Hsiao-Chi Lu

Absorption Cross Sections of NH3, NH2D, NHD2, and ND3 in the Spectral Range 140-220 nm and Implications for Planetary Isotopic Fractionation

Cross sections for photoabsorption of NH_3, NH_2D, NHD_2, and ND_3 in the spectral region 140-220 nm were determined at ~298 K using synchrotron radiation. Absorption spectra of NH_2D and NHD_2 were deduced from spectra of mixtures of NH_3 and ND_3, of which the equilibrium concentrations for all four isotopologues obey statistical distributions. Cross sections of NH_2D, NHD_2, and ND_3 are new. Oscillator strengths, an integration of absorption cross sections over the spectral lines, for both A ← X and B ← X systems of NH_3 agree satisfactorily with previous reports; values for NH_2D, NHD_2, and ND_3 agree with quantum chemical predictions. The photolysis of NH_3 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 NH_3 and NH_2D into the Caltech/JPL photochemical diffusive model for the atmosphere of Jupiter, we find that the photolysis efficiency of NH_2D is lower than that of NH_3 by as much as 30%. The D/H ratio in NH_2D/NH_3 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

With radiation from a synchrotron we measured the spectra of several small molecular species, in the solid phase at 10K, either pure--O2, NO, CO2, N2O, H2O and NH3--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 Ar. 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.

PHOTOLYSIS OF ETHYNE IN SOLID NEON AND SYNTHESIS OF LONG-CHAIN CARBON CLUSTERS WITH VACUUM-ULTRAVIOLET LIGHT

The absorption spectrum of ethyne, C2H2, in solid Ar was measured in the wavelength region 107-220 nm with light from a synchrotron. Based on that absorption, irradiation of samples of ethyne dispersed in neon with vacuum-ultraviolet (VUV) radiation yielded various products that were identified through their infrared absorption spectra including C n (n = 3-12), C2H, C2H3, C4H, C4H2, C8H–, and C8H2. The efficiency of photolysis of ethyne and the nature of photoproducts depend on the selected wavelength of VUV light. Information about the photodissociation of C2H2 with various photon energies and the formation and identification of large carbon clusters and hydrides at low temperature might be useful in photochemical models to simulate the composition of the atmosphere of Titan and as a source of aerosols.

Production of N3 upon Photolysis of Solid Nitrogen at 3 K with Synchrotron Radiation

The photodissociation of gaseous molecular nitrogen has been investigated intensively, but the corresponding knowledge in a solid phase is lacking. Irradiation of pure solid nitrogen at 3 K with vacuum-ultraviolet light from a synchrotron produced infrared absorption lines of product l-N3 at 1657.8 and 1652.6 cm(-1). The threshold wavelength to generate l-N3 was determined to be (143.7±1.8) nm, corresponding to an energy of (8.63±0.11) eV. Quantum-chemical calculations support the formation of l-N3 from the reaction N2 +N2, possibly through an activated complex l-N4 upon photoexcitation with energy above 8.63 eV. The results provide a possible application to an understanding of the nitrogen cycle in astronomical environments.

Vacuum-ultraviolet photolysis of methane at 3 K: synthesis of carbon clusters up to C20.

Samples of pure methane and of methane dispersed in solid neon at 3 K subjected to irradiation at wavelengths less than 165 nm with light from a synchrotron yielded varied products that were identified through their infrared absorption spectra, including CH3, C2H2, C2H3, C2H4, C2H6, C4H2, C4H4, C5H2, C8H2, CnH with n = 1-5, and carbon chains Cn with n = 3-20. The efficiency of photolysis of methane and the nature of the photoproducts depended on the concentration of methane and the wavelength selected for irradiation; an addition of H2 into solid neon enhanced the formation of long carbon chains.

