Ammonium salts are a reservoir of nitrogen on a cometary nucleus and possibly on some asteroids.

The measured nitrogen-to-carbon ratio in comets is lower than for the Sun, a discrepancy which could be alleviated if there is an unknown reservoir of nitrogen in comets. The nucleus of comet 67P/Churyumov-Gerasimenko exhibits an unidentified broad spectral reflectance feature around 3.2 micrometers, which is ubiquitous across its surface. On the basis of laboratory experiments, we attribute this absorption band to ammonium salts mixed with dust on the surface. The depth of the band indicates that semivolatile ammonium salts are a substantial reservoir of nitrogen in the comet, potentially dominating over refractory organic matter and more volatile species. Similar absorption features appear in the spectra of some asteroids, implying a compositional link between asteroids, comets, and the parent interstellar cloud.

Electronic spectrum of the protonated diacetylene cation (H2C4H+).

The B̃1A1←X̃1A1 electronic band system of the protonated diacetylene cation (H2C4H+) is measured over the 230-295 nm range by photodissociating H2C4H+ ions stored in a cryogenic ion trap and by photodissociating H2C4H+ tagged with Ar and N2 in a tandem mass spectrometer. The B̃1A1←X̃1A1 band system has an origin at 34 941 cm-1 for H2C4H+, 34 934 cm-1 for H2C4H+-Ar, and 34 920 cm-1 for H2C4H+-N2. The spectra of H2C4H+, H2C4H+-Ar, and H2C4H+-N2 display similar vibronic structure, which is assigned using ab initio calculations to progressions in two symmetric a1 C-C stretch vibrational modes (ν6 and ν4), with band spacings of 860 and 1481 cm-1, respectively.

Twisted Intramolecular Charge Transfer in Protonated Amino Pyridine.

The excited state properties of protonated ortho (2-), meta (3-), and para (4-) aminopyridine molecules have been investigated through UV photofragmentation spectroscopy and excited state coupled-cluster CC2 calculations. Cryogenic ion spectroscopy allows recording well-resolved vibronic spectroscopy that can be reproduced through Franck-Condon simulations of the ππ* local minimum of the excited state potential energy surface. The excited state lifetimes have also been measured through a pump-probe excitation scheme and compared to the estimated radiative lifetimes. Although protonated aminopyridines are rather simple aromatic molecules, their deactivation mechanisms are indeed quite complex with unexpected results. In protonated 3- and 4-aminopyridine, the fragmentation yield is negligible around the band origin, which implies the absence of internal conversion to the ground state. Besides, a twisted intramolecular charge transfer reaction is evidenced in protonated 4-aminopyridine around the band origin, while excited state proton transfer from the pyridinic nitrogen to the adjacent carbon atom opens with roughly 500 cm(-1) of excess energy.

Development and optimization of an analytical system for volatile organic compound analysis coming from the heating of interstellar/cometary ice analogues.

This contribution presents an original analytical system for studying volatile organic compounds (VOC) coming from the heating and/or irradiation of interstellar/cometary ice analogues (VAHIIA system) through laboratory experiments. The VAHIIA system brings solutions to three analytical constraints regarding chromatography analysis: the low desorption kinetics of VOC (many hours) in the vacuum chamber during laboratory experiments, the low pressure under which they sublime (10(-9) mbar), and the presence of water in ice analogues. The VAHIIA system which we developed, calibrated, and optimized is composed of two units. The first is a preconcentration unit providing the VOC recovery. This unit is based on a cryogenic trapping which allows VOC preconcentration and provides an adequate pressure allowing their subsequent transfer to an injection unit. The latter is a gaseous injection unit allowing the direct injection into the GC-MS of the VOC previously transferred from the preconcentration unit. The feasibility of the online transfer through this interface is demonstrated. Nanomoles of VOC can be detected with the VAHIIA system, and the variability in replicate measurements is lower than 13%. The advantages of the GC-MS in comparison to infrared spectroscopy are pointed out, the GC-MS allowing an unambiguous identification of compounds coming from complex mixtures. Beyond the application to astrophysical subjects, these analytical developments can be used for all systems requiring vacuum/cryogenic environments.

