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  • 1
    Electronic Resource
    Electronic Resource
    Line mixing and nonlinear density effects in the ν3 and 3ν3 infrared bands of CO2 perturbed by He up to 1000 bar (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 7306-7316 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present high density experimental and theoretical results on CO2–He absorption in the ν3 and 3ν3 infrared bands. Measurements have been made at room temperature for pressures up to 1000 bar in both the central and wing regions of the bands. Computations are based on an impact line-mixing approach in which the relaxation operator is modeled with the energy corrected sudden (ECS) approximation. Comparisons between experimental and calculated results demonstrate the accuracy of the ECS approach when applied to band wings and band centers at moderate densities. On the other hand, small but significant discrepancies appear at very high pressures. They are attributed to a number of reasons which include nonlinear density dependence due to the finite volume of the molecules, neglected contributions of vibration to the relaxation matrix, and incorrect modeling of interbranch mixing. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469042
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  • 2
    Electronic Resource
    Electronic Resource
    Experimental and theoretical study of line mixing in methane spectra. IV. Influence of the temperature and of the band (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 5776-5783 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Line-mixing effects are studied in infrared bands of CH4 perturbed by N2 at various pressures. The effects of temperature are investigated in the ν3 region whereas spectral shapes of the ν2, ν4, and ν3 bands are compared at room temperature. The theoretical approach proposed in preceding papers is used in order to model and analyze the influence of collisions on the spectral shape. All model parameters are now fixed to values determined in the previous studies. Comparisons between measurements and spectra computed with and without the inclusion of line mixing are made. They show that our approach satisfactorily accounts for the effects of temperature, pressure, and of rotational quantum numbers on the absorption by the ν3 band. Furthermore, the effects of collisions on spectra in the ν4 region at room temperature are also correctly calculated. On the other hand, the proposed approach fails in modeling the evolution with increasing pressure of absorption in the spectral range containing the ν2 band. This result is attributed to the Coriolis coupling between the ν2 and ν4 vibrational states and to a contribution whose physical origin remains unclear. The latter, which is negligible when CH4–He mixtures are considered, behaves as collision-induced absorption. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1289243
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  • 3
    Electronic Resource
    Electronic Resource
    Simple modeling of line-mixing effects in IR bands. II. Nonlinear molecules applications to O3 and CHClF2 (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 2185-2191 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A simple semiempirical approach is developed in order to model the shape of infrared absorption bands. It is based on use of the strong collision model and of a classical representation of rotational levels. The absorption coefficient then has a simple analytical expression whose wavenumber and pressure dependencies are computed by using eleven parameters which depend on the considered vibrational transition, the temperature, and the nature of the perturber only. These quantities, which are band-averaged values of the detailed spectroscopic and collisional parameters of the molecular system, can be deduced from direct fits of measured spectra. The model thus requires no previous knowledge of the characteristics of the molecules and is thus applicable to complex systems; in particular it seems a promising approach for very dense molecular spectra for which only absorption cross sections are now available. Tests are presented in the case of O3 and CHClF2 bands perturbed by N2 at room temperature for which new measurements have been made. They demonstrate the accuracy of our semiempirical approach in predicting the spectral shape in a wide range of density provided that effective parameters are used. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470974
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  • 4
    Electronic Resource
    Electronic Resource
    Temperature, pressure, and perturber dependencies of line-mixing effects in CO2 infrared spectra. III. Second order rotational angular momentum relaxation and Coriolis effects in Π←Σ bands (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 7733-7744 
    Details
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.478723
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  • 5
    Electronic Resource
    Electronic Resource
    Collision-induced absorption by H2 pairs in the second overtone band at 298 and 77.5 K: Comparison between experimental and theoretical results (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 4750-4756 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The collision-induced spectra of hydrogen in the region of the second overtone at 0.8 μm have been recorded at temperatures of 298 and 77.5 K and for gas densities ranging from 100 to 800 amagats. The spectral profile defined by the absorption coefficient per squared density varies significantly with the density, so that the binary absorption coefficient has been determined by extrapolations to zero density of the measured profiles. Our extrapolated measurements and our recent ab initio quantum calculation are in relatively good agreement with one another. Taking into account the very weak absorption of the second overtone band, the agreement is, however, not as good as it has become (our) standard for strong bands. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477813
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  • 6
    Electronic Resource
    Electronic Resource
