G. Martinet

Statistical universal branching ratios for cosmic ray dissociation, photodissociation, and dissociative recombination of the C, CH and C3H2 neutral and cationic species

Context. Fragmentation-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 ,C n Ho r C 3H2. Aims. Our aim is to provide astrochemical databases with more realistic branching ratios for Cn (n = 2t o 10), Cn H( n = 2t o 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. Methods. High velocity collision in an inverse kinematics scheme was used to measure the complete fragmentation pattern of electronically excited Cn (n = 2 to 10), Cn H( n = 2t o 4), and C 3H2 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. Results. 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. Conclusions. We recommend astrochemists to use these new branching ratios. The data published here have been added to the online database KIDA (KInetic Database for Astrochemistry, http://kida.obs.u-bordeaux1.fr).

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

Anion production cross sections in collisions between Cn+, 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 Cn− production, namely double electron capture (DEC) onto Cn+ cations and single electron capture onto neutral (SECN) Cn. 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 C2− 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 Cn+–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 Cn− species after the collision, competing with fragmentation and depending on the size.

Fragmentation of multiply-charged small hydrocarbon molecules in CnHq+ (n = 1-3, q = 2-6) produced in high velocity collisions: Branching Ratios and associated Kinetic Energy Releases of the H+ fragment

Dissociation Branching Ratios of CnHq+ molecules formed by multi-ionization of incident CnH+ projectiles colliding in high velocity collisions with a Helium atom have been measured. In addition, the KER of the H+ fragment for each channel was extracted. A striking feature that we obtained is the fact that the KER (Kinetic Energy Release) is always far below predictions of the point charge coulomb model (PCCM) even at large q values. For CHq+, we could explain this result on the basis of electronic state calculations and taking into account the fact that 1s ionization of the carbon atom occurs and has its own dynamics.

Ion-pair dissociation of highly excited carbon clusters, size and charge effects

Ion-pair dissociation of a highly excited molecule is a relaxation process giving rise to emission of anionic and cationic fragments. We present first measurements of ion-pair dissociation of carbon clusters. We found that ion- pair relaxation is an ubiquitous, although very small, relaxation channel common to all sizes and charges of Cq+n species produced in high velocity C+n-He collisions. Quantitative interpretation of measured branching ratios is conducted on the basis of a statistical approach i.e through listing of all possible final states.