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  • 1
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A detailed analysis of the rotational and hyperfine structure of the (0,0) band of the B 3Φ–X 3Δ electronic transition of NbN has been performed from sub-Doppler spectra taken with linewidths of about 50 MHz. The Nb hyperfine structure is impressively wide in both states, but particularly so in X 3Δ where one of the unpaired electrons occupies a σ orbital derived from the metal 5s orbital. The electron spin and hyperfine structures do not follow the expected case (aβ ) coupling because of extensive second order spin-orbit effects. It is shown that the asymmetry in the spin–orbit structure of X 3Δ is explained almost quantitatively by interaction with a 1 Δ state from the same electron configuration (which lies at 5197 cm−1); also cross terms between the spin–orbit and Fermi contact interactions in the matrix element 〈3Δ2||H||1Δ〉 produce a large correction to the apparent coefficient of the I⋅L magnetic hyperfine interaction in X 3Δ2. The hyperfine structure in a triplet state turns out to be extremely sensitive to the details of the electron spin coupling, and reversals in the sense of the hyperfine structure in the 3Φ4–3Δ3 and 3Φ2–3Δ1 subbands are shown to be consistent with the3Δ state being a regular spin–orbit multiplet (A〉0). Particular care has been taken with the calibration, which has meant that extra terms have needed to be added to the magnetic hyperfine Hamiltonian to account for the spin–orbit distortions: instead of the usual three parameters needed in case (aβ ) coupling, the B 3Φ state has required four parameters and the X 3Δ state has required five. The model explains the data very well, and the standard deviation in the least-squares fit to more than 1000 hyperfine line frequencies was 0.000 58 cm−1 (17 MHz).
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 4827-4835 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photoisomerization of cis-stilbene in liquid solution was studied by time-resolved excited-state absorption spectroscopy using 306 nm pump and 612 nm probe pulses of 100 fs width. Transient absorption signals were found to decrease exponentially with time. Decay rate constants were determined over the pressure range 1–4000 bars at temperatures of 295 and 390 K in a series of alkane solvents as well as in methanol, acetonitrile, and in polymethylmethacrylate (PMMA). The viscosity dependence confirms the existence of two pathways of the reaction, one leading to ground-state cis- and trans-stilbene (C/TS), the other to dihydrophenanthrene (DHP). Whereas the DHP component shows only little viscosity dependence up to 4 kbar, the C/TS component is characterized by rate constants which are inversely proportional to the solvent viscosity. This is in contrast to earlier conclusions from studies with solvents at 1 bar. The C/TS process shows practically no temperature dependence apart from that of the viscosity, whereas the DHP process has an apparent activation energy of 8.5 kJ mol−1. The results are interpreted in terms of a Kramers–Smoluchowski equation for low-barrier processes as well as by solvent modifications of the low-barrier potential. Alternative interpretations by rate theory for barrierless processes are considered as well.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 4805-4816 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photoisomerization of diphenylbutadiene was studied by picosecond absorption spectroscopy over wide pressure and temperature ranges in liquid and supercritical alkanes, CO2, SF6, and He. The reaction shows typical features of a thermal unimolecular reaction on the S1 potential energy surface. The rate can be expressed by a combination of standard unimolecular rate theory and Kramers–Smoluchowski theory. However, multidimensional behavior manifests itself in the transition to the gas phase low pressure range as well as to the high density Kramers–Smoluchowski range: in the former case, the low pressure limit of a unimolecular reaction of the polyatomic molecule is approached; in the latter case, the effective imaginary barrier frequency shows a marked apparent temperature dependence. The experiments also suggest contributions of reactant–solvent cluster interactions, which modify the barrier height even in nonpolar solvents.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 3283-3291 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Rayleigh–Brillouin spectra of aqueous solutions of hydroxyl ammonium nitrate (HAN) and triethanol ammonium nitrate (TEAN) were studied at atmospheric pressure over a temperature range from 205 to 298 K. The hypersonic velocity and absorption coefficient of the phonons were calculated from the spectra as a function of temperature. The absorption due to viscosity and internal energy exchange through the relaxation mechanisms were considered. We find that both mechanisms are responsible for the absorption, but at the lower temperature, the one related to internal energy dominates. The free volume model was used to estimate the activation energy. This value is quite consistent with values calculated from the relaxation time. The effects of relaxation and the water concentration on mixtures of aqueous solutions of HAN and TEAN are also discussed.