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Absorption oscillator strengths have been determined from high-resolution spectra in the 967–972 Å region of three CO isotopomers for transitions to the Rydberg levels 4pπ(0), 3dπ(1) and 4pσ(0), as well as to the mixed E(6)level recently characterized by Eidelsberg et al. ([CITE]). Synchrotron radiation from the Super-ACO electron storage ring at Orsay (LURE) was used as a light source. Oscillator strengths were extracted from the recorded spectra by least-squares fitting of the experimental profiles with synthetic spectra taking into account the homogeneous and heterogeneous interactions of the four levels. Column densities were derived from fits to the 3pπ(0) absorption band whose oscillator strength is well established. These are the first reported measurements for 13C18O. For 12C16O, our results are consistent with the larger values obtained in the most recent laboratory and astronomical studies.

The adsorption of two isomers, acetic acid (CH3COOH) and methyl formate (HCOOCH3), on water ice surfaces is investigated using temperature-programmed desorption and infrared spectroscopy in the 80–200 K temperature range. For each compound, multilayer and submonolayer adsorption regimes are distinguished. In the multilayer regime, the adsorption conformations of the adlayer molecules as a function of the sample temperature are analyzed, providing the identification of phase transitions and associated thermal desorptions. In the submonolayer regime, different behaviors are found in the adsorption of each isomer. The acetic acid first layer desorbs simultaneously with the water substrate itself, whereas the methyl formate monolayer desorption is found 15 K below the onset of water sublimation. This difference in adsorption energies is discussed and associated with the ability of each isomer to perform hydrogen bonding with the dangling O–H surface water bonds. Such a differential adsorption process between organic isomers might be of interest in systems where these molecules can be found in interactions with water ices, such as stratospheric or interstellar media. Indeed, it might play a role in the observation of gas-phase relative abundances of these isomers in such media.

Vacuum-UV (VUV) photodissociation of gaseous formic acid (HCOOH) has been studied in the 6−23 eV range using photofragment fluorescence spectroscopy and synchrotron radiation excitation. Previous studies were limited to the region below 11.4 eV. Emission of OH (A), CH (A, B), HCOO, and H Balmer radiation is observed at various excitation energies in the VUV. Bands in the corresponding fluorescence excitation spectra can be associated with specific absorption bands of HCOOH. The HCOO fluorescence between 330 and 480 nm is assigned to the ²B1 → ²A‘ ‘ transition. The quantum yield of photoionization, γ, measured from 10 to 23 eV, reaches unity at 18 ± 0.1 eV. The energy domain, ΔE, of competitive nonionic decay processes of superexcited molecular states above the ionization threshold is thus found to be ΔE(HCOOH) = 6.68 ± 0.1 eV. Between 15.7 and 17 eV, Rydberg absorption bands converging to the 3²A‘ state of HCOOH⁺ are mirrored in the γ(Eexc) spectrum, from which an autoionization rate kai ≈ 7.5 × 10¹³ s^-1 was determined for the Rydberg levels. Below the astrophysical HI limit (13.6 eV), the neutral processes HCOOH → OH + HCO and HCOOH → CO2 + 2H dominate the fragmentation of formic acid. The quantum yield of ionic dissociation processes HCOOH⁺ → COOH⁺ + H and HCOOH⁺ → HCO⁺ + OH is determined to be less than 25% in this energy region. Although the ionic processes become more important between 13.6 and 18 eV, neutral dissociation processes with large quantum yields still occur. The astrophysical relevance of the photophysical properties of formic acid is briefly discussed.

Rotational and vibrational state distributions have been measured for the OH/OD(A²Σ⁺) fragment produced by the photodissociation of H2O/D2O at a set of wavelengths in the 133−119 nm range that lead to excitation of the &Btilde;(¹A1) state, either directly or through particular absorption features of the &Ctilde;¹B1 ← &Xtilde;¹A1 and &Dtilde;¹A1 ← &Xtilde;¹A1 Rydberg transitions. The experimental distributions are obtained from the A²Σ⁺−X²Π fluorescence spectra using a truncated single value decomposition method (TSVD). They are compared to distributions calculated in a complete three-dimensional quantum mechanical treatment by using two sets of diabatic potential energy surfaces obtained by Dobbyn and Knowles (DK) and by van Harrevelt and van Hemert (Leiden). The measured vibrational branching ratios seem to be better accounted for by the former and the rotational distributions by the latter. However, a more detailed analysis indicates that the DK surface would presumably account for the experiment if the room-temperature rotational distribution of the parent water molecule were introduced in the calculation. The experimental results are therefore interpreted as a confirmation of the more direct and shorter-lived trajectories favored by the DK surface.

The components of the 5s + 4d Rydberg complex of C2H2 and C2D2 have been studied by (3 + 1) photon ionization spectroscopy and fragment fluorescence excitation spectroscopy following synchrotron radiation VUV excitation. Thanks to the remarquable spectral resolution of the new synchrotron beam line SU5 at Super-ACO (12 mÅ in the range 248−65 nm), rotationally resolved spectra of Rydberg states of acetylene could be observed through their photofragment visible fluorescence. Unlike the lower 4s + 3d Rydberg complex, all Rydberg components of this complex exhibit rotational line widths typical of a 1−10 ps predissociation. The ¹Φu 4dδg and ¹Δu 4dπg components have been characterized for both isotopic species. The relative electronic band intensities of the REMPI spectra have been interpreted within a semi-united atom model and by taking into account predissociation.

Context.Over 20 of the  ~150 different species detected in the interstellar and circumstellar media have also been identified in icy environments. For most of the species observed so far in the interstellar medium (ISM), the most abundant isomer of a given generic chemical formula is the most stable one (minimum energy principle – MEP) with few exceptions such as, for example, CH3COOH/HCOOCH3and CH3CH2OH/CH3OCH3, whose formation is thought to occur on the icy mantles of interstellar grains.

Aims.We seek to study the abundances of H2, HD, and D2adsorbed onto ice-covered dust grains in dark molecular clouds in the interstellar medium.

Context. The synthesis of water is one necessary step in the origin and development of life. It is believed that pristine water is formed and grows on the surface of icy dust grains in dark interstellar clouds. Until now, there has been no experimental evidence whether this scenario is feasible or not on an astrophysically relevant template and by hydrogen and oxygen atom reactions.Aims. We present here the first experimental evidence of water synthesis by such a process on a realistic analogue of grain surface in dense clouds, i.e., amorphous water ice.Methods. Atomic beams of oxygen and deuterium are aimed at a porous water ice substrate (H2O) held at 10 K. Products are analyzed by the temperature-programmed desorption technique.Results. We observe the production of HDO and D2O, indicating that water is formed under conditions of the dense interstellar medium from hydrogen and oxygen atoms. This experiment opens up the field of a little explored complex chemistry that could occur on dust grains,which is believed to be the site where key processes lead to the molecular diversity and complexity observed in the Universe.

Context. The synthesis of water is one necessary step in the origin and development of life. It is believed that pristine water is formed and grows on the surface of icy dust grains in dark interstellar clouds. Until now, there has been no experimental evidence whether this scenario is feasible or not on an astrophysically relevant template and by hydrogen and oxygen atom reactions.