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  • 11
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 1335-1343 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The shape of the ν1 Raman Q branch of CH4 perturbed by Ar and He at room temperature has been studied. Stimulated Raman spectroscopy (SRS) experiments have been made in the 2915–2918 cm−1 spectral region for total pressures from 0.4 to 70 atm and mixtures of (approximate)5% CH4 with He and Ar. Analysis of the spectra demonstrates that the shape of the Q branch is significantly influenced by line mixing and much narrower than what is predicted by the addition of individual line profiles. For the first time, a model is proposed for the calculation and analysis of the effects of collisions on the considered spectra. In this approach, the rotational part of the relaxation matrix is constructed, with no adjustable parameter, starting from semiclassical state-to-state rates. Two empirical constants which account for the shift and broadening of the branch due to vibrational effects are introduced and their values are determined from fits of measured spectra. Comparisons between measurements and results computed with and without the inclusion of line mixing are made. Although not perfect, our model satisfactory accounts for most effects of pressure at low densities, where rotational transfers are dominant, as well as at high densities, where the profile is strongly influenced by vibrational contributions. It is shown that collisions with He and Ar lead to different behaviors at elevated pressure. The influence of the perturbation introduced by the Fermi coupling between the ν1 and ν2+ν4 levels is discussed and the rotational and vibrational contributions to the spectral shape are pointed out. © 2000 American Institute of Physics.
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  • 12
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 9315-9324 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The shapes of the ν1 and 2ν2 isotropic Raman Q-branch of CO2 perturbed by argon and helium have been measured by Stimulated Raman Spectroscopy (SRS) or coherent anti-Stokes Raman Spectroscopy (CARS) techniques. The data have been successfully analyzed with an energy corrected sudden (ECS) approximation model based on basic rates determined independently. Finally comparison of the present data with time resolved double resonance experiments allows us to discuss the physical origin of the two empirical constants which account for the shift and broadening of the branch due to vibrational effects. © 1999 American Institute of Physics.
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  • 13
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The energy corrected sudden approach is used in order to deduce collisional parameters and to model infrared quantities in Π←Σ bands of CO2–He and CO2–Ar mixtures in the 200–300 K temperature range. Measured line-broadening coefficients and absorption in the Q-branch of the ν2 band at moderate pressure are first used for the determination (from a fit) of the time constant associated with the relaxation of the second order traceless tensor of the rotational angular momentum (all other collisional quantities have been determined previously). The results obtained are consistent with previous (calculated) temperature dependent values of the depolarized Rayleigh cross sections. The model is then successfully tested through computations of absorption in the ν2 and (ν1+ν2)I bands at elevated densities. Analysis of line-mixing effects is made, including study of the influence of interbranch transfers and of Coriolis coupling. Differences between the effects of collisions with He and Ar are pointed out and explained. © 1999 American Institute of Physics.
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  • 14
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 6684-6690 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Two Q branches of N2O near 579.3 and 2798 cm−1 belonging to the 2ν20e−ν21f and ν2+ν3 bands, respectively, of Σ←Π and Π←Σ symmetry, have been studied for He and N2 perturbers at pressures ranging from 0.1 to 2 atm, using a tunable diode laser and a difference-frequency laser spectrometer. To interpret the line-mixing effects in these spectra, we have applied a model based on the energy corrected sudden approximation whose parameters have been only derived from line-broadening data for N2O–He and also from the measured absorption by the Q branches for N2O–N2. This model provides a satisfactory agreement with experimental band shapes, whatever the band, the perturber and the pressure considered. Significantly larger line-mixing effects are shown for N2O–He with respect to N2O–N2. Finally, the assumption made in the calculations to treat separately the couplings in the even and odd j levels appears to have a negligible influence on the resulting band shapes. © 1998 American Institute of Physics.
