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  • 2000-2004  (12)
  • 1990-1994  (48)
  • 1985-1989  (66)
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  • 1
    Electronic Resource
    Electronic Resource
    Solvent interaction with the (2p3s) Rydberg state of hexamethylenetetramine: Energetics and relaxation dynamics (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 118-125 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The (1+1) mass resolved excitation spectra are reported for the (2p3s)←(2p)2 Rydberg transition of the tricyclic, high symmetry molecule hexamethylenetetramine [HMT (C6H12N4)] and its van der Waals clusters. The solvent molecules employed include both nonpolar (Ar, CH4) and polar (NH3, CH3OCH3) species. HMT and its clusters are generated and cooled in a supersonic expansion. The observed electronic transition is assigned as T2←A1 within the Td print group. A transition blue shift of 52 cm−1 for each Ar atom and 65 cm−1 for each methane molecule in the HMT van der Waals cluster is characterized. These shifts are caused by an excited state repulsive interaction between the excited Rydberg electron and the closed shell solvent which reduces the attractive dispersion interaction between the HMT and nonpolar solvent species in the van der Waals cluster. A transition red shift of more than 600 cm−1 for NH3 and CH3OCH3 solvent/HMT clusters is observed; this large increase in interaction energy for the excited Rydberg state of HMT with respect to the ground state of HMT is associated with the delocalization of the excited electron into available (virtual) Rydberg orbitals of the solvent molecules. The interaction is characterized as an electron transfer interaction. These results and assignments are consistent with previously reported ones for DABCO/solvent clusters. Relaxation dynamics of excited HMT and its clusters are investigated via a pump/probe (ionization) technique. Relaxation of the clusters is dominated by an intersystem crossing mechanism resulting in an excited state singlet lifetime of ∼5 ns compared to a bare molecule HMT excited state lifetime of ∼1.0 μs. A triplet state of HMT lies 255 cm−1 below the first excited singlet Rydberg state as determined by two-color threshold ionization studies.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468494
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  • 2
    Electronic Resource
    Electronic Resource
    (σ3s) Rydberg states of cyclohexane, bicyclo[2.2.2]octane, and adamantane (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 8625-8632 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this effort the effect of Rydberg electronic excitation on the structure of cyclic and polycyclic alkanes is investigated. Two-photon resonant, one-photon ionization, mass-resolved excitation spectroscopy is employed to observe the (σ3s)←(σ)2 Rydberg transitions of cyclohexane, bicyclo[2.2.2]octane, and adamantane cooled in a supersonic jet expansion. Rydberg spectra of these three molecules display sharp, well-resolved vibronic structure. Analysis of the spectra is assisted by isotopic substitution, circular/linear polarization, vibronic feature widths (rotational selection rules), as well as comparison to the ground-state vibrational energies. A significant reduction of vibrational energies in the excited electronic state and a 381 cm−1 blue shift of the transition origin upon deuterium isotope substitution for cyclohexane are interpreted as due to the promotion of an electron from a σ-bonding orbital to a nonbonding Rydberg orbital upon optical excitation. Extensive vibronic coupling is observed for both cyclohexane and adamantane in their excited (σ3s) Rydberg electronic states. Jahn–Teller splitting is small for adamantane but quite substantial for cyclohexane. This difference is attributed to the basic stability difference for the two different ring systems (mono- and tri-cyclic). A progression in a nontotally symmetric mode is observed in the Rydberg spectrum of bicyclo[2.2.2]octane suggesting a change in the geometry of this molecule upon (σ3s)←(σ)2 excitation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466716
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  • 3
    Electronic Resource
    Electronic Resource
    Chemical reactions in isolated clusters: Excited state electron transfer in 3- and 4-dimethylaminobenzonitrile (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 871-878 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Two-color time-of-flight mass spectra of 3- and 4-dimethylaminobenzonitrile (3- and 4-DMABN) bare molecules and clusters with methane, water, acetone, dichloromethane, and acetonitrile are reported and discussed. The clusters and molecules are isolated and cooled in a supersonic expansion. Both 3- and 4-DMABN bare molecules display significant changes in geometry, associated with rotation–inversion coordinates of the dimethylamino group, upon excitation from the ground electronic state S0 to the first excited singlet state S1. Cluster spectra for the monosolvates [CH4, H2O, (CH3)2CO, CH2Cl2, CH3CN] of 3- and 4-DMABN evidence two general types of behavior. (1) Cluster spectra have both red and blue shifts from their respective bare molecule origins which are relatively small (in general less than 200 cm−1). These cluster spectra are nearly identical with the bare molecule spectra; the solvation process seems to have little effect on the DMABN molecule, especially the –N(CH3)2 moiety, for these clusters or cluster states. (2) Cluster spectra have shifts that are large (∼500–1000 cm−1) and to low energy of the bare molecule spectra. These cluster spectra are composed of both sharp and broad features which bear little resemblance to those observed in the respective bare DMABN molecule spectra. Results of calculations of cluster geometry suggest that thedifferent cluster spectra may be associated with clusters of different geometry. These data are interpreted in terms of solution behavior of 3- and 4-DMABN. The relationship between the twisted intramolecular charge transfer (TICT) state model proposed for 4-DMABN in dipolar aprotic solvents and these cluster results is explored.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454166
