Mohammed Bahou

Photo-induced fractionation of water isotopomers in the Martian atmosphere

The history and size of the water reservoirs on early Mars can be constrained using isotopic ratios of deuterium to hydrogen. We present new laboratory measurements of the ultraviolet cross-sections of H2O and its isotopomers, and modeling calculations in support of a photo-induced fractionation effect (PHIFE), that reconciles a discrepancy between past theoretical modeling and recent observations. This supports the hypothesis that Mars had an early warm atmosphere and has lost at least a 50-m global layer of water. Likely applications of PHIFE to other planetary atmospheres are sketched.

Experimental and theoretical studies on vacuum ultraviolet absorption cross sections and photodissociation of CH3OH, CH3OD, CD3OH, and CD3OD

Absorption cross sections of CH3OH, CH3OD, CD3OH, and CD3OD are measured in a 107–220 nm spectral region using synchrotron radiation. Spectra of improved quality for four deuterated isotopomers, coupled with extensive calculations on low-lying excited states of methanol using time-dependent density functional theory with a large cc-pV5Z basis set, enable us to improve assignments of observed spectral features and to better understand the nature of these electronic transitions. Energies and oscillator strengths of all transitions predicted with calculations are consistent with experimental results. Observed isotopic shifts clearly indicate that absorption features in the 163–220 nm region (transition 1 1A″–X 1A′) are associated mainly with breaking of the O–H bond, consistent with theoretical predictions. In the 151–163 nm region (transition 2 1A″–X 1A′), observed small vibrational spacings (806 cm−1 for CH3OH) associated with the C–O stretching mode can be rationalized with a broad double-well-like potent...

Enhancement of Deuterated Ethane on Jupiter

We report laboratory measurements of cross sections of CH3D and C2H5D in the extreme ultraviolet. The results are incorporated in a photochemical model for the deuterated hydrocarbons up to C2 in the upper atmosphere of Jupiter, taking into account the fast reactions for exchanging H and D atoms between H2 and CH4 ,H , . Since there is no reliable kinetics measurement for the reaction, HD ↔ D HC H D ↔ CH D H 23 2 , we use Yung et al.’s estimate for its rate constant. The strong temperature dependence CH D H r CH D 23 for this reaction leads to large isotopic fractionation for CH 3D and C2H5D in the upper atmosphere of Jupiter, where their production rates depend on the abundance of deuterated methyl radical. The model predicts that the D/H ratio in deuterated ethane is about 15 times that of the bulk atmosphere. A confirmation of this result would provide a sensitive test of the photochemistry of hydrocarbons in the atmosphere of Jupiter.

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.

Infrared spectra of free radicals and protonated species produced in para-hydrogen matrices.

The quantum solid para-hydrogen (p-H2) has emerged as a new host for matrix isolation experiments. Among several unique characteristics, the diminished cage effect enables the possibility of producing free radicals via either photolysis in situ or bimolecular reactions of molecules with atoms or free radicals that are produced in situ from their precursors upon photo-irradiation. Many free radicals that are unlikely to be produced in noble-gas matrices can be produced readily in solid p-H2. In addition, protonated species can be produced upon electron bombardment of p-H2 containing a small proportion of the precursor during deposition. The application of this novel technique to generate protonated polycyclic aromatic hydrocarbons (PAH) and their neutral counterparts demonstrates its superiority over other methods. The technique of using p-H2 as a matrix host has opened up many possibilities for the preparation of free radicals and unstable species and their spectral characterization. Many new areas of applications and fundamental understanding concerning the p-H2 matrix await further exploration.

A new method for investigating infrared spectra of protonated benzene (C6H7+) and cyclohexadienyl radical (c-C6H7) using para-hydrogen

We use protonated benzene (C(6)H(7)(+)) and cyclohexadienyl radical (c-C(6)H(7)) to demonstrate a new method that has some advantages over other methods currently used. C(6)H(7)(+) and c-C(6)H(7) were produced on electron bombardment of a mixture of benzene (C(6)H(6)) and para-hydrogen during deposition onto a target at 3.2 K. Infrared (IR) absorption lines of C(6)H(7)(+) decreased in intensity when the matrix was irradiated at 365 nm or maintained in the dark for an extended period, whereas those of c-C(6)H(7) increased in intensity. Observed vibrational wavenumbers, relative IR intensities, and deuterium isotopic shifts agree with those predicted theoretically. This method, providing a wide spectral coverage with narrow lines and accurate relative IR intensities, can be applied to larger protonated polyaromatic hydrocarbons and their neutral species which are difficult to study with other methods.

