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  • 1
    Electronic Resource
    Electronic Resource
    An electron spin-echo envelope modulation study of the lowest triplet state of pyridine-d5: Spin-density distribution and structure (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 6549-6565 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Recently we have shown that the lowest triplet state (T0) of pyridine, incorporated in a single crystal of benzene, may be studied by electron spin-echo (ESE) spectroscopy. From the nitrogen hyperfine structure in the ESE detected electron paramagnetic resonance (EPR) spectra, we were able to conclude that pyridine, a planar molecule in the ground state, becomes nonplanar upon excitation into T0. Here we report the results of a detailed investigation of this distortion and of the electronic nature of the lowest triplet state of pyridine-d5. We have performed electron spin–echo envelope modulation (ESEEM) spectroscopy. From the modulation spectra, the electron-nuclear double resonance (ENDOR) frequencies corresponding to the various deuterium nuclei are obtained. Analysis of the dependence of these frequencies on the orientation of the magnetic field with respect to the triplet fine-structure axes system allows for a determination of the deuterium hyperfine and quadrupole tensors. From these tensors and the known nitrogen hyperfine tensor, the structure and spin-density distribution of pyridine in its lowest triplet state are deduced. Pyridine adopts upon excitation into T0 a boatlike structure, in which the nitrogen atom is tilted by about 40° with respect to the plane through the ortho- and meta-carbon atoms and the para-carbon/para-deuterium fragment by about 10° with respect to this plane. Thereby the hybridization of the ortho-carbon atomic orbitals strongly deviates from that for aromatic hydrocarbons and becomes almost sp3; the hybridization of the atomic orbitals on the other carbon atoms changes much less, while the nitrogen atomic orbitals remain sp2 hybridized. Approximately half of the spin density is found to be localized on the nitrogen atom with a remarkable distribution over the atomic orbitals: the π orbital carries 40%, the n orbital 10%. The rest of the spin density is distributed over the para- (30%) and ortho-carbon atoms (10% each). The lowest triplet state of pyridine is neither an nπ* state nor a ππ* state, but a state of mixed character as the result of a strong vibronic coupling between the 3B1 (nπ*) and 3A1 (ππ*) states.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457374
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  • 2
    Electronic Resource
    Electronic Resource
    Absorption cross sections, saturated vapor pressures, sublimation energies, and evaporation energies of some organic laser dye vapors (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 1403-1409 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new technique of transmission measurement of overheated dye vapors is applied to determine absolute absorption cross-section spectra of three active dyes for vapor phase dye lasers. The investigated compounds are 1,4-di[2-(5-phenyloxazolyl)]-benzene (POPOP), 1,4-di[2-(4-methyl-5-phenyl-oxazolyl)]-benzene (dimethyl-POPOP), and 2,5-diphenylfuran (PPF). The vapor absorption spectra are compared with liquid solution spectra in order to obtain information on the dye–solvent interaction. The saturated vapor densities are determined by transmission measurements after knowing the absolute absorption cross-section spectra. The latent heats of sublimation, evaporation, and melting are derived by analyzing the dependences of the saturated vapor densities on the vapor temperature.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457152
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  • 3
    Electronic Resource
    Electronic Resource
    Vibron band structure in chlorinated benzene crystals: Lattice dynamics calculations and Raman spectra of 1,2,4,5-tetrachlorobenzene (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 4023-4034 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: From a combined theoretical and experimental study of 1,2,4,5-tetrachlorobenzene (TCB) we conclude that this crystal shows many interesting effects. In agreement with earlier optical measurements, which probe the complete vibron band structure of TCB, we calculate that several of the vibron modes have the dispersion of a one-dimensional crystal with stacks of molecules along the a axis. The inclusion of fractional atomic charges in the atom–atom potential used in the calculations is absolutely necessary to obtain the correct vibron bandwidths. Also the sign of the vibrational coupling matrix elements, which is given correctly by the calculations, is determined by these charges. For other properties, such as the crystal stability, the phonon frequencies, the site splitting in the vibron bands, and also the dispersion of the lower frequency vibrations, the interactions between the "one-dimensional'' stacks are essential, however. The calculations predict further, in qualitative agreement with the Raman spectra, that the splitting between the pairs of inequivalent vibron bands in α-TCB (the site splitting) is significantly larger than the factor group splitting in β-TCB. Finally we have found that, for those vibrations where the conditions are shown by the calculations to be favorable, the 35Cl/37Cl isotope effects are clearly visible in the Raman spectra.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454835
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  • 4
    Electronic Resource
    Electronic Resource
    One-dimensional vibrational excitons in 1,2,4,5-tetrachlorobenzene (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 3719-3723 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have studied the line shapes of vibronic transitions in the phosphorescence spectrum of the one-dimensional triplet exciton of TCB between 4.2 and 0.4 K. It is shown that also the vibrational excitons are highly one dimensional. This finding is confirmed by time-resolved emission spectra. Observed vibron bandwidths vary up to 15 cm−1 with k=0 either at the top or at the bottom of the band. In contrast to the fundamental vibrations, no overtones or combination bands give evidence of a well-defined one-dimensional quasimomentum.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452976
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  • 5
    Electronic Resource
    Electronic Resource
    Magnetic and optical properties of triplet excitons and traps in 1,4-dibromobenzene (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 1521-1527 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An optical and magnetic resonance study on neat 1,4-dibromobenzene crystals shows that the molecules in their lowest triplet state are distorted into a chair-like shape and confirms previous conclusions about the one-dimensional character of the triplet excitons. From a study of the phosphorescence spectrum at various temperatures it is found that the band width of the triplet excitons is (−1.0±0.15) cm−1 and that the k=0 state is at the bottom of the band.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457112