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  • 4
    Electronic Resource
    Electronic Resource
    Geometry and torsional motion of biphenyl in the ground and first excited singlet state (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 7337-7347 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The lowest excited singlet state of biphenyl (BP) and a number of its isotopically and chemically substituted analogs have been studied by supersonic jet laser spectroscopy. The symmetry species of this excited state in BP can be classified as B+2u in the G16 extended molecular symmetry group G16 (EM). The symmetry-allowed origin of the biphenyl -h10 S1←S0 electronic transition occurs at 35 268 cm−1. The frequency of the torsional motion in S1 is determined to be ∼65 cm−1. The potential parameters for this motion in S1 are V2=1195 cm−1, V4=−190 cm−1, and V6=18 cm−1. The torsional motion for the ground state (∼50 cm−1) can be described by V2=50 cm−1 and V4=−148 cm−1. The change in the dihedral angle between the two benzene rings in BP upon S0 to S1 excitation is determined to be ∼44° based on a Franck–Condon factor calculation. Several fundamentals of the molecular vibrations are assigned in the S1 state. The exciton interaction between the coupled benzene rings in biphenyl is suggested to be large (〉103 cm−1).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454345
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  • 5
    Electronic Resource
    Electronic Resource
    Spectroscopic studies of cryogenic fluids: Benzene in argon and helium (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 2457-2465 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Energy shifts and bandwidths of the 610 vibronic feature of the 1B2u←1A1g optical absorption spectrum of benzene dissolved in supercritical argon and helium, and in liquid argon are reported as a function of pressure, temperature, and density. Benzene/Ar solutions display red shifts of the 610 transition with increasing density but the dependence is found to be nonlinear at high densities. Benzene/He solutions evidence blue shifts of the 610 transition as a function of increasing density which also becomes nonlinear at high densities. Only small spectral shifts are recorded if the density is kept constant and pressure and temperature are varied simultaneously. In addition, a small density independent temperature effect on the transition energy shift is identified. Experimental results are compared to dielectric (Onsager–Böttcher and Wertheim) and quantum statistical mechanical (Schweizer–Chandler) theories of solvent effects on solute absorption energy. Reasonably good agreement between experiment and theory is found only for the benzene/Ar system at relatively low densities. The theory fails to predict energy shifts for both the benzene/He and high density benzene/Ar systems. This result is different from the findings for the benzene/N2 and benzene/C3H8 solutions and can be interpreted qualitatively in terms of competition between dispersive attractive and repulsive interactions as a function of density. The failure of the theory to describe these transition energy shifts is attributed to the omission of explicit repulsive interactions terms in the theoretical models employed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453086
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  • 6
    Electronic Resource
    Electronic Resource
    Spectroscopic studies of cryogenic fluids: Benzene in nitrogen (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 4783-4789 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Energy shifts and bandwidths for the 1B2u←1A1g optical absorption of benzene in supercritical nitrogen are presented as a function of pressure, temperature, and density. The pressure and density dependence of energy shifts of room temperature emission of benzene in nitrogen fluid is also reported. Both absorption and emission spectra exhibit shifts to lower energy as a function of density, whereas almost no spectral shifts are observed if the density is kept constant and temperature and pressure varied simultaneously. Thus, density is the fundamental microscopic parameter for energy shifts of optical transitions in supercritical nitrogen. This result is analogous to the findings for the liquid benzene/propane system and can be interpreted qualitatively in terms of changes occurring in the intermolecular potential; however, in the benzene/supercritical nitrogen system an additional small density independent temperature effect on the transition energy has been identified. Experimental results are compared to dielectric (Onsager–Böttcher and Wertheim) and microscopic quantum statistical mechanical (Schweizer–Chandler) theories of solvent effects on solute electronic spectra. Reasonably good agreement between experiment and theory is found. The results demonstrate that liquid state theory can be used to describe the supercritical nitrogen fluid.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452700
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  • 7
    Electronic Resource
