Denis Duflot

Dissociations of the ethyne dication C2H2+2

Dissociations of the ethyne dication following its production by photoionization in the photon energy range of 35–65 eV have been investigated by the photoelectron–ion–ion coincidence technique using both synchrotron radiation and laboratory light sources. New quantum mechanical calculations identify and locate the electronic states of the molecular dication in this energy range and show that the dissociation products are formed in their ground states by heterogeneous processes. Five reaction channels leading to three molecular fragments have been identified and are interpreted as sequential processes, several faster than fragment rotation and one possibly involving dissociation of CH+ to H+ with a lifetime of the order of 25 fs.

Water ice deuteration: a tracer of the chemical history of protostars

Context. Millimetric observations have measured high degrees of molecular deuteration in several species seen around low-mass protostars. The Herschel Space Telescope, launched in 2009, is now providing new measures of the deuterium fractionation of water, the main constituent of interstellar ices. Aims: We aim at theoretically studying the formation and the deuteration of water, which is believed to be formed on interstellar grain surfaces in molecular clouds. Methods: We used our gas-grain astrochemical model GRAINOBLE, which considers the multilayer formation of interstellar ices. We varied several input parameters to study their impact on water deuteration. We included the treatment of ortho- and para-states of key species, including H 2 , which affects the deuterium fractionation of all molecules. The model also includes relevant laboratory and theoretical works on the water formation and deuteration on grain surfaces. In particular, we computed the transmission probabilities of surface reactions using the Eckart model, and we considered ice photodissociation following molecular dynamics simulations. Results: The use of a multilayer approach allowed us to study the influence of various parameters on the abundance and the deuteration of water. Deuteration of water is found to be very sensitive to the ortho-to-para ratio of H 2 and to the total density, but it also depends on the gas/grain temperatures and the visual extinction of the cloud. Since the deuteration is very sensitive to the physical conditions, the comparison with sub-millimetric observation towards the low-mass protostar IRAS 16293 allows us to suggest that water ice is formed together with CO 2 in molecular clouds with limited density, whilst formaldehyde and methanol are mainly formed in a later phase, where the condensation becomes denser and colder. Appendices are available in electronic form at http://www.aanda.org

Ab initio study of the acetylene and vinylidene dications fragmentation

The C2H++2 fragmentation processes have been studied using the complete active space self‐consistent field method followed by a multireference perturbative configuration interaction, in order to interpret recent charge separation spectroscopy experiments. For two‐body processes, the calculated appearance thresholds of the C2H+/H+ and CH+/CH+ fragment pairs are in good agreement with the experimental data. It is shown that the C2H++2→CH++CH+ dissociation occurs with an important rotation of the CH+ ions. The presence of the CH+2 ion is explained by a preliminary isomerization of acetylene to vinylidene dication. This reaction has been studied for the lowest lying states of C2H++2 (3Σ−g and 1Δg) and compared with other acetylenic ions isomerizations (C2H2, C2H+2, C2H−2). For three‐body processes, the calculations are consistent with the mechanisms proposed by the experimentalists.

The core excitation of pyridine and pyridazine: An electron spectroscopy and ab initio study

The carbon and nitrogen K-shell excitation spectra of gaseous pyridine and pyridazine were recorded using the electron-energy loss spectroscopy under electric-dipole conditions (2 keV, small angle) with a resolution of 0.2 eV. Ab initio Configuration interaction calculations in the frame of the equivalent core model were performed in order to help in the assignment of the spectral features. The spectra are dominated by the transitions to the 1π* and σ* type orbitals. The C1s spectra of both molecules are close to that of benzene: The intensity of Rydberg transitions are enhanced by an important valence σC–H* character; the 1s→3π* transition is mixed with double excitations and give rise to several states, some of them lying above the ionization thresholds. Finally, the N1s spectra of both molecules are similar to the s-triazine one.

Ab initio potential energy surfaces for C2H→C2+H photodissociation

The C2H photodissociation processes have been studied using ab initio multireference perturbative configuration interaction methods. The calculated structural parameters of the two linear lowest lying states X 2Σ+ and A 2Π were found to be in good agreement with previous theoretical and experimental works. Construction of the correlation diagram indicates that C2H cannot photodissociate into C+CH. Therefore, two‐dimensional potential energy surfaces of the six lowest lying A’ and A‘ doublet states have been calculated as functions of the bond angle and the C–H bond distance, keeping the C–C distance at the equilibrium C2H value. It appears that the 1 2A’, 2 2A’, and 1 2A‘ states correlating to the X 2Σ+ and A 2Π states in linear geometry remain bound at all angles, whereas the 3 2A’, 4 2A’, and 4 2A‘ are dissociative. However, the structure of the energy surfaces is complicated by the presence of numerous avoided crossings between these states. Transition dipole moments connecting the excited states with ...