    Experimental and theoretical study of line mixing in methane spectra. II. Influence of the collision partner (He and Ar) in the v3 IR band (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 6850-6863 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Line mixing effects are studied in the v3 band of CH4 perturbed by Ar and He at room temperature. Experiments have been made in the 2800–3200 cm−1 spectral region using four different setups. They cover a wide range of total densities, including low (0.25–2 atm), medium (25–100 atm), and high (200–1000 atm) pressure conditions. Analysis of the spectra demonstrates that the spectral shapes (of the band, the Q branch, the P and R manifolds,...) are significantly influenced by line mixing. The theoretical approach proposed in the preceding paper is used in order to model and analyze these effects. As done previously, semiclassical state-to-state rates are used together with a few empirical constants. Comparisons between measurements and spectra computed with and without the inclusion of line mixing are made. They prove the quality of the approach which satisfactorily accounts for the effects of pressure and of rotational quantum numbers on the spectral shape. It is shown that collisions with He and Ar lead to different line-coupling schemes (e.g., more coupling within the branches and less between branches) and hence to different shapes. The influence of line coupling between different branches and manifolds is evidenced and studied using high pressure spectra and absorption in the band wings. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480095
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  • 7
    Electronic Resource
    Electronic Resource
    Experimental and theoretical study of line mixing in methane spectra. I. The N2-broadened ν3 band at room temperature (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 7717-7732 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Line-mixing effects have been studied in the ν3 band of CH4 perturbed by N2 at room temperature. New measurements have been made and a model is proposed which is not, for the first time, strictly empirical. Three different experimental set ups have been used in order to measure absorption in the 2800–3200 cm−1 spectral region for total pressures in the 0.25–2 and 25–80 atm ranges. Analysis of the spectra demonstrates the significant influence of line mixing on the shape of the Q branch and of the P and R manifolds. A model is proposed which is based on state-to-state collisional transfer rates calculated from the intermolecular potential surface with a semiclassical approach. The line-coupling relaxation matrix is constructed from these data and two additional parameters which are fitted on measured absorption. Comparisons between measurements and spectra computed accounting for and neglecting line mixing are made. They prove the quality of the approach which satisfactory accounts for the effects of pressure and of rotational quantum numbers on the spectral shape under conditions where modifications introduced by line mixing are important. For high rotational quantum number lines, the main features induced by collisions are predicted but some discrepancies remain; the latter may be due to improper line-coupling elements but there is strong evidence that the use of inaccurate line broadening parameters also contributes to errors in calculated spectra. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.478724
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  • 8
    Electronic Resource
    Electronic Resource
    The EPR spectra of Gd3+ and Eu2+ in glassy systems (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 4285-4299 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A critical analysis is given of the EPR spectrum exhibited by the rare-earth S-state ions, Gd3+ and Eu2+, in glassy and disordered polycrystalline materials. The analysis of this spectrum and of its previous interpretations is based on (a) a set of criteria derived from a wide range of experimental EPR and optical data, and (b) a first principles computer simulation method which explicitly incorporates broad distributions in the crystal field interaction parameters. It is found that all four previous interpretations of the glassy spectrum are unsatisfactory, each failing to satisfy two or more of the criteria imposed by the full range of data. The correct general solution to the spectrum is unequivocally established and shown to be a convolution of (a) a broad and essentially unimodal distribution of second-order crystal field parameters, b02, with a maximum in the approximate range 0.051(approximately-less-than)b02 (approximately-less-than)0.056 cm−1, and (b) a broad distribution of asymmetry parameters, λ'=b22/b02, with appreciable probability over the whole range 0.0≤ λ'≤1.0. The prominent features in the X-band spectrum at g∼6.0 and 2.8 are found to be the result of specific EPR transitions that are stationary with respect to b02, λ', and the orientation angles of the applied field H over a wide range. The quantitive results indicate that the site symmetries of the RE ions are essentially very low and disordered, and are best characterized by a single low-symmetry "glassy type'' site.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.449041
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  • 9
    Electronic Resource
    Electronic Resource
    Collision-induced absorption in the ν2 fundamental band of CH4. II. Dependence on the perturber gas (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 123-127 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The integrated intensities of the collision-induced enhancement spectra of the ν2 band of CH4 perturbed by rare gases and linear molecules (N2, H2, and CO2) are calculated theoretically using the quadrupole transition moment obtained from an analysis of CH4–Ar spectra. In addition to the isotropic quadrupole mechanism responsible for the enhancement in CH4-rare gases, there is additional absorption arising from the anisotropic quadrupole mechanism in the case of molecular perturbers. This latter effect involves the matrix element of the anisotropic polarizability for the ν2 transition in CH4 that is available from the analysis of the depolarized Raman intensity measurements. Overall, the theoretical values for the slope of the enhancement spectra with respect to the perturber density are in reasonably good agreement with the experimental results, thus confirming that the collision-induced absorption arises primarily through the quadrupolar induction mechanism. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1424311
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  • 10
    Electronic Resource
    Electronic Resource
    Molecular motions in liquid HSiCl"3 and DSiCl"3 by Raman and IR spectroscopy
    Amsterdam : Elsevier
    Journal of Molecular Structure 46 (1978), S. 459-463 
    Details
    ISSN: 0022-2860
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://linkinghub.elsevier.com/retrieve/pii/0022-2860(78)87165-8
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