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  • Articles  (128)
  • 2000-2004  (128)
  • Physics  (128)
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  • Articles  (128)
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  • 1
    Electronic Resource
    Electronic Resource
    Water-chain clusters: Vibronic spectra of 7-hydroxyquinoline⋅(H2O)2 (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 1192-1203 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Mass- and isomer-selected S1←S0 resonant two-photon ionization and S1→S0 fluorescence spectra were obtained for the supersonically cooled 7-hydroxyquinoline⋅(H2O)2 cluster. UV/UV-holeburning measurements show that 〉98% of the spectrum is due to a single "water-chain" cluster isomer, although two different tautomers (7-keto- and 7-hydroxyquinoline), two different rotamers (cis- and trans-hydroxy), and two torsional conformers of the chain are possible. Ab initio calculations of structures and vibrations of five different tautomers/ rotamers/ conformers of this cluster are reported. These predict that the cis-7-hydroxyquinoline⋅(H2O)2 "up/down" water-chain form is the most stable cluster. The experimentally observed S0 and S1 state vibrational frequencies agree well with those calculated for this isomer. We find no evidence for either the trans-rotamer or the keto tautomer clusters. S1←S0 excitation leads to contraction of all three hydrogen-bonds along the hydrogen-bonded water chain, inducing intermolecular stretching vibrations, but no proton transfer. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480672
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  • 2
    Electronic Resource
    Electronic Resource
    Vibrational spectroscopy and photodissociation of jet-cooled ammonia (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 4955-4961 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Vibrationally mediated photodissociation action spectroscopy provides rotation-vibration spectra of jet-cooled ammonia in the 2.3 μm and 3.0 μm regions by detecting the emission of electronically excited NH2(A˜ 2A1) produced by the photodissociation of the vibrationally excited molecules. Vibrational excitation changes the relative photofragmentation yield of NH2(A˜ 2A1) markedly. Isoenergetic photolysis of ammonia molecules with one quantum of antisymmetric N–H stretching excitation (ν3) or two quanta of bend (2ν4) yields three times more excited state NH2(A˜ 2A1) than photolysis of NH3 with a quantum of symmetric N–H stretch excitation (ν1). By contrast, the relative yield is insensitive to initial vibrational excitation of the combination bands ν1+ν2 and ν2+ν3 that contain the umbrella (inversion) motion ν2. The vibrational mode dependence of the NH2(A˜ 2A1) photofragment yield arises from either enhanced Franck–Condon factors for electronic excitation or from an increased probability for the competing nonadiabatic dissociation to form the ground state NH2(X˜ 2B1) product. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1450550
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  • 3
    Electronic Resource
    Electronic Resource
    Semiclassical treatment of charge transfer in molecule-surface scattering (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 6396-6403 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have treated the ionization probability of iodine molecules scattered from diamond by a semiclassical surface hopping scheme, namely Tully's fewest-switches algorithm [J. Chem. Phys. 93, 1061 (1990)]. The interaction is described by a model potential that has been adjusted to empirical data. We start with a one-dimensional two-state model in which just the molecular distance from the surface and the neutral and negatively charged state of I2 are considered. We determine the ionization probability within the adiabatic and diabatic representation and compare it with exact quantum calculations. For this particular problem we find that the diabatic picture shows too little coherence, while the adiabatic representation yields satisfactory results. In the second part we have successively increased the complexity of the simulation by additionally taking a surface oscillator coordinate, the molecular rotation and vibration into account. Including more degrees of freedom damps out the Stückelberg oscillations present in the one-dimensional model. Our results qualitatively reproduce the observed dependence of the ionization probability on the incident energy of the molecules. This dependence is not given by the electronic coupling per se, but rather due to energy transfer to substrate and internal degrees of freedom during the scattering event. Finally, we are also able to reproduce the measured dissociation probability which can be explained in a centrifugal model. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1356457
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  • 4
    Electronic Resource
    Electronic Resource
    Water–wire clusters: Vibronic spectra of 7-hydroxyquinoline⋅(H2O)3 (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 9032-9043 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The supersonically cooled 7-hydroxyquinoline⋅(H2O)3 cluster was investigated by mass- and isomer-selected S1←S0 resonant two-photon ionization and S1→S0 fluorescence spectroscopy. UV(ultraviolet)/UV-holeburning measurements prove that a single cluster isomer is formed, although different tautomers (7-keto- and 7-hydroxyquinoline), rotamers (cis and trans) and isomers (cyclic and water–wire) are possible. Ab initio calculations of structures and vibrations of different tautomers and isomers of this cluster predict that the cis-enol-7-hydroxyquinoline⋅(H2O)3 "water–wire" cluster is the most stable species. The experimental S0 and S1 inter- and intramolecular vibrational frequencies are in good agreement with the calculated harmonic frequencies. S1←S0 excitation leads to contraction of all four hydrogen bonds along the hydrogen bonded water wire, inducing intense intermolecular stretching vibrations of the O–H(centered ellipsis)O and HO–H(centered ellipsis)N(quinoline) hydrogen bonds which terminate the water–wire. There are no spectroscopic signs of proton transfer. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1319352
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  • 5
    Electronic Resource
