Abstract. 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

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.

Cross sections for photoabsorption of NH 3 , NH 2 D, NHD 2 , and ND 3 in the spectral region 140Y220 nm were determined at $298 K using synchrotron radiation. Absorption spectra of NH 2 D 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 2 D, 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 2 D, 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 2 D into the Caltech /JPL photochemical diffusive model for the atmosphere of Jupiter, we find that the photolysis efficiency of NH 2 D is lower than that of NH 3 by as much as 30% . The D/H ratio in NH 2 D/ NH 3 for tracing the microphysics in the troposphere of Jupiter is also discussed.

We report laboratory measurements of cross sections of CH 3 D and C 2 H 5 D 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 H 2 and CH 4 , H ϩ ,. Since there is no reliable kinetics measurement for the reaction, HD ↔ D ϩ H CH ϩ D ↔ CH D ϩ H 2 3 2 , we use Yung et al.'s estimate for its rate constant. The strong temperature dependence CH D ϩ H r CH ϩ D 2 3 for this reaction leads to large isotopic fractionation for CH 3 D and C 2 H 5 D 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 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 potential-energy curve resulting from avoided crossing of Rydberg states 11A″(2a″→3s) and 2 1A″(2a″→3p); with isotopic data, further vibrational progressions are identified. Absorption lines in the 140–151 nm region with regular vibrational spacing (∼1046 cm−1 for CH3OH), likely associated with the CH2 twisting mode, are assigned to nearly degenerate transitions 3 1A″–X 1A′ and 3 1A′–X 1A′; the 3 1A″ and 3 1A′ states are associated with excitations 2a″→3p′ and 2a″→3p″, respectively. Progressions associated with the torsional mode of the excited state are observed for the first time. For wavelengths smaller than 140 nm, Rydberg transitions with n⩾3 are tentatively assigned in accord with their quantum defects that are identical for all isotopomers in each Rydberg series.