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  • 1
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Polymer solution dynamics may be inferred from light scattering spectra of dissolved optical probe particles. We compare a variety of probes in solutions of several polymers. In the "overlapping'' concentration/molecular weight regime, the Stokes–Einstein equation fails by up to a factor of 2, while the probe diffusion coefficient D follows a scaling law D/D0=exp(−aMγcνRδ) (c, M, and R are the polymer concentration, molecular weight, and the probe radius, respectively). Experimentally, γ=0.8±0.1, ν=0.6–1.0, and δ=−0.1 to 0, contrary to the theoretical predictions γ=0 and δ=1. With very high molecular-weight polymers, we observe a further "entangled'' regime, characterized by huge (104) failures of the Stokes–Einstein equation and the appearance of "fast'' modes in the scattering spectrum.
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 9517-9525 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A line-tunable CO laser has been used to record infrared absorption profiles for (NO)n+ and (NO)n+.Ar clusters for n in the range 2–19 and at wavelengths between 1600 and 1900 cm−1. The presence of an argon atom is shown to have the effect of reducing the internal energy content of the ions, and for the case of (NO)2+. Ar is the only circumstance under which infrared absorption can be detected for the dimer ion. The trimer ion, (NO)3+, is found to have a very strong absorption feature at ∼1700 cm−1, which remains both static and comparatively narrow in subsequent cluster ions containing up to 19 nitric oxide molecules. It is concluded that (NO)3+ forms a stable core in larger clusters and that there is very little additional charge delocalization. © 1999 American Institute of Physics.
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 9526-9535 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The orbital momentum profiles and binding energy spectra for the complete valence shell of propane are reported. The experiment has been performed using a high energy resolution (ΔE=0.95 eV FWHM) multichannel (e,2e) electron momentum spectrosocopy spectrometer at an impact energy of 1200 eV plus the binding energy. The measured binding energy spectra are compared and consistent with PES data available in the literature and also with the predictions of Hartree–Fock, Green's function and MRSD-CI methods. A strong splitting observed in the inner valence energy spectra due to electron correlation and ion relaxation effects is confirmed by MRSD-CI calculations. The experimental momentum profiles have been compared with calculations obtained using the target Hartree–Fock method with a minimum basis set and also a very large basis set. Density functional theory calculations using B3LYP functionals as well as large basis set MRSD-CI calculations are also reported. The agreement between theory and experiment for the shape of the orbital electron momentum distributions is generally good. © 1999 American Institute of Physics.
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 9536-9548 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: With the high accuracy afforded by the sextuple correlation consistent basis set of Dunning, we have calculated energy levels, dissociation energies, equilibrium distances, and other spectroscopic constants for eleven valence and four Rydberg states of the CH radical. Comparisons with experimental and previous theoretical results are made for each state that has been treated. An understanding of their binding is attempted by means of simple valence bond–Lewis diagrams. © 1999 American Institute of Physics.
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 9560-9567 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The effect of electronic polarizability on the absorption of infrared photons by diatomic molecules in an intense laser field is investigated by numerically simulating the time evolution of the system. The absorption spectrum has a frequency shift proportional to the intensity; its dependence on polarizability and anharmonicity is established. It is shown that for pulsed fields the effect of polarizability is irregularly dependent on the pulse length. Resonances in the half-frequency range are strongly enhanced by the electronic polarizability. © 1999 American Institute of Physics.
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  • 6
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The effect of rotationally inelastic collisions on the orientation of the angular momentum is investigated for C2H2 (ν2=1, j). The orientation of the angular momentum in initial rotational states ji=7 to 13 is prepared by stimulated Raman pumping using circularly polarized light and S-branch transitions. After allowing an appropriate time for collisions to occur the orientation is probed by laser-induced fluorescence in the A˜ 1Au←X˜ 1Σg transition. For acetylene the theoretically calculated and the measured initial orientation, A0(1), ranges between 0.7 and 0.9. A substantial part of the orientation is conserved in rotationally inelastic collisions, which could be measured up to |Δj|=6 with sufficient signal-to-noise ratio. These results are compared with previous measurements on the conservation of alignment in collisions. Rotationally inelastic collisions with |Δj|=2 result in an average rotation of the orientation vector by about 30°. The amount of orientation that remains after a rotationally inelastic collision is found to depend primarily on |Δj|. A Δmj=0 propensity rule often used in models of rotational energy transfer is found to be inconsistent with the measurements. Also a model which proposes a minimum change of the classical angle φ between j(vector) and the quantization axis does not reproduce well the experimental data. Finally, a microscopic model for Δmj-transitions is presented which reconciles both alignment and orientation measurements. © 1999 American Institute of Physics.
