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

¹ Intitut de Physique Nucléaire d’Orsay, IN2P3-CNRS and Université Paris-Sud, 91406 Orsay Cedex, France
e-mail: chabot@ipno.in2p3.fr
² Institut des Sciences Moléculaires d’Orsay, CNRS and Université Paris-Sud, 91405 Orsay Cedex, France
³ Centre de Spectroscopie Nucléaire et de Spectrométrie de Masse, Université Paris-Sud and IN2P3-CNRS, 91405 Orsay Cedex, France
⁴ Centre d’Étude Spatiale des Rayonnements, Université Paul Sabatier, UMR5187, 31028 Toulouse Cedex 9, France
⁵ Laboratoire de Chimie Physique, Université Paris-Sud and CNRS, 91405 Orsay Cedex, France
⁶ Laboratoire Univers et Théories, CNRS et Observatoire de Paris, Place J. Janssen, 92195 Meudon Cedex, France
⁷ Université de Bordeaux, Observatoire Aquitain des Sciences de l’Univers, BP 89, 33271 Floirac Cedex, France
⁸ CNRS, UMR 5804, Laboratoire dAstrophysique de Bordeaux, BP 89, 33271 Floirac Cedex, France

Received: 18 May 2010
Accepted: 3 August 2010

Abstract

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 C_n, C_nH or C₃H₂.

Aims. Our aim is to provide astrochemical databases with more realistic branching ratios for C_n (n = 2 to 10), C_nH (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.

Methods. High velocity collision in an inverse kinematics scheme was used to measure the complete fragmentation pattern of electronically excited C_n (n = 2 to 10), C_nH (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.

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 C_n and hydrocarbons at a visual extinction A_V 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).