Formation of hydroxyacetonitrile (HOCH2CN) and polyoxymethylene (POM)-derivatives in comets from formaldehyde (CH2O) and hydrogen cyanide (HCN) activated by water.

Studying chemical reactivity is an important way to improve our understanding of the origin of organic matter in astrophysical environments such as molecular clouds, protoplanetary disks, and possibly, as a final destination, in our solar system bodies such as in comets. Laboratory simulations on the reactivity of ice analogs can provide important insights into this complex reactivity. Here, the role of water as a catalytic agent is investigated under the conditions of simulated interstellar and cometary grains in the formation of complex organic molecules: the hydroxyacetonitrile (HOCH2CN) and formaldehyde polymers (polyoxymethylene POM). Using infrared spectroscopy and mass spectrometry, we show that HCN reacts with CH2O only in the presence of H2O, whereas in the absence of H2O, HCN is not sufficiently reactive to promote this reaction. Furthermore, depending on the dilution of CH2O and HCN in the water matrix, 1-cyanopolyoxymethylene polymers can also be formed (H-(O-CH2)n-CN, POM-CN), as confirmed by mass spectrometry using the HC(15)N isotopologue. Moreover, quantum chemical calculations allowed us to suggest mechanistic proposals for these reactions, the first step being the activation of HCN by water forming H3O(+) and CN(-), which subsequently condense on a neighbouring CH2O promoting the formation of (-)OCH2CN. Once (-)OCH2CN is formed, it can either recover a proton by reacting with H3O(+) or condense on CH2O molecules leading to POM-CN structures. Implications of this work for the forthcoming Rosetta mission are also addressed.

The mechanism of hexamethylenetetramine (HMT) formation in the solid state at low temperature.

There is convincing evidence that the formation of complex organic molecules occurred in a variety of environments. One possible scenario highlights the universe as a giant reactor for the synthesis of organic complex molecules, which is confirmed by numerous identifications of interstellar molecules. Among them, precursors of biomolecules are of particular significance due to their exobiological implications, and some current targets concern their search in the interstellar medium as well as understanding the mechanisms of their formation. Hexamethylenetetramine (HMT, C(6)H(12)N(4)) is one of these complex organic molecules and is of prime interest since its acid hydrolysis seems to form amino acids. In the present work, the mechanism for HMT formation at low temperature and pressure (i.e. resembling interstellar conditions) has been determined by combining experimental techniques and DFT calculations. Fourier transform infra-red spectroscopy and mass spectrometry techniques have been used to follow experimentally the formation of HMT as well as its precursors from thermal reaction of NH(3):H(2)CO:HCOOH and CH(2)NH:HCOOH ice mixtures, from 20 K to 330 K. DFT calculations have been used to compute the mechanistic steps through which HMT can be formed starting from the experimental reactants observed in solid phase. The fruitful interplay between theory and experiment has allowed establishing that the mechanism in the solid state at low temperature is different from the one proposed in liquid phase, in which a new intermediate (1,3,5-triazinane, C(3)H(9)N(3)) has been identified. In the meantime, aminomethanol has been unambiguously confirmed as the first intermediate whereas the hypothesis of methylenimine as the second is further strengthened.

Dipole moment of water in highly vibrationally excited states: analysis of photofragment quantum-beat spectroscopy measurements using a local-mode hamiltonian.

