Reactions forming C(0,+)n=2,10, Cn=2,4H(0,+) and C3H(0,+) in the gas phase: semi empirical branching ratios

The aim of this paper is to provide a new set of branching ratios for interstellar and planetary chemical networks based on a semi empirical model. We applied, instead of zero order theory (i.e. only the most exoergic decaying channel is considered), a statistical microcanonical model based on the construction of breakdown curves and using experimental high velocity collision branching ratios for their parametriza- tion. We applied the model to ion-molecule, neutral-neutral, and ion-pair reactions implemented in the few popular databases for astrochemistry such as KIDA, OSU and UMIST. We studied the reactions of carbon and hydrocarbon species with electrons, He+, H+, CH+, CH, C, and C+ leading to intermediate complexes of the type Cn=2,10, Cn=2,4 H, C3 H2, C+n=2,10, Cn=2,4 H+, or C3 H+2 . Comparison of predictions with measurements supports the validity of the model. Huge deviations with respect to database values are often obtained. Effects of the new branching ratios in time dependant chemistry for dark clouds and for photodissociation region chemistry with conditions similar to those found in the Horsehead Nebula are discussed

Relaxation of the 4D3/2 and 4D5/2 levels of sodium atom perturbed by noble gases

The sodium atom is excited selectively in the 4D3/2 or 4D 5/2 level using Doppler-free two-photon absorption. We analyse the fluorescence in intensity and polarization in presence of noble gas atoms. We measure the cross-sections corresponding to the transfer of population between 4D3/2 and 4D5/2 and between 4D and 4F, the relaxation of alignment in both fine sublevels and the transfer of alignment. In particular, we have observed a transfer of alignment in the case of helium and neon perturbers. These results are compared with semi-classical calculations performed using the potentials of Pascale and Vandeplanque.L'atome de sodium est excité sélectivement dans le niveau 4D3/2 ou le niveau 4D5/2 par la méthode d'absorption à deux photons sans élargissement Doppler. Nous analysons la fluorescence en intensité et en polarisation en présence de gaz rares. Nous en déduisons les sections efficaces correspondant au transfert de population entre 4D3/2 et 4D5/2 et entre 4D et 4F, la relaxation de l'alignement dans chacun des sous-niveaux ainsi que le transfert d'alignement. Nous avons d'ailleurs observé un transfert d'alignement dans le cas de collisions contre l'hélium et le néon. Ces résultats sont comparés à un calcul semi-classique utilisant les potentiels de Pascale et Vandeplanque

Relaxation of the sodium atom in the 4D level by collisions with noble gases. II. Theoretical aspects

In previous papers [1-3] we have reported experimental measurements of the relaxation rates of the k-multipole moments in the 4D level of sodium and the 3S-4D optical coherence. In the present paper we compare the experimental values with those obtained theoretically for different types of potentials. We show that the Van der Waals C6R -6 interaction is not sufficiently anisotropic to describe the relaxation of the k multipole moments. For Ar, Kr and Xe, much better results are obtained by taking into account the next term C8R-8 in the expansion of the long-range interaction. The potentials of Pascale and Vandeplanque [4] give very good results for helium. The results are also satisfactory for Ar, Kr and Xe, but the agreement could still be improved.Dans des articles précédents [1-3], nous avons donné les résultats de mesures expérimentales sur les taux de relaxation des multipôles d'ordre k dans le niveau 4D du sodium et pour la cohérence 3S-4D. Ici, nous comparons ces valeurs expérimentales avec les valeurs théoriques que nous calculons à partir de différents types de potentiel. Nous montrons que l'interaction de Van der Waals en C6R-6 n'est pas assez anisotrope pour décrire la relaxation des moments multipolaires d'ordre k. Pour l'argon, le krypton et le xénon, on obtient de bien meilleurs résultats en tenant compte du terme suivant en C8R -8 dans le développement de l'interaction à longue distance. Les potentiels de Pascale et Vandeplanque [4] donnent de très bons résultats pour l'hélium. Les résultats sont également satisfaisants pour l'argon, le krypton et le xénon, mais l'accord pourrait encore être amélioré

Relaxation of the sodium atom in the 4D level by collisions with noble gases. I. Experimental aspects

The sodium atom is excited selectively in the 4D3/2 or 4D 5/2 level using Doppler-free two-photon absorption. We analyse the fluorescence in intensity and polarization in presence of noble gas atoms. We measure the cross-sections corresponding to the transfer of population between 4D3/2 and 4D5/2 and between 4D and 4F, the relaxation of alignment in both fine sublevels and the transfer of alignment. In particular, we have observed a transfer of alignment in the case of helium and neon perturbers.L'atome de sodium est excité sélectivement dans le niveau 4D3/2 ou le niveau 4D5/2 par la méthode d'absorption à deux photons sans élargissement Doppler. Nous analysons la fluorescence en intensité et en polarisation en présence de gaz rares. Nous en déduisons les sections efficaces correspondant au transfert de population entre 4D3/2 et 4D5/2 et entre 4D et 4F, la relaxation de l'alignement dans chacun des sous-niveaux ainsi que le transfert d'alignement. Nous avons d'ailleurs observé un transfert d'alignement dans le cas de collisions contre l'hélium et le néon

