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  • 1
    ISSN: 1573-0662
    Keywords: J NO 2 ; NO2 photolysis ; atmospheric photochemistry ; J NO 2measurement
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences
    Notes: Abstract An instrument, specifically designed for measurements from a balloon platform in the stratosphere, has been used to obtain ground-level values of the atmospheric photodissociation coefficient of nitrogen dioxide, J NO 2.A typical clear-sky value is 8.0×10-3 s-1 when the solar zenith angle is 40°. Measurements were made as a function of solar zenith angle and correlated with a calibrated Eppley UV radiometer. It is shown that J NO 2may be expressed as a simple function of the radiometer output so that estimates of J NO 2can be made using just an upward looking radiometer to an accuracy of about 20%. The measurements are also found to be in good agreement with calculations of J NO 2using a simplified isotropic multiple scattering computer routine.
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  • 2
    ISSN: 1573-0662
    Keywords: Atmosphere ; photolysis ; nitrogen dioxide ; photodissociation coefficient ; temperature effect
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences
    Notes: Abstract The photodissociation coefficient, J NO2 of NO2 in the atmosphere was calculated at 235 and 298 K using the measured temperature dependences of the absorption cross-sections and quantum yields. These calculations gave a ratio J NO2(298 K)/J NO2(235 K)=1.155±0.010 which is only weakly dependent on altitude, surface albedo and solar zenith angle.
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  • 3
    ISSN: 1573-0662
    Keywords: J NO 2 ; NO2 photolysis ; atmospheric photochemistry ; J NO 2 measurement
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences
    Notes: Abstract The photodissociation coefficient of NO2, J NO 2, has been measured from a balloon platform in the stratosphere. Results from two balloon flights are reported. High Sun values of J NO 2 measured were 10.5±0.3 and 10.3±0.3×10-3 s-1 at 24 and 32 km respectively. The decrease in J NO 2 at sunset was monitored in both flights. The measurements are found to be in good agreement with calculations of J NO 2 using a simplified isotropic multiple scattering computer routine.
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  • 4
    ISSN: 1573-0662
    Keywords: active nitrogen ; ozone ; radicals ; snow chemistry ; Arctic ; surface layer
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences
    Notes: Abstract Measurements of NOx (NO +NO2) and the sum of reactive nitrogenconstituents, NOy, were made near the surface atAlert (82.5°N), Canada during March and April1998. In early March when solar insolation was absentor very low, NOx mixing ratios were frequentlynear zero. After polar sunrise when the sun was abovethe horizon for much or all of the day a diurnalvariation in NOx and NOy was observed withamplitudes as large as 30–40 pptv. The source ofactive nitrogen is attributed to release from the snowsurface by a process that is apparently sensitized bysunlight. If the source from the snowpack is a largescale feature of the Arctic then the diurnal trendsalso require a competing process for removal to thesurface. From the diurnal change in the NO/NO2ratio, mid-April mixing ratios for the sum of peroxyand halogen oxide radicals of ≤10 pptv werederived for periods when ozone mixing ratios were inthe normal range of 30–50 ppbv. Mid-day ozoneproduction and loss rates with the active nitrogensource were estimated to be ∼1–2 ppbv/day and in nearbalance. NOy mixing ratios which averaged only295±66 pptv do not support a large accumulation inthe high Arctic surface layer in the winter and springof 1998. The small abundance of NOy relative tothe elevated mixing ratios of other long-livedanthropogenic constituents requires that reactivenitrogen be removed to the surface during transport toor during residence within the high Arctic.
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  • 5
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 77 (2000), S. 990-992 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Electron populations induced by spontaneous and piezoelectric polarization in semiconductor heterostructures can be estimated simply by using elementary electrostatic theory. The method is illustrated for the AlGaN/GaN system in which the AlGaN barrier is either undoped, or doped n type, and the effect of a GaN overlayer is described. © 2000 American Institute of Physics.
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  • 6
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 73 (1998), S. 2485-2487 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A simple rigorous analytical theory of two-dimensional (2D) nonequilibrium electrons occupying an arbitrary number of subbands in a quantum well is developed. The electric-field dependence of electron mobility and the average kinetic energy for AlN/GaN quantum wells are presented. At temperatures below 200 K the electron mobility is controlled mainly by the acoustic phonon scattering and it is a nonmonotonous function of the electric field, which has a maximum. At room and higher temperatures the interaction with both acoustic and polar optical phonons determine the hot-electron mobility and it depends very weakly on the electric field. Both the mobility and average energy of 2D electrons are smaller than that for three-dimensional (3D) electrons in the bulk semiconductor. Our theory provides a self-consistent transition from the 2D to the 3D regime of electron transport with increasing electric field accompanied by the occupation of an increasingly large number of subbands by the electrons. © 1998 American Institute of Physics.
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  • 7
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: For high-power electronics applications, GaN is a promising semiconductor. Under high electric fields, electrons can reach very high energies where polar optical phonon (POP) emission is the dominant scattering mechanism. So, we undertake a full-band analysis of POP scattering of conduction-band electrons based on an empirical pseudopotential band structure. To uncover the directional variations, we compute POP emission rates along high-symmetry directions for the zinc-blende (ZB) crystal phase of GaN. We also compare the results with those of the wurtzite phase. In general, the POP scattering rates in the zinc-blende phase are lower than the wurtzite phase. Our analysis also reveals appreciable directional dependence, with the Γ–L direction of ZB GaN being least vulnerable to POP scattering, characterized by a scattering time of 11 fs. For both crystal phases, we consider the negative differential conductivity possibilities driven by the negative effective mass part of the band structure. According to our estimation, for the ZB phase the onset of this effect requires fields above ∼1 MV/cm. © 2000 American Institute of Physics.
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  • 8
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 66 (1995), S. 3633-3635 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The continuum theory of optical-mode hybrids in multilayered structures is applied to the case of a quantum well containing a centrally placed monolayer of a semiconductor whose optical frequencies are very different from those of the host. It is shown that antisymmetric modes are strongly modified whereas symmetric modes are not modified at all. As a consequence, the intersubband scattering rate is expected to be reduced. © 1995 American Institute of Physics.
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  • 9
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A theoretical model that uses simple, analytic valence band equations to calculate the differential gain in strained layer InGaAs/GaAs quantum wells shows good agreement with experimental differential gain values obtained from multiple quantum well strained layer lasers. The differential gain in these devices is 7 times greater than in bulk, p-type doped InGaAsP lasers. Calculations including nonlinear damping effects indicate that modulation bandwidths exceeding 60 GHz should be achievable in strained layer quantum well lasers.
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  • 10
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The suppression of longitudinally polarized optical-phonon (LOP) electron scattering in multiple quantum wells (MQWs) was sought in short periodic AlAs/GaAs with well widths of 12, 15, and 20 monolayers and AlAs barrier widths of 2 and 4 monolayers, based on a study of electron mobility in the plane of the MQW. Two-dimensional electron-gas structures with MQWs of up to eight wells in their channel were grown. Their mobilities at room temperature were slightly reduced, as compared to samples without MQW channel, due to interaction with interface polaritons from AlAs barriers, while mobility at temperatures 〈50 K improved due to reduction of remote ionized impurity scattering. The theoretical analysis of the results based on the model of hybridon-electron interaction in an infinite superlattice is presented. The reduction of room-temperature mobility in the MQWs is believed to be caused by the interaction of electrons with both barrier interface-polariton (IP) -like modes and the well LOP-IP hybrids. An alternative explanation of the results of a similar experiment done elsewhere is offered denying the evidence of strong suppression of LOP scattering there.
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