We present here the analysis of experimental Stark effect measurements made using photofragment quantum beat spectroscopy on the |4,0(-)>, |5,0(-)>, |8,0(+)> and |4,0(-)>|2> vibrational states of H(2)O [ Callegari , A. ; et al. Science 2002 , 297 , 993.]. To link the measured Stark coefficients with the dipole surface, we analyze our results using a coupled anharmonic oscillator model, which takes into account the local-mode nature of higly excited OH stretching vibrations in water, and the tunneling between the two equivalent bonds. The large inertial frame tilt associated with the local-mode bond stretching results in a complex interaction between rotational-, vibrational-, and tunneling-motion, all of which become deeply entangled in the Stark coefficients. A perturbational approach makes it possible to analyze the problem at increasingly higher levels of approximation and to disentangle the different contributions, according to the different time scales involved. This simple model reproduces most experimental values to within a few percent, even for these highly vibrationally excited levels, and gives valuable insight into the complex rotational and vibrational motions that link the dipole moment surface with the Stark coefficients.

Formation of neutral methylcarbamic acid (CH3NHCOOH) and methylammonium methylcarbamate [CH3NH3+][CH3NHCO2-] at low temperature.

We study low temperature reactivity of methylamine (CH3NH2) and carbon dioxide (CO2) mixed within different ratios, using FTIR spectroscopy and mass spectrometry. We report experimental evidence that the methylammonium methylcarbamate [CH3NH3(+)][C3NHCO2(-)] and methylcarbamic acid (CH3NHCOOH) are formed when the initial mixture CH3NH2:CO2 is warmed up to temperatures above 40 K. An excess of CH3NH2 favors the carbamate formation while an excess of CO2 leads to a mixture of both methylammonium methylcarbamate and methylcarbamic acid. Quantum calculations show that methylcarbamic acid molecules are associated into centrosymmetric dimers. Above 230 K, the carbamate breaks down into CH3NH2 and CH3NHCOOH, then this latter dissociates into CH3NH2 and CO2. After 260 K, it remains on the substrate a solid residue made of a well-organized structure coming from the association between the remaining methylcarbamic acid dimers. This study shows that amines can react at low temperature in interstellar ices rich in carbon dioxide which are a privileged place of complex molecules formation, before being later released into "hot core" regions.

Dipole moments of HDO in highly excited vibrational states measured by Stark induced photofragment quantum beat spectroscopy.

We report here a measurement of electric dipole moments in highly vibrationally excited HDO molecules. We use photofragment yield detected quantum beat spectroscopy to determine electric field induced splittings of the J=1 rotational levels of HDO excited with 4, 5, and 8 quanta of vibration in the OH stretching mode. The splittings allow us to deduce mua and mub, the projections of dipole moment onto the molecular rotation inertial axes. We compare the measured HDO dipole moment components with the results of quantitative calculations based on Morse oscillator wave functions and an ab initio dipole moment surface. The vibrational dependence of the dipole moment components reflect both structural and electronic changes in HDO upon vibrational excitation; principally the vibrational dependence of the O-H bond length and bond angle, and the resulting change in orientation of the principal inertial coordinate system. The dipole moment data also provide a sensitive test of theoretical dipole moment and potential energy surfaces, particularly for molecular configurations far from equilibrium.

Laser spectroscopy of Si3C.

The C 1B1<--X 1A1 band system of the potential interstellar species Si3C has been recorded in a silane/acetylene discharge by resonant two-color two-photon ionization spectroscopy. The origin band is located near 24,925 cm-1 (3.09 eV). Several other features in the spectrum are assigned to progressions in the Si-Si stretching modes as well as to sequence and hot band transitions. The assignment was facilitated by ab initio calculations, which also indicate that this is the strongest electronic transition of Si3C in the visible region of the spectrum. Features in the spectrum are broadened considerably (ca. 10 cm-1), and suggest an excited state lifetime of a few picoseconds. Possible reasons for the short-lived nature of the excited state are discussed.

Dipole moments of highly vibrationally excited water.

The intensity of water absorption in the region of the solar spectrum plays a dominant role in atmospheric energy balance and hence strongly influences climate. Significant controversy exists over how to model this absorption accurately. We report dipole moment measurements of highly vibrationally excited water, which provide stringent tests of intensities determined by other means. Our measurements and accompanying calculations suggest that the best currently available potential and dipole surfaces do not accurately model intensities in the optical spectrum of water.