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

摘要

Fragmentation branching ratios of electronically excited molecular speciesare of first importance for the modeling of gas phase interstellar chemistry.Despite experimental and theoretical efforts that have been done during thelast two decades there is still a strong lack of detailed information on thosequantities for many molecules such as Cn, CnH or C3H2. Our aim is to provideastrochemical databases with more realistic branching ratios for Cn (n=2 to10), CnH (n=2 to 4), and C3H2 molecules that are electronically excited eitherby dissociative recombination, photodissociation, or cosmic ray processes, whenno detailed calculations or measurements exist in literature. High velocitycollision in an inverse kinematics scheme was used to measure the completefragmentation pattern of electronically excited Cn (n=2 to 10), CnH (n=2 to 4),and C3H2 molecules. Branching ratios of dissociation where deduced from thoseexperiments. The full set of branching ratios was used as a new input inchemical models and branching ratio modification effects observed inastrochemical networks that describe the dense cold Taurus Molecular Cloud-1and the photon dominated Horse Head region. The comparison between thebranching ratios obtained in this work and other types of experiments showed agood agreement. It was interpreted as the signature of a statistical behaviorof the fragmentation. The branching ratios we obtained lead to an increase ofthe C3 production together with a larger dispersion of the daughter fragments.The introduction of these new values in the photon dominated region model ofthe Horse Head nebula increases the abundance of C3 and C3H, but reduces theabundances of the larger Cn and hydrocarbons at a visual extinction Av smallerthan 4.