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  • 6
    Electronic Resource
    Electronic Resource
    Single molecules as nanoprobes. A study of the Shpol'skii effect (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 117 (2002), S. 3383-3390 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a comparative study, down to the single-molecule level, of the linewidth and multiplet structure of the optical transitions of 2.3,8.9-dibenzanthanthrene (DBATT) in shock-frozen solutions of n-decane, n-undecane, and n-tetradecane. The n-alkane matrices show weak structural ordering and distinct incorporations of the DBATT molecules have been recognized. The order does not increase, nor does the ensemble optical linewidth decrease going from bulk to sub-volumes of less than 1 μm3. The relation between lateral position, orientation of the transition moment, and transition frequency has been investigated using single-molecule fluorescence-excitation and polarization spectroscopy. The distribution of the orientation is found not to depend on frequency and vice versa. Structural ordering on a microscopic scale is found not to be decisive as regards the linewidth and multiplet structure. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1494424
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  • 7
    Electronic Resource
    Electronic Resource
    Rotational relaxation in the H2CO A˜ 1A2 state by transient gain spectroscopy (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 82 (1985), S. 5755-5756 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new, pulsed pump–cw probe technique, transient gain spectroscopy, has been used to measure the zero pressure lifetime (279±10 ns) and self-collision depopulation rate (110.5±3.1 μs−1 Torr−1) for the H2CO A˜ 1A2 41 JKa,Kc =132,12 level as well as the 122,11←132,12(31±8 μs−1 Torr−1) and 142,13←132,12(40±6 μs−1 Torr−1) rotational relaxation rates. The dominant collisional process is rotational energy transfer, which, for the H2CO(A˜)+H2CO(X˜) process examined here, appears to follow dipole–dipole scaling and propensity rules. The 110.5 μs−1 Torr−1 depopulation rate in the A˜ state agrees with the vibrationless X˜ state rate obtained from microwave power broadening scaled by the 1.46 D/2.33 D electric dipole moment ratio. The two strongest a-dipole rotational state-to-state rates account for more than 50% of the total removal rate from H2CO A˜ 41 132,12.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.448564
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  • 8
    Electronic Resource
    Electronic Resource
    Microwave-frequency alternating current scanning tunneling microscopy by difference frequency detection: Atomic resolution imaging on graphite (1999)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 70 (1999), S. 3377-3380 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We present a detailed description of an experimental setup for alternating current scanning tunneling microscopy, in which two slightly detuned high frequency signals are mixed at the tunneling junction and the resulting difference frequency signal is amplified using conventional scanning tunneling microscope electronics. This signal is used to control the distance between the microscope tip and the sample. With graphite as a model surface atomic resolution images have been obtained. It is demonstrated that the origin of the generated signal on graphite is the nonlinearity of the static current–voltage characteristics. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1149922
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  • 9
    Electronic Resource
    Electronic Resource
    Dependence of the magnetization and remanence of single-domain particles on the second cubic anisotropy constant (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 90 (2001), S. 6243-6250 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Magnetization and remanent magnetization curves for noninteracting single-domain particles with cubic magnetocrystalline anisotropy have been calculated, taking into account the first two anisotropy constants. The dependencies of the saturation remanence, coercivity, remanence coercivity, and δM plots on the anisotropy constants ratio have been discussed. It has been found that the calculated saturation remanence shows maxima in the vicinity of the spin-reorientation transition points for both negative and positive first cubic anisotropy constants. The remanent magnetization for the case when the face diagonals 〈110〉 are the easiest magnetization orientations has been analytically determined as well. It has been shown that when more than one type of easiest directions coexist, the remanence of the system can only be obtained numerically because of the history dependence of the remanent magnetization for some particles' configurations. The predicted remanence behavior for systems representing spin-reorientation has been compared with that of a Tb0.6Ho0.4Fe2 powder sample, and an excellent agreement between numerical and experimental data has been found. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1415062
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  • 10
    Electronic Resource
    Electronic Resource
    A stabilized vapor liquid interface in deuterated cyclohexane–methanol mixtures (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 3631-3635 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ellipticity of the vapor–liquid interface above mixtures of methanol (CH3OH) and deuterated cyclohexane (C6D12) has been measured in the vicinity of the consolute point. The data above and below Tc are consistent with models for the structure of the interface adapted from the theory of Widom and Ramos-Gomez and Widom. As three-phase coexistence is approached, the quantity of methanol adsorbed at the interface increases whether ordinary or deuterated cyclohexane is used in the mixture. If ordinary cyclohexane were used for studies within the three-phase region below Tc, thick, unstable, lenticular films of the methanol-rich liquid phase would form at the interface and lead to nonreproducible data. In the present measurements lenticular films do not form when deuterated cyclohexane is used in the mixture. (The entire methanol-rich phase then forms on top.) The gravitationally stable interface above the deuterated mixture can be studied reliably in the three-phase region below Tc even though the liquid phases are nearly density matched.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.450934
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