C. Romanzin

CH radical production from 248nm photolysis or discharge-jet dissociation of CHBr3 probed by cavity ring-down absorption spectroscopy

The A-X bands of the CH radical, produced in a 248 nm two-photon photolysis or in a supersonic jet discharge of CHBr(3), have been observed via cavity ring-down absorption spectroscopy. Bromoform is a well-known photolytic source of CH radicals, though no quantitative measurement of the CH production efficiency has yet been reported. The aim of the present work is to quantify the CH production from both photolysis and discharge of CHBr(3). In the case of photolysis, the range of pressure and laser fluences was carefully chosen to avoid postphotolysis reactions with the highly reactive CH radical. The CH production efficiency at 248 nm has been measured to be Phi=N(CH)N(CHBr(3))=(5.0+/-2.5)10(-4) for a photolysis laser fluence of 44 mJ cm(-2) per pulse corresponding to a two-photon process only. In addition, the internal energy distribution of CH(X (2)Pi) has been obtained, and thermalized population distributions have been simulated, leading to an average vibrational temperature T(vib)=1800+/-50 K and a rotational temperature T(rot)=300+/-20 K. An alternative technique for producing the CH radical has been tested using discharge-induced dissociation of CHBr(3) in a supersonic expansion. The CH product was analyzed using the same cavity ring-down spectroscopy setup. The production of CH by discharge appears to be as efficient as the photolysis technique and leads to rotationally relaxed radicals.

X-ray photodesorption from water ice in protoplanetary disks and X-ray-dominated regions

Water is the main constituent of interstellar ices, and it plays a key role in the evolution of many regions of the interstellar medium, from molecular clouds to planet-forming disks1. In cold regions of the interstellar medium, water is expected to be completely frozen out onto the dust grains. Nonetheless, observations indicate the presence of cold water vapour, implying that non-thermal desorption mechanisms are at play. Photodesorption by ultraviolet photons has been proposed to explain these observations2,3, with the support of extensive experimental and theoretical work on ice analogues4–6. In contrast, photodesorption by X-rays, another viable mechanism, has been little studied. The potential of this process to desorb key molecules such as water, intact rather than fragmented or ionized, remains unexplored. We experimentally investigated X-ray photodesorption from water ice, monitoring all desorbing species. We found that desorption of neutral water is efficient, while ion desorption is minor. We derived yields that can be implemented in astrochemical models. These results open up the possibility of taking into account the X-ray photodesorption process in the modelling of protoplanetary disks or X-ray-dominated regions.The X-ray-induced photodesorption of water from astrophysical ices, intact, has been little studied. However, it could be a key process in producing the cold water vapour that is seen in these regions. Here, the yield of such a mechanism is experimentally quantified.

Experimental and ab initio characterization of HC3N+ vibronic structure. I. Synchrotron-based threshold photo-electron spectroscopy

Threshold-photoionization spectroscopy of cyanoacetylene (HC3N) and its 15N isotopologue has been investigated in the vacuum-ultraviolet range with a synchrotron-based experiment allowing to record threshold-photoelectron spectrum and photoion yield over a large energy range (from 88u2009500 to 177u2009500 cm-1, i.e., from 11 to 22 eV). Adiabatic ionization energies towards the three lowest electronic states X+2Π, A+ Σ+2, and B+ Π2 are derived from the threshold-photoelectron spectrum. A detailed description of the vibrational structure of these states is proposed leading to the determination of the vibrational frequencies for most modes. The vibrational assignments and the discussion about the electronic structure are supported by multireference ab initio calculations (CASPT2, MRCI). Unprecedented structures are resolved and tentatively assigned in the region of the B+← X transition. Exploratory calculations highlight the complexity of the electronic landscape of the cation up to approximately 10 eV above its ground state.

Laboratory experiments as support to the built up of Titan’s theoretical models and interpretation of Cassini-Huygens data

To interpret the concentrations of the products measured in Titan’s atmosphere and to better understand the associated chemistry, many theoretical models have been developed so far. Unfortunately, large discrepancies are still found between theoretical and observational data. A critical examination of the chemical scheme included in these models points out some problems regarding the reliability of the description of critical reaction pathways as well as the accuracy of kinetic parameters. Laboratory experiments can be used to reduce these two sources of uncertainty. It can be: i) experimental simulations: in our laboratory (LISA), representative Titan’s simulation experiments are planned to be carried out in a reactor where the initial gas mixture will be exposed, for the first time, to both electrons and photons. Thus, the chemistry between N atoms and CH3 , CH2 , CH fragments, issued from electron dissociation of N2 and photo-dissociation of CH4 respectively, will be initiated. Thank to a time resolved technique, we will be able to analyse “in situ”, qualitatively and quantitatively, the stable species as well as the short life intermediates. Then, the implied chemistry will be determined precisely, and consequently, its description will be refined in theoretical models. The current status of this program will be given. ii) specific experiments: they are devoted, for example, to determine kinetic rate constants and low temperature VUV spectra that will be used to feed models and to interpret observational data. Such experiments performed in LISA and in Rennes’ laboratory concern polyynes and cyanopolyynes as these compounds could link the gaseous and the solid phase in planetary atmosphere. Results concerning C4H hydrocarbons kinetic rate constants and VUV cross section of HC3N and HC5N will be detailed.