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 9574-9576 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A continuous wave (cw) tunable single mode laser beam was propagated perpendicular to a collimated effusive molecular beam of NaI, and the excitation spectra of the A0+(v′,J′)−X 1Σ+(v″,J″) transitions of the NaI molecule were measured by monitoring the intensity of the molecular fluorescence. A large variation of the linewidth and line intensity with the rotational quantum number J was observed. The linewidths of rotationally resolved lines, attributable to lifetime broadening originating from predissociation, were obtained by a fitting of the observed spectral line shape to a Voigt profile. © 1999 American Institute of Physics.
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 9568-9573 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The pulsed-field ionization zero-electron kinetic-energy (PFI-ZEKE) threshold photoionization spectrum of NO2 from 9.58 to 20 eV is obtained using vacuum ultraviolet synchrotron radiation by means of the Chemical Dynamics Beamline at the Lawrence Berkeley National Laboratory Advanced Light Source. The high resolution afforded by PFI threshold discrimination yields new or refined spectroscopic constants for a number of known excited states of the cation, including the first estimate of the A rotational constant in the a 3B2 state, as well as new fundamental frequencies for the A 1A2 and B 1B2 states, a precise determination of the singlet–triplet splitting in the c 3B1–C 1B1 complex and the first observations of the states, d 3A1 and D 1B2. Most significantly, ZEKE photoelectron detection resolves vibrational structure in the linear X 1Σg+ ground state of NO2+. Vibrational positions in the first electron volt of the spectrum are found to conform with the predictions of a Hamiltonian that includes Fermi resonance and other anharmonic terms derived from earlier multiresonant laser spectroscopic experiments on the lower bending excited states. © 1999 American Institute of Physics.
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 9577-9586 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Cluster growth dynamics of vanadium oxide and titanium oxide clusters produced by laser ablation of vanadium and titanium metal in a He gas flow seeded with up to 2% O2 are studied by covariance mapping time-of-flight mass spectrometry. Covariance mapping enables the recognition of two different distribution components in the overall homogeneous mass spectra for both vanadium oxide and titanium oxide cluster systems. The oxygen-rich component Or shows small correlated fluctuations while the oxygen-poor component Op shows large correlated fluctuations. These two cluster distribution components are observed at low ablation laser powers and low expansion gas concentrations. Fluctuations of small vanadium oxide clusters (V2O, V2O2, and V2O3) and small titanium oxide clusters (Ti2O2 and Ti2O3) are covariance determining. The less fluctuating V2O3 and Ti2O3 clusters are "nuclei" for the oxygen-rich components Or. The more fluctuating V2O and Ti2O2 are "nuclei" for the oxygen poor components Op. Correlated fluctuations or covariances within each distribution component are constant. Covariances for the different distribution components are different. Studies of mass spectra and covariances as functions of ablation laser power and expansion gas concentration imply that V2O and Ti2O2 clusters are formed in different regions of the ablation plasma plume than V2O3 and Ti2O3. We suggest that V2O3 and Ti2O3 are formed in the hot and optically dense region near the ablated metal surface and that V2O and Ti2O2 are formed in the colder plasma region farther away from the ablated metal surface. Larger vanadium oxide and titanium oxide clusters grow from these small clusters by very specific pathways which involve only uptake of VO or VO2, and TiO2, respectively. © 1999 American Institute of Physics.
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  • 10
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 9587-9592 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The microwave spectrum of CsO has been observed and analyzed, not only in the ground vibrational state, but also in the v=1–3 excited vibrational states. The CsO radical was generated by the reaction of N2O with Cs vapor, which was produced by the reaction of Li metal with CsCl at 500–530 °C. The observed spectra were found to conform to those expected for a 2Σ diatomic molecule, thereby establishing the ground electronic state of CsO to be of 2Σ. The observed rotational and centrifugal distortion constants yielded the equilibrium bond length and the harmonic vibrational frequency to be 2.300 745 (16) Å and 356.78 (11) cm−1, respectively, based on the Born–Oppenheimer approximation. A careful examination of the observed spectral pattern definitely concluded that the spin-rotation interaction constant was positive, at variance with the expectation from a simple 2Σ/2Π two-states interaction. This observation was interpreted by assuming positive contributions from higher excited electronic states which superseded a negative contribution from the 2Π lowest excited state; the latter state was responsible for the large dependence of the spin-rotation interaction constant on the vibrational quantum number and was estimated from this vibrational dependence to be located at 1225 cm−1 above the ground electronic state. In reverse to the spin-rotation splitting, the hyperfine splitting was found to increase with the vibrational excitation; in the v=3 state the hyperfine structure was found completely resolved. However, the hyperfine coupling constants did not vary much with the vibrational quantum number, namely the vibrational dependence of the hyperfine splitting was caused primarily by that of the spin-rotation splitting. The observed hyperfine interaction constants indicated that CsO was an ionic molecule. © 1999 American Institute of Physics.
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