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  • 1
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A simple optical multiple reflection system is developed with two cylindrical concave mirrors at an appropriate spacing. The two cylindrical mirrors have different focal lengths and their principal sections are orthogonal. The alternate focusing of the two cylindrical mirrors at different direction keep the reflecting spots small. The reflecting spots fall on Lissajous patterns on the cylindrical mirrors. The mathematics for this optical system is described and the calculated coordinates of beam spots are very close matches of the experimental observations. The cylindrical mirror optical system is easy to construct and align, with a suitable method for obtaining long optical paths and a large number of passes in small volumes. In a photoacoustic spectrometer the beam family enhance the effective power in the photoacoustic cell and thus the signal-to-noise ratio of photoacoustic signal. An experimental result for photoacoustic spectrum of HDSe gas is given. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
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  • 3
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We report a new design for an acoustically resonant photoacoustic cell with high performance. A cylindrical cell with suitable size was selected so that the resonant frequency of the first radial mode was equal to that of a longitudinal higher mode. The ends of the cell have a pair of thin coaxial tubes which lengths are 1/2 and 1/4 of acoustic wavelength λ, respectively. The λ/4 acoustic resonance makes a coupling between the first radial resonance and the higher longitudinal resonance, then leads to an acoustic energy concentrating in the middle of the cell. Thus, the surface loss was decreased, the acoustic quality factor and pressure amplitude increased obviously as compared with conventional radial resonant cylindrical cell. The ability to measure absorption coefficients as low as 2.43×10−9 cm−1 (1:1 signal to noise level) has been achieved. © 2002 American Institute of Physics.
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  • 4
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: By optimizing the size of a photoacoustic cell for decreasing acoustic loss and noise, and using a proper multipass arrangement for increasing light power in the cell and optical coupling to the acoustic wave, a high-resolution external laser photoacoustic spectrometer with a detection sensitivity of 6.35×10−9 cm−1 (1:1 signal to noise level) has been developed. Using this apparatus, the high quality Doppler-limited overtone spectra of AsH3 (600) and H2Se (60) have been observed at room temperature in the wave number regions 11 500–11 650 and 12 600–12 925 cm−1, respectively. Results are presented which highlight the applicability of this apparatus to the spectroscopy of weak optical transitions. © 2000 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 112 (2000), S. 9624-9631 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An ab initio quartic force field of PH3 is derived using the coupled-cluster theory with all singles and doubles supplemented with quasiperturbative treatment of the connected triple excitations [CCSD(T)], and Dunning's correlation consistent polarized valence triple zeta cc-pVTZ basis set. Improved geometry and quadratic force constants are evaluated, respectively, with the correlation consistent polarized valence quadruple zeta cc-pVQZ, core–valence quadruple zeta cc-pCVQZ, and weighted core–valence quadruple zeta cc-pwCVQZ basis sets. In the latter two cases, all electrons are correlated to account for the core correlation effects. Core–valence correlation effect on geometry is found to be significant. By comparing the cc-pCVQZ and cc-pwCVQZ results with the cc-pVQZ ones, the bond length re is reduced by 0.0045 or 0.0049 Å and the bond angle θe by 0.07° or 0.06°, respectively. Cubic and quartic force fields are further determined with the correlation consistent polarized weighted core–valence triple zeta cc-pwCVTZ basis set. Vibrational analysis based on second-order perturbation theory is carried out with the calculated force constants. Computed fundamentals of PH3 agree to better than 4 cm−1 on average with the experimental data. Spectroscopic constants are also predicted for a number of symmetric and asymmetric top isotopomers of PH3. With a minor empirical adjustment of our best force field, agreement of 1 cm−1 between the computed and experimental fundamentals is found for all isotopomers. © 2000 American Institute of Physics.
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  • 6
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Here we report the preliminary analysis of the high resolution spectra of 70GeH4 (6000) and (7000) stretch overtone bands obtained by intracavity laser absorption spectroscopy (ICLAS). The observed P(J+1), Q(J), and R(J−1) transitions agree well with the combination difference relations, indicating that the Coriolis coupling in the degenerate states is negligible. This is an important feature for a spherical top molecule near the local mode limit. The vibration–rotational energy levels of upper states as well as those of some perturber states are obtained. The improved local mode parameters of GeH4 are also derived.
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 3104-3108 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Acetylene and its evolution products (CCH, C2, and CCH2) on the Cu (001) surface are studied using the density functional method with cluster models. The binding energies, geometries, STM images, and vibrational frequencies for the adsorbates are obtained. The results agree well with the available experimental results. By comparing the calculated frequencies with the experimental ones, we identify the thermal evolution product of acetylene on Cu(001) in the HREELS experiment as CCH2, and establish its orientation relative to the surface. CCH2 is found to be more stable than C2H2 on Cu(001). © 2002 American Institute of Physics.
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  • 8
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The infrared spectra of PH3 molecule were recorded on a Bruker IFS 120HR Fourier transform spectrometer from 4000 to 9500 cm−1. The P–H stretching vibrational frequencies and intensities were derived from the experimental data. The Morse oscillator parameters De and α in the anharmonically coupled anharmonic oscillator local mode model were determined by the least-squares fitting with the observed vibrational band centers. The ab initio three-dimensional P–H stretching dipole moment surfaces were calculated by the density functional theory method. The dipole moment vectors were projected to three kinds of molecule-fixed reference systems. The corresponding dipole moment components were fitted to polynomial functions in terms of the P–H bond length displacements with the molecular symmetry taken into account. The absolute band intensities were obtained and then compared with the experimental data. The results showed that a proposed improved bond dipole model can predict the absolute band intensities within a factor of 2 for most of the observed transitions, indicating a reasonably good agreement. © 2001 American Institute of Physics.
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  • 9
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Relative absorption intensities of the Fermi resonance polyads of isolated CH chromophore for the CHCl3 molecule were calculated with one-dimensional and two-dimensional dipole moment surfaces, which are obtained by the ab initio density functional method. The calculation showed an unusual strong absorption at the second Fermi resonance polyad, which agrees well with the experimental data. Such an intensities anomaly can be attributed to the nonlinearity of the dipole moment surface in the vicinity of the equilibrium configuration. By taking advantage of the two-dimensional dipole moment surface where both CH stretching and bending vibrations are taken into consideration, the ν1 and 2ν4 bands which constitute the first Fermi resonance polyad and have little wave function mixture, are found to be of almost equal intensity. Such a calculation agrees with observations. © 2000 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 105 (1996), S. 8011-8019 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Present work studies the effective rotational Hamiltonians and their vibration–rotational parameters for XH2 and XH3 type molecules near the local mode limit by including the diagonal matrix elements of coordinate operators when the bond anharmonicity is significant. An improved "α relation'' is given for the local mode limit by taking the anharmonic bond oscillator wave function as the basis function. Then the rotational tunneling approach is extended to model the effect of nonzero interbond coupling for XH2 and XH3 type molecules, which provides a dynamical view of the rovibrational structure of the local mode states. © 1996 American Institute of Physics.
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