Theoretical Investigation of the Isomerization of trans-HCOH to H2CO: An Example of a Water-Catalyzed Reaction

A concerted hydrogen atom transfer mechanism has been elucidated for the isomerization of trans-HCOH to H(2)CO using a variety of ab initio and density functional theory methods. This work places specific emphasis on the role water molecules can play as a catalyst for this reaction and the mechanism by which this is achieved. This is of particular importance in the context of molecular ices in the interstellar medium because the presence of water in this reaction reduces the activation energy by at least 80%, which is accompanied by a significant enhancement of the reaction rate, at ≤300 K.

Electronic excitation of gaseous pyrrole and pyrazole by inner-shell electron energy loss spectroscopy

The spectroscopy of the unoccupied molecular orbitals of gaseous pyrrole and pyrazole has been studied by electron impact under electric dipole scattering conditions (2 KeV, small angle) and the inner-shell electron energy loss method. The core-excitation spectra have been recorded at the C1s and N1s edges with 0.2 eV resolution, resolution allowing to observe separately the transitions from the carbon atoms with different chemical environment. Ab initio calculations, using the equivalent core model, were performed to help in the interpretation of the spectral features. The spectra are dominated by transitions to the π* and σ* molecular orbitals. The calculations confirm that the 3b1(π*) orbital is the lowest-energy unoccupied one in pyrrole. In several cases, the intensity of the Rydberg excitations is enhanced by an important valence σ* character of the Rydberg orbitals.

Limonene: electronic state spectroscopy by high-resolution vacuum ultraviolet photoabsorption, electron scattering, He(I) photoelectron spectroscopy and ab initio calculations

Electronic state spectroscopy of limonene has been investigated using vacuum ultraviolet photoabsorption spectroscopy in the energy range 5.0-10.8 eV. The availability of a high resolution photon beam (~0.075 nm) enabled detailed analysis of the vibrational progressions and allowed us to propose, for the first time, new assignments for several Rydberg series. Excited states located in the 7.5-8.4 eV region have been studied for the first time. A He(I) photoelectron spectrum has also been recorded from 8.2 to 9.5 eV and compared to previous low resolution works. A new value of 8.521 ± 0.002 eV for the ground ionic state adiabatic ionisation energy is proposed. Absolute photoabsorption cross sections were derived in the 10-26 eV range from electron scattering data. All spectra presented in this paper represent the highest resolution data yet reported for limonene. These experiments are complemented by new ab initio calculations performed for the three most abundant conformational isomers of limonene, which we then used in the assignment of the spectral bands.

Core shell excitation of 2-propenal (acrolein) at the O 1s and C 1s edges: An experimental and ab initio study

The carbon and oxygen K-shell spectra of gaseous 2-propenal (acrolein) have been measured using the inner-shell electron energy loss spectroscopy method. Large scale ab initio configuration interaction calculations have been carried out to enable firm assignments of the observed bands. The overall shapes of the spectra are similar to previous low resolution monolayer and multilayer phases NEXAFS spectra recorded by photoabsorption of synchrotron radiation, but the spectral bands are much better resolved than the earlier ones. The spectra are dominated by excitation of π* type states and by interaction between the C=C and C=O π* orbitals.

The H + CO ⇌ HCO reaction studied by ab initio benchmark calculations

The title reaction has been calculated using complete active space self-consistent field and internally contracted multi-reference configuration interaction, including Davidson correction, calculations. Dunnings correlation consistent atomic basis sets, together with several complete basis set extrapolation schemes, were employed. Core-valence and scalar relativistic effects were also taken into account, as well as anharmonicity of the vibrational modes. Core-valence correlation appears to have a large impact on the calculated frequencies, spectroscopic constants, and on the energetics. In particular, the best estimate for the HCO (DCO) formation barrier height at 0 K, 4.54 ± 0.14 (4.43 ± 0.14) kcal mol(-1) is larger than previous theoretical works and well above the usually accepted value of 2.0 ± 0.4 kcal mol(-1), measured at room temperature. Inclusion of temperature and entropy at 298 K does not seem to be able to solve this discrepancy. The present theoretical barrier height is therefore the recommended value. The exo-ergicity of the HCO (DCO) dissociation reaction, predicted to be -13.36 ± 0.57 (-14.72 ± 0.57) kcal mol(-1), is slightly below the experimental value. Finally, all tested density functionals fail to reproduce accurately both the formation and dissociation barriers.