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  • 15
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 6467-6478 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A simple approach is developed in order to model the influence of collisions on the shape of infrared absorption by linear molecules. It accounts for line-mixing effects within, as well as between, the different branches (P,Q,R) of the band. It is based on use of the strong collision model, of a classical representation of rotational levels, and of the rigid rotor approximation. The absorption coefficient then has a very simple analytical expression; its wave number and pressure dependencies are computed by using eight parameters which depend on the considered vibrational transition, the temperature, and the nature of the perturber only. These quantities are band-averaged values of the detailed spectroscopic and collisional parameters of the molecular system. Tests of the model are presented in the ν3 and 3ν3 bands of CO2 perturbed by He and Ar at elevated pressures. They demonstrate the accuracy of our approach in accounting for the effects of collisions on the spectral shape in a wide density range; indeed, the superposition of Lorentzian individual lines at low pressure, as well as the collapse (narrowing) of the band at very high pressure are satisfactory predicted. © 1995 American Institute of Physics.
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  • 16
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 3009-3010 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Comparisons are made between different methods of introducing the principle of detailed balance in far-wing line shape theories. In particular, it is shown that by symmetrizing the operators in the quasistatic formulation of Ma and Tipping [J. Chem. Phys. 95, 6290 (1991)], one can obtain the same results for the symmetrized matrix elements of the relaxation operator and their frequency detuning that were introduced in an ad hoc way in the resonant quasistatic formulations of Boulet and co-workers [J. Chem. Phys. 91, 2163 (1989); 94, 6406 (1991)]. © 1995 American Institute of Physics.
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  • 17
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 81 (1997), S. 2966-2972 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A photothermal microscope that provides micrometer lateral and submicrometer depth resolution was designed. Thermal conductivity measurements with modulation frequencies up to 12 MHz on single grains in polycrystalline diamond demonstrate its lateral resolution power even for a highly conducting material. Measured conductivities strongly depend on the averaged volume and values up to 2200 W/mK are found in the high frequency limit where the properties inside a grain are sampled. The capability of the instrument to measure thermal parameters on thin films is demonstrated for gold films evaporated on quartz with a thickness ranging from 20 to 1500 nm. Measurements reveal a strong thickness dependence for both thin film conductivity and the contact resistance between film and substrate. Thermal conductivity decreases monotonically from 230 to 30 W/mK whereas the contact resistance rises from 2×10−7 to 8×10−6 m2K/W with decreasing film thickness. © 1997 American Institute of Physics.
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  • 18
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 72 (1998), S. 3151-3153 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report on a self-regulated method for the growth of tilted superlattices. It relies on the reconstructed surfaces alternatively stabilized during the atomic layer epitaxy (ALE) of compound semiconductors. The c(2×2)+(2×1) Cd-stabilized and the (2×1) Te-stabilized surfaces alternatively formed during the ALE of CdTe and CdMn(Mg)Te ensure a self-regulation of the growth at 0.5 monolayer deposited per ALE cycle for both CdTe and CdMn(Mg)Te. We are thus able to overcome the problem of precise flux control inherent to tilted superlattices. © 1998 American Institute of Physics.
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  • 19
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 70 (1997), S. 1113-1115 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have grown by atomic layer epitaxy CdTe/MnTe tilted and serpentine superlattices. These heterostructures are formed by depositing in the step-flow growth mode fractional monolayer superlattices (CdTe)m(MnTe)n, with p=m+n∼1, onto 2 °A and 2 °B Cd0.95Zn0.05Te vicinal substrates. Transmission electron microscopy images reveal a good in-plane CdTe/MnTe separation and a uniform short-range superlattice period. The very existence of those superlattices imply that Te-based vicinal surfaces present a regular array of monomolecular steps, with no important step meandering and no step bunching. © 1997 American Institute of Physics.
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  • 20
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 80 (1996), S. 2013-2018 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The thermoreflectance technique is applied for imaging electric current distributions and thermal transfer in a temperature reference resistor heated by an alternating current. High-frequency scans (30 kHz) allow imaging of the current density distribution in conducting strips of the resistor while scans of amplitude and phase of the surface temperature variation at lower frequencies reveal plane, cylindrical, and spherical thermal waves. We investigate wave dimensionality as a function of heating geometry and thermal length, and present a simple method allowing a quantitative thermal analysis by exploiting the phase profile of cylindrical thermal waves. © 1996 American Institute of Physics.
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