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 2393-2404 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The pressure and temperature dependence of the photoisomerization rate coefficient of trans-stilbene in the S1 state have been measured in the solvents C2H6, C3H8, C4H10, Xe, Co2, SF6, and CHF3. At constant temperature, the pressure dependences up to 6 kbar can be well represented by the Kramers–Smoluchowski model. The comparison of results in different solvents clearly indicates the importance of reactant–solvent cluster formation modifying the height and imaginary frequency of the barrier. The change of the temperature dependence with pressure points towards a multidimensional barrier of nonseparable character. Multidimensional barrier effects manifest themselves most clearly via the temperature dependence of the rate coefficient in the Kramers–Smoluchowski limit.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 3205-3212 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Dielectric potting materials (encapsulants) are used to prevent air breakdown in high-voltage electrical devices. We report breakdown strengths in void-filled encapsulants, stressed with unipolar voltage pulses of the order of 10 μs duration. High strengths, on the order of 100 kV mm−1, are measured under these test conditions. The materials studied include low-density open celled gel-derived foams with cell sizes of 4 μm or less, closed celled CO2-blown polystyrene and urethane foams, and epoxies containing 48 vol % of hollow glass microballoon (GMB) fillers. These last specimens varied the void gas (N2 or SO2) and also the void diameters (tens to hundreds of μm). Our measurements are thought to be directly sensitive to the rate of field-induced ionization events in the void gas; however, the breakdown strengths of the materials tested appeared to vary in direct proportion with the conventional Paschen-law gas-discharge inception threshold, the electric stress at which gas-ionization avalanches become possible. The GMB-epoxy specimens displayed this type of dependence of breakdown strength on the void-gas density and void size, but the measurements were an order of magnitude above the conventional predictions. Small-celled foams also showed increased breakdown strengths with decreased cell size, although their irregular void geometry prevented a direct comparison with the more uniformly structured microballoon-filled encapsulants. The experimental observations are consistent with a breakdown mechanism in which the discharge of a few voids can launch a full breakdown in the composite material. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5962-5971 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report breakdown strengths in a void-filled dielectric material, epoxy containing 48 vol % hollow glass microballoon filler, which is stressed with unipolar voltage pulses of the order of 10 μs duration. The microballoon voids had mean diameters of approximately 40 μm and contained SO2 gas at roughly 30% atmospheric pressure. This void-filled material displays good dielectric strength (of the order of 100 kV mm−1) under these short-pulse test conditions. Results from a variety of electrode geometries are reported, including arrangements in which the electric stress is highly nonuniform. Conventional breakdown criteria based on mean or peak electric stress do not account for these data. A statistics-based predictive breakdown model is developed, in which the dielectric is divided into independent, microballoon-sized "discharge cells" and the spontaneous discharge of a single cell is presumed to launch full breakdown of the composite. We obtain two empirical parameters, the mean and standard deviation of the spontaneous discharge field, by fitting breakdown data from two electrode geometries having roughly uniform fields but with greatly differing volumes of electrically stressed material. This model accounts for many aspects of our data, including the inherent statistical scatter and the dependence on the stressed volume, and it provides informative predictions with electrode geometries giving highly nonuniform fields. Issues related to computational spatial resolution and cutoff distance are also discussed. