studied experimentally over a wide range of gas velocities and power levels. Depending on the system parameters the following discharge regimes were observed: low power nonequilibrium discharge; thermal quasiequilibrium discharge; and gliding discharge with equilibrium to nonequilibrium transition. The effect of system parameters on discharge characteristics is analyzed. The equilibrium to nonequilibrium transition was experimentally observed as a change of voltage increase rate with discharge length growth. The local electric field, defined as dV/dl, increased up to three times, indicating the change of plasma conditions. However, previously reported phenomenon of length explosion was not supported by our experimental data. The co-existence of equilibrium and nonequilibrium phases is also discussed in the frame of phenomenological theory, assuming formation of a growing nonequilibrium fragment inside the gliding discharge channel. It was found that high flow velocities provide intensive cooling, an increase of electric field, and a decrease of gas temperature, promoting equilibrium to nonequilibrium transition at high power levels. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.372071
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