Browsing by Author "Chen, Hong-Kai"

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Absorption cross sections of NH3, NH2D, NHD2, and ND3 in the spectral range 140-220 nm and implications for planetary isotopic fractionation
(2006-08-20)
Cheng, Bing-Ming
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Lu, Hsiao-Chi
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Chen, Hong-Kai
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Bahou, Mohammed
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Lee, Yuan-Pern
Cross sections for photoabsorption of NH3, NH2D, NHD2, and ND3 in the spectral region 140-220 nm were determined at similar to 298 K using synchrotron radiation. Absorption spectra of NH2D and NHD2 were deduced from spectra of mixtures of NH3 and ND3, of which the equilibrium concentrations for all four isotopologues obey statistical distributions. Cross sections of NH2D, NHD2, and ND3 are new. Oscillator strengths, an integration of absorption cross sections over the spectral lines, for both A X and B X systems of NH3 agree satisfactorily with previous reports; values for NH2D, NHD2, and ND3 agree with quantum chemical predictions. The photolysis of NH3 provides a major source of reactive hydrogen in the lower stratosphere and upper troposphere of giant planets such as Jupiter. Incorporating the measured photoabsorption cross sections of NH3 and NH2D into the Caltech/JPL photochemical diffusive model for the atmosphere of Jupiter, we find that the photolysis efficiency of NH2D is lower than that of NH3 by as much as 30%. The D/H ratio in NH2D/NH3 for tracing the microphysics in the troposphere of Jupiter is also discussed.
Absorption spectra in the vacuum ultraviolet region of small molecules in condensed phases
(2008-12-15)
Lu, Hsiao-Chi
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Chen, Hong-Kai
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Cheng, Bing-Ming
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Ogilvie, J. F.
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材料科學與工程學系
With radiation from a synchrotron we measured the spectra of several small molecular species in the, solid phase at 10K, either pure -O(2), NO, CO(2), N(2)O, H(2)O and NH(3) - or, for NH3 also dispersed in Ar at, molar ratio 1/250, from the onset of absorption in the ultraviolet region until the limits of transmission by crystalline LiF or solid At. In a quantitative treatment of spectral data, we fitted the total absorption profile divided by wavenumber to Gaussian curves of minimal number, and made tentative assignments of electronic transitions and vibrational structure by comparison with spectra of gaseous species. These results illuminate the nature of electronic spectra of samples in solid phases in the vacuum ultraviolet region. (C) 2008 Elsevier B.V. All rights reserved.
Complementing Crystallography with Ultralow-Frequency Raman Spectroscopy: Structural Insights into Nitrile-Functionalized Ionic Liquids
(2016-01-04)
Chen, Hong-Kai
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Srivastava, Nitin
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Saha, Satyen
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Shigeto, Shinsuke
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應用化學系
Functionalized ionic liquids are a subclass of ionic liquids that are tailored for a specific application. Structural characterization in both solid and liquid phases is central to understanding their physical properties. Here, we used ultralow-frequency Raman spectroscopy, which can measure Raman spectra down to approximately 5cm(-1), to study the structures and physical properties of 1-(4-cyanobenzyl)-3-methylimidazolium salts with five different anions. A comparison of the observed low-frequency Raman spectral patterns enabled us to predict the crystal symmetry of one of the synthesized salts for which single-crystal X-ray diffraction data were unobtainable. Real-time tracking of the low-frequency Raman spectral changes during melting revealed peak shifts indicative of different degrees of microscopic heterogeneity in the ionic liquids. The results show that our method provides a facile means that is complementary to X-ray crystallography, for obtaining structural information of ionic liquids.
Isotopic fractionation of nitrogen in ammonia in the troposphere of Jupiter
(2007-03-10)
Liang, Mao-Chang
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Cheng, Bing-Ming
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Lu, Hsiao-Chi
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Chen, Hong-Kai
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Alam, M. S.
Laboratory measurements of the photoabsorption cross section of (NH3)-N-15 at wavelengths between 140 and 220 nm are presented for the first time. Incorporating the measured photoabsorption cross sections of (NH3)-N-15 and (NH3)-N-14 into a one-dimensional photochemical diffusive model, we find that at 400 mbar, the photolytic efficiency of (NH3)-N-15 is about 38% greater than that of (NH3)-N-14. In addition, it is known that ammonia can condense in the region between 200 and 700 mbar, and the condensation tends to deplete the abundance ratio of (NH3)-N-15 and (NH3)-N-14. By matching the observed ratio of (NH3)-N-15 and (NH3)-N-14 at 400 mbar, the combined effect of photolysis and microphysics produces the ratio of (2.42 +/- 0.34) x 10(-3) in the deep atmosphere, in excellent agreement with the Galileo spacecraft measurements. The usefulness of the isotopic composition of ammonia as a tracer of chemical and dynamical processes in the troposphere of Jupiter is discussed.
Photoabsorption cross sections of NH3, NH2D, NHD2, and ND3 in the spectral range 110-144 nm
(2007-10-21)
Wu, Yu-Jong
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Lu, Hsiao-Chi
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Chen, Hong-Kai
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Cheng, Bing-Ming
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Lee, Yuan-Pern
Cross sections for photoabsorption of NH3, NH2D, NHD2, and ND3 near 298 K were measured in the spectral range of 110-144 nm using radiation from a synchrotron. Absorption cross sections and oscillator strengths of NH3 agree satisfactorily with previous reports; those of ND3 are improved over those in a previous report, whereas those of NH2D and NHD2 are new. The oscillator strengths of transitions to D, D-', D-'', F, and G states are nearly the same among all four isotopic variants, but those to D-''' and E states vary substantially. Observed absorption bands are arranged into vibrational progressions in accord with known Rydberg transitions. All progressions show a common trend of vibrational intervals increasing with vibrational quantum numbers. The Rydberg orbitals for states D(3de('')), D-'(4sa(1)(')), D-''(3da(1)(')), D-'''(4pe(')), and E(4de('')) are readily assigned with quantum defects determined in these experiments, but assignments for F(5de('')) and G(6de('')) are uncertain. Absorption cross sections of dissociative continua underneath discrete structures are larger for NH2D and NHD2 than for NH3 and ND3, indicating that the rate of dissociation of ammonia might increase when its symmetry is broken. (C) 2007 American Institute of Physics.