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 65 (1994), S. 2033-2035 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: New experimental results on Raman scattering from porous silicon and silicon and gallium arsenide nanocrystals are reported. In all of these systems, almost all vibrational modes become Raman active and are remarkably soft. A carrier-induced strain model is proposed to explain the optical properties of these nanocrystal systems. According to this carrier-induced strain model, the selection rule of crystal momentum conservation for Raman scattering is greatly relaxed in Si and GaAs nanocrystals due to the dilatation strain caused by coupling of excited free carriers with the particle lattice and the optical properties of such systems are dominated by multiphonon assisted free-electron transition processes. © 1994 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 101 (1994), S. 7566-7579 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The viscosity dependence of the photoisomerization of trans-stilbene in compressed liquid ethanol shows deviations from a simple power law description in the viscosity range from 1 to 4 mPa s. Corresponding deviations are observed in the solvents methanol, n-propanol, and n-butanol. This behavior is attributed to a competition between solvent relaxation and barrier crossing in the S1 state of trans-stilbene. The relative time scales of barrier crossing and solvent relaxation change as the pressure increases, because the dielectric relaxation rate of the solvent decreases more rapidly with increasing viscosity than the barrier crossing rate. Consequently, the reaction takes place in an increasingly retarded solvent environment which no longer relaxes completely around the changing charge distribution of the solute along its reaction path, giving rise to "dielectric friction.'' In contrast to trans-stilbene, the corresponding reaction of diphenylbutadiene in n-alkanols shows a much weaker sensitivity to solute-solvent interaction and, consequently, a simple inverse viscosity dependence of the photoisomerization rate is observed in all alkanols such as described by the Kramers–Smoluchowski theory. This significant difference is probably caused by smaller sudden polarization effects along the reaction path in diphenylbutadiene. The observed dependence of the trans-stilbene barrier crossing rate on pressure is compared either to a model with density dependent effective barrier height, or to a simple continuum model of the frequency dependence of the dielectric friction in the limit of weak coupling. Neither model works well unless a very strong viscosity dependence of the dielectric relaxation time of the solvent (τD∝η10) is employed to obtain agreement with the observed viscosity dependence of the barrier crossing rate.
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  • 10
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 6240-6262 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The (0,0) band of the B 4Π–X 4Σ− transition of NbO, near 6600 A(ring), has been analyzed from spectra taken at sub-Doppler resolution. The transition is notable for the great width of its Nb nuclear hyperfine structure, which is caused principally by the unpaired 5sσ electron in the ground state interacting with the large magnetic moment of the 4193Nb nucleus (I=9/2). A fit to the ground-state combination differences, including four very precise microwave lines measured by Suenram et al. [J. Mol. Spectrosc. 148, 114 (1991)], has given a comprehensive set of rotational, spin, and hyperfine parameters. Prominent among these are the third-order spin–orbit distortions of the spin-rotation interaction and the Fermi contact interaction, which are large and well determined, reflecting different degrees of spin–orbit contamination of the the 4Σ1/2− and 4Σ3/2− components of the ground state.The δ 2π B 4Π state was hard to fit, for a number of reasons. First, its spin–orbit structure is asymmetric, because of strong perturbations by a 2Π state which has been identified in this work, from among the various weak bands in the NbO spectrum near 7000 A(ring); the result is that many high order centrifugal distortion terms are needed in an effective Hamiltonian model for the rotation. Second, the hyperfine structure is perturbed, not only by this 2Π state, but by distant Σ and Δ states at higher energy. The δ 2σ* C 4Σ− state at 21 350 cm−1 appears to be one of these. The distant states generate large apparent nuclear spin-rotation interactions, both within and between the Λ components of the Π state, as a result of cross terms between matrix elements of the operators −2BJ⋅L and aI⋅L. Similar cross terms arising from the operators AL⋅S and aI⋅L produce corrections to the Fermi contact matrix elements and are responsible for the unexpected negative sign of the magnetic hyperfine parameter d. The "off-diagonal'' quadrupole parameter e2Qq2 is very large, and causes some of the higher J line shapes of the B–X system to be noticeably asymmetric at Doppler limited resolution; its value is consistent with the electron configuration of the B 4Π state being δ 2π.
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