    Electronic Resource
    Proton transfer in 7-hydroxyquinoline⋅(NH3)n solvent clusters (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 560-565 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Mass- and isomer-selected S1←S0 resonant two-photon ionization and S1→S0 fluorescence spectra were obtained for the supersonically cooled 7-hydroxyquinoline⋅(NH3)n clusters with n=2–16. For n=2 and 3, the absorption and emission spectra exhibit discrete and narrow bands, characteristic of nonreactive hydrogen bonded ammonia-chain clusters. For n≥4, the S1←S0 R2PI spectra are completely broadened, with an onset at (approximate)355 nm. For n=4–7, a weak fluorescence emission is observed, Stokes-shifted by (approximate)185 nm, with a maximum at (approximate)540 nm, which shows discrete structure on a broad background. From comparison to fluorescence emission observed in bulk solution, we conclude that S1 state enol→keto tautomerization occurs. For the n≥7 or 8 clusters, the fluorescence emission spectra become completely unstructured and shift to the blue, peaking at 435–450 nm. This emission indicates the occurrence of either S1-state proton transfer to the ammonia solvent cluster and formation of the 7-HQ anion, or of the ground-state intracluster enol→keto tautomerization reaction. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480548
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  • 6
    Electronic Resource
    Electronic Resource
    A new heterocylic ring system in (4SR,6SR)-6-(2-phenylethyl)-4-phenyl-1,4,3-oxathiazin-2(6H)-one (2001)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 57 (2001), S. o252-o253 
    Details
    ISSN: 1600-5368
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The title compound, C17H17NO2S, is formed by reaction of 2-azidocarbonyloxy-4-phenyl-1-thiophenylbutane with iron(II) chloride as catalyst. This is the first example of the 1,4,3-oxathiazin-2(6H)-one ring system.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S160053680100304X
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  • 7
    Electronic Resource
    Electronic Resource
    (±)-(1RS,2SR)-1,2-Bis(2′-methyl-3′-oxocyclohex-1′-enyl)-1,2-diphenylethane (2001)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 57 (2001), S. o1145-o1146 
    Details
    ISSN: 1600-5368
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The title compound, C28H30O2, was obtained by an unprecedented osmium tetraoxide-mediated dimerization of (2′-methyl-3′-oxocyclohex-1′-enyl)phenylethane. The trans stereochemistry was revealed by X-ray crystallography. The molecule exhibits a crystallographic center of inversion.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S1600536801018384
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  • 8
    Electronic Resource
    Electronic Resource
    Ubiquitination-dependent cofactor exchange on LIM homeodomain transcription factors (2002)
    [s.l.] : Macmillian Magazines Ltd.
    Nature 416 (2002), S. 99-103 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The interactions of distinct cofactor complexes with transcription factors are decisive determinants for the regulation of gene expression. Depending on the bound cofactor, transcription factors can have either repressing or transactivating activities. To allow a switch between these different ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/416099a
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  • 9
    Electronic Resource
    Electronic Resource
    Return to equilibrium (2000)
    College Park, Md. : American Institute of Physics (AIP)
    Journal of Mathematical Physics 41 (2000), S. 3985-4060 
    Details
    ISSN: 1089-7658
    Source: AIP Digital Archive
    Topics: Mathematics , Physics
    Notes: We study an atom with finitely many energy levels in contact with a heat bath consisting of photons (blackbody radiation) at a temperature T〉0. The dynamics of this system is described by a Liouville operator, or thermal Hamiltonian, which is the sum of an atomic Liouville operator, of a Liouville operator describing the dynamics of a free, massless Bose field, and a local operator describing the interactions between the atom and the heat bath. We show that an arbitrary initial state that is normal with respect to the equilibrium state of the uncoupled system at temperature T converges to an equilibrium state of the coupled system at the same temperature, as time tends to +∞ (return to equilibrium). © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.533334
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  • 10
    Electronic Resource
    Electronic Resource
    A computer-controlled apparatus for performing pressure–composition–temperature measurements on metal hydrides with protium, deuterium, and tritium (2001)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 72 (2001), S. 1775-1780 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: An apparatus for measuring protium, deuterium, and tritium pressure–composition–temperature (PCT) thermodynamic equilibrium behavior on metal hydrides has been designed, constructed, and operated with deuterium at the Weapons Engineering Tritium Facility at Los Alamos National Laboratory. The apparatus is capable of performing PCT measurements with the three hydrogen isotopes at a maximum pressure of 3.45 MPa, and sample temperatures in the range 25–300 °C. A personal computer that controls the system provides the capability for manual or automated operation. Two deuterium PCT isotherms on palladium at 51 and 70 °C were measured with the apparatus in the automated mode to demonstrate its operation. Comparison of the resulting isotherms with literature data showed a plateau pressure at 70 °C in close agreement with the literature value (57.06 kPa versus literature value of 57.33 kPa). The deuterium capacity at 70 °C in palladium at the high end of the isotherm was measured both by the PCT apparatus and by weighing the sample before and after the PCT measurement. The deuterium capacity determined by the PCT apparatus (D/M=0.624 at 144.52 kPa) agreed well with the value determined by the weight difference measurement (D/M=0.628, at 144.52 kPa). These values agreed closely with the literature value (D/M=0.629 at 144.52 kPa). © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1347979
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