    Electronic Resource
    Spectroscopic studies of cryogenic fluids: Benzene in propane (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 3197-3206 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Energy shifts and bandwidths for the 1B2u↔1A1g optical absorption and emission transitions of benzene dissolved in propane are presented as a function of pressure, temperature, and density. Both absorption and emission spectra exhibit shifts to lower energy as a function of density, whereas no shifts are observed if density is kept constant and temperature and pressure are varied simultaneously. Density is thus the fundamental microscopic parameter for energy shifts of optical transitions. The emission half-width is a linear function of both temperature and pressure but the absorption half-width is dependent only upon pressure. These results are interpreted qualitatively in terms of changes occurring in the intermolecular potentials of the ground and excited states. Both changes in shape of and separation between the ground and excited state potentials are considered as a function of density. Classical dielectric (Onsager–Böttcher), microscopic dielectric (Wertheim) and microscopic quantum statistical mechanical (Schweizer–Chandler) theories of solvent effects on solute electronic spectra are compared with the experimental results. Calculations suggest limited applicability of dielectric theories but good agreement between experiment and microscopic theory. The results demonstrate the usefulness of cryogenic solutions for high pressure, low temperature spectroscopic studies of liquids.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452030
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  • 8
    Electronic Resource
    Electronic Resource
    van der Waals clusters of pyridazine and isoquinoline: The effect of solvation on chromophore electronic structure (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 6707-6716 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: van der Waals clusters of pyridazine and isoquinoline with CH4, NH3, H2O, and CH3OH are generated in a supersonic molecular jet expansion and investigated by two-color time-of-flight mass spectroscopy. As is the case for the other diazine systems, no spectra could be observed for pyridazine (H2O)n or (CH3OH)n clusters. Both chromophore molecules are reported to have close lying, vibronically coupled S1 and S2 excited states: nπ* for pyridazine and nπ* (S1) and ππ* (S2) for isoquinoline. Cluster spectra for pyridazine methane and ammonia clusters do not favor the presence of two nπ* transitions in the S1←S0 transition region but rather suggest that the "S2 origin'' is a vibronic feature of the S1←S0 transition. Isoquinoline clusters that are only weakly or not at all hydrogen bonded (CH4 and NH3) display a complicated spectrum indicative of S1 (nπ*)–S2 (ππ*) vibronic coupling and not the usual shifted isolated molecular spectrum. Isoquinoline clusters with substantial hydrogen bonding (H2O and CH3OH) display relatively simple spectra indicative of only a single electronic transition S2 (ππ*)←S0 in the region and no interstate vibronic coupling. These results are compared and contrasted with each other and the spectra of the other diazine clusters. Potential energy calculations are also employed to help understand the clustering in these systems.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452369
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  • 9
    Electronic Resource
    Electronic Resource
    On the pyrazine and pyrazine–pyrimidine dimers (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 3243-3250 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Spectra of the pyrazine-d4, pyrazine-h4–pyrazine-d4, and pyrazine-d4–pyrimidine dimer are obtained and analyzed with the help of Lennard-Jones–hydrogen-bonding (LJ–HB) potential energy calculations. The pyrazine isotopic hetero and homo dimers possess nearly identical spectra with the exception that the perpendicular dimer features are displaced to the red by ∼11 cm−1. Exchange or exciton interactions in this system are vanishingly small (less than 1 cm−1). The geometries suggested by the isotopically substituted pyrazine dimer spectra are the same as those found for the pyrazine-h4 homo dimer: a parallel planar hydrogen bonded and a perpendicular dimer. The pyrazine-d4– and pyrazine-h4–pyrimidine dimer spectra are quite complicated. These spectra can be assigned as arising from one parallel stacked head-to-tail displaced dimer, one parallel planar dimer, and three perpendicular dimers based on comparisons with spectra of the pyrazine and pyrimidine dimers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.450994
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  • 10
    Electronic Resource
    Electronic Resource
    The S2 ← S0 laser photoexcitation spectrum and excited state dynamics of jet-cooled acetophenone (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 2365-2367 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The S2 ← S0 photoexcitation spectrum of jet-cooled acetophenone is presented. The observed homogeneously broadened linewidths indicate S2 lifetimes ≤0.26 ps while the measured emission lifetimes range from 540±30 ns for S2 000 excitation to 130±50 ns for S2 1210 excitation. The dynamics revealed by the spectrum are discussed in terms of the known photochemical and photophysical rates of the excited states of acetophenone. Arguments are presented which identify the emitting state as a known triplet state of acetophenone.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.451090
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