Doppler-free two-photon spectroscopy of neon. III. Isotopic shift and fine structure for the 2p5 4d and 2p5 5s configurations

Using Doppler-free two-photon spectroscopy, we have measured the isotopic shift 20Ne-22Ne for two-photon transitions between the 3s[3/2]2 metastable state and the excited states of the 2p5 4d and 2p5 5s configurations. The results are analyzed using the specific mass shift theory. We show that we can deduce from the expérimental data the expansion of the 1P Russel-Saunders term over the true eigenstates of the atom. We also obtain experimental determinations of the radial integrals J(4d, 2p) and J(5s, 2p). All the experimental results are in good agreement with theoretical predictions of Bauche and Liberman.En utilisant la spectroscopie d'absorption à deux photons sans élargissement Doppler, nous avons mesuré l'écart isotopique 20Ne-22Ne pour des transitions reliant le niveau métastable 3s[3/2]2 et les niveaux excités des configurations 2p5 4d et 2p5 5s. Les résultats sont analysés en utilisant la théorie du déplacement de masse spécifique. Nous montrons que nous pouvons déduire des données expérimentales un développement du terme 1P de Russel-Saunders sur les états propres du hamiltonien atomique. Nous obtenons également des déterminations expérimentales des intégrales radiales J(4d, 2p) et J(5s, 2p). Les résultats expérimentaux sont en bon accord avec des prévisions théoriques de Bauche et Liberman

Anion production in high-velocity cluster-atom collisions; the electron capture process revisited

Anion production cross sections in collisions between C-n(+), Cn carbon clusters (n <= 5) and helium atoms have been measured in high-velocity collisions (v = 2.25 and 2.6 au). This paper focuses on two of the three processes responsible for the C-n(-) production, namely double electron capture (DEC) onto C-n(+) cations and single electron capture onto neutral (SECN) C-n. They were experimentally distinguished from a gaseous thickness dependence study. Dissociative and non-dissociative cross sections were measured and, in the case of DEC, all dissociative branching ratios measured; for these small systems, the C-2(-) fragment was found magical. Data concerning electron capture in neutral-neutral collisions are extremely rare, especially at high velocity. Introduction of this measured process in the independent atom and electron (IAE) model allowed us to revisit and satisfactorily reproduce the so-far unexplained size evolution of single electron capture (SEC) cross sections in 2.6 au C-n(+)-He (n <= 10) collisions (Chabot et al 2006 J. Phys. B: At. Mol. Opt. Phys. 39 2593-603). IAE calculations for DEC cross sections and their comparison with experiment suggest a loss of electron in anionic C-n(-) species after the collision, competing with fragmentation and depending on the size

Statistical universal branching ratios for cosmic ray dissociation, photodissociation, and dissociative recombination of the C(n=2-10), C(n=2-4)H and C3H2 neutral and cationic species

Accepted to Astronomy & AstrophysicsInternational audienceFragmentation branching ratios of electronically excited molecular species are of first importance for the modeling of gas phase interstellar chemistry. Despite experimental and theoretical efforts that have been done during the last two decades there is still a strong lack of detailed information on those quantities for many molecules such as Cn, CnH or C3H2. Our aim is to provide astrochemical databases with more realistic branching ratios for Cn (n=2 to 10), CnH (n=2 to 4), and C3H2 molecules that are electronically excited either by dissociative recombination, photodissociation, or cosmic ray processes, when no detailed calculations or measurements exist in literature. High velocity collision in an inverse kinematics scheme was used to measure the complete fragmentation pattern of electronically excited Cn (n=2 to 10), CnH (n=2 to 4), and C3H2 molecules. Branching ratios of dissociation where deduced from those experiments. The full set of branching ratios was used as a new input in chemical models and branching ratio modification effects observed in astrochemical networks that describe the dense cold Taurus Molecular Cloud-1 and the photon dominated Horse Head region. The comparison between the branching ratios obtained in this work and other types of experiments showed a good agreement. It was interpreted as the signature of a statistical behavior of the fragmentation. The branching ratios we obtained lead to an increase of the C3 production together with a larger dispersion of the daughter fragments. The introduction of these new values in the photon dominated region model of the Horse Head nebula increases the abundance of C3 and C3H, but reduces the abundances of the larger Cn and hydrocarbons at a visual extinction Av smaller than 4