Quantitative spectroscopic and theoretical study of the optical absorption spectra of H2O, HOD, and D2O in the 125–145 nm region

The room temperature absorption spectra of water and its isotopomers D2O and HOD have been determined in absolute cross section units in the 125 to 145 nm wavelength region using synchrotron radiation. The experimental results for these B band spectra are compared with results from quantum mechanical calculations using accurate diabatic ab initio potentials. A Monte Carlo sampling over the initial rotational states of the molecules is applied in order to calculate the cross sections at a temperature of 300 K. The overall rotation of the water molecule is treated exactly. Both for the experimental and for the theoretical spectrum an analysis is made in terms of a component attributed to rapid direct dissociation processes and a component attributed to longer-lived resonances. The agreement between the results from experiment and theory is excellent for H2O and D2O. In the case of HOD in the results of theory two more resonances are found at low energy. It is demonstrated that the width of the resonances of 0.04 eV is the result of overlapping and somewhat narrower resonances in the spectra of molecules differing in rotational ground state.

ABSORPTION CROSS SECTIONS OF HCl AND DCl AT 135-232 NANOMETERS: IMPLICATIONS FOR PHOTODISSOCIATION ON VENUS

Cross sections for photoabsorption of HCl and DCl are determined in the spectral region of 135-232 nm using radiation from a synchrotron light source. At wavelengths near the onset of absorption (λ > 200 nm), cross sections of HCl are approximately 5-10 times larger than those of DCl. These data are used to calculate rates of photodissociation of HCl and DCl in the Venusian atmosphere. For the entire wavelength region measured, the rate of photodissociation of DCl is only 16% that of HCl. The difference in rates of photodissociation contributes to the exceptionally large [D]/[H] ratio of the Venusian atmosphere.

Isomers of SNO2: Production and infrared spectra of cis- and trans-OSNO from irradiated inert matrices containing OCS and NO2

New species cis- and trans-OSNO, designated c-OSNO and t-OSNO, respectively, are produced and identified with infrared absorption spectra when an argon or nitrogen matrix containing OCS and NO2 is irradiated with laser emission at 248 nm. Lines at 1156.1 and 1454.4 cm−1 are assigned to c-OSNO and those at 1178.0 and 1459.0 cm−1 are assigned to t-OSNO in solid N2. Lines at 1154.9 and 1450.8 cm−1 are assigned to c-OSNO and those at 1181.2 and 1456.0 cm−1 are assigned to t-OSNO in solid Ar; further lines associated with minor matrix sites are identified. Assignments of spectral lines are based on results of both experiments with 15N- and 18O-isotopic substitution and theoretical calculations using density-functional theories, B3LYP with an aug-cc-pVTZ basis set; these calculations predict the geometry, energy, vibrational frequencies, and infrared intensities of SNO2 as four isomers: C2v-SNO2, t-SONO, t-OSNO, and c-OSNO, in increasing order of stability. Mechanisms are proposed to rationalize that c-OSNO and...

Infrared Spectra of Protonated Coronene and Its Neutral Counterpart in Solid Parahydrogen: Implications for Unidentified Interstellar Infrared Emission Bands

Large protonated polycyclic aromatic hydrocarbons (H(+) PAHs) are possible carriers of unidentified infrared (UIR) emission bands from interstellar objects, but the characterization of infrared (IR) spectra of large H(+) PAHs in the laboratory is challenging. IR absorption spectra of protonated coronene (1-C24 H13 (+) ) and mono-hydrogenated coronene (1-C24 H13 (.) ), which were produced upon electron bombardment of parahydrogen containing a small proportion of coronene (C24 H12 ) during matrix deposition, were recorded. The spectra are of a much higher resolution than those obtained by IR multiphoton dissociation by Dopfer and co-workers. The IR spectra of protonated pyrene and coronene collectively appear to have the required chromophores for features of the UIR bands, and the spectral shifts on an increase in the number of benzenoid rings point in the correct direction towards the positions of the UIR bands. Larger protonated peri-condensed PAHs might thus be key species among the carriers of UIR bands.