Isotopic Fractionation of Nitrogen in Ammonia in the Troposphere of Jupiter

Laboratory measurements of the photoabsorption cross section of ^(15)NH_3 at wavelengths between 140 and 220 nm are presented for the first time. Incorporating the measured photoabsorption cross sections of ^(15)NH_3 and ^(14)NH_3 into a one-dimensional photochemical diffusive model, we find that at 400 mbar, the photolytic efficiency of ^(15)NH_3 is about 38% greater than that of ^(14)NH_3. 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 ^(15)NH_3 and ^(14)NH_3. By matching the observed ratio of ^(15)NH_3 and ^(14)NH_3 at 400 mbar, the combined effect of photolysis and microphysics produces the ratio of (2.42 ± 0.34) × 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.

Absorption Cross Section of Gaseous Acetylene at 85?K in the Wavelength Range 110-155?nm

Absorption spectra and absorption cross sections of gaseous acetylene, C2H2, at 298 and 85?K were measured in the wavelength range 110-155?nm with a slit-jet system coupled to a synchrotron as a source of vacuum ultraviolet light. Using published spectral parameters of C2H2, we simulated the absorption profile for the Rydberg transition to state 4R 0 in the range 124.6-125.1?nm, according to which the temperature of the jet-expanded sample at stagnation pressure 200 Torr is 85?? 5?K. Our cross sections of C2H2 are applicable for determining properties sensitive to temperature for diagnostic work on Saturn and Titan.

Infrared and Ultraviolet Spectra of Diborane(6): B2H6 and B2D6.

We recorded absorption spectra of diborane(6), B2H6 and B2D6, dispersed in solid neon near 4 K in both mid-infrared and ultraviolet regions. For gaseous B2H6 from 105 to 300 nm, we report quantitative absolute cross sections; for solid B2H6 and for B2H6 dispersed in solid neon, we measured ultraviolet absorbance with relative intensities over a wide range. To assign the mid-infrared spectra to specific isotopic variants, we applied the abundance of (11)B and (10)B in natural proportions; we undertook quantum-chemical calculations of wavenumbers associated with anharmonic vibrational modes and the intensities of the harmonic vibrational modes. To aid an interpretation of the ultraviolet spectra, we calculated the energies of electronically excited singlet and triplet states and oscillator strengths for electronic transitions from the electronic ground state.

Quantitative analysis of nitrogen defect N4 in diamond with photoluminescence excited in the 170-240 nm region.

Upon excitation at 170-240 nm, diamonds emit strong luminescence in wavelength range of 300-700 nm. The spectral features observed in the photoluminescence excitation (PLE) spectra show two vibrational progressions, A and B, related to nitrogen defects N2 and N4, respectively. We used PLE spectra excited in region 170-240 nm to identify the type of diamond and demonstrate quantitative analysis of the B center as a N4 nitrogen defect in diamonds; the least detectable concentration of the N4 nitrogen defect is about 13 ppb, and the sensitivity of PLE is about 30 times than that practicable with infrared absorption spectra.

FORMATION OF N{sub 3}, CH{sub 3}, HCN, AND HNC FROM THE FAR-UV PHOTOLYSIS OF CH{sub 4} IN NITROGEN ICE

The irradiation of pure solid N{sub 2} at 3 K with far-ultraviolet light from a synchrotron produced infrared absorption lines at 1657.7, 1655.6, and 1652.4 cm{sup −1} and an ultraviolet absorption line at 272.0 nm, which are characteristic of the product N{sub 3}. The threshold wavelength at which N{sub 3} was generated was 145.6 ± 2.9 nm, corresponding to an energy of 8.52 ± 0.17 eV. The photolysis of isotopically labeled {sup 15}N{sub 2} at 3 K consistently led to the formation of {sup 15}N{sub 3} with the same threshold wavelength of 145.6 ± 2.9 nm for its formation. The photolysis of CH{sub 4} in nitrogen ice in low concentrations also led to the formation of N{sub 3}, together with CH{sub 3}, HCN, and HNC, with the same threshold wavelength of 145.6 ± 2.9 nm. These results indicate that N{sub 3} radicals may play an important role in the photochemistry of nitrogen ices in astronomical environments.