Low‐Temperature Experiments on the Formation of Deuterated C3H+3

Abstract

Manydeuteratedmoleculeshavebeendiscoveredininter-andcircumstellarregions.Insomecases,theobserved abundances can be explained with simple thermodynamic models; often, however, isotope enrichment is more complicated. This has been seen recently in detailed low-temperature experiments performed for the fundamental systemsH þ =H2D þ andCH þ =CHn� 1D þ .An unsolvedproblem is toexplainthelarge abundanceofC3H2andlarger hydrocarbons and their deuterated variants observed in cold, dark interstellar clouds. In this work a variabletemperature22poletrapisutilizedforcloselyscrutinizingvariousion-moleculereactionsthatmaycontributetothe formation of C3H2D þ or C3HD þ and, via dissociative recombination, C3HD. The experimental study of the promising candidate C3H þ þ HD ! C3H2D þ þ H2, which has already been excluded by theory, corroborates that this exothermic H-D exchange does not occur at all. A careful analysis of the data reveals that the 15 K rate coefficient is smaller than 4 ; 10 � 16 cm 3 s � 1 . In contrast, quite efficient routes have been found in the lowtemperature experiments, starting with C þ and proceeding via deuterated C3H þ to C3H þ and C3H þ . Formation of C3D þ in C þ þ HD collisions is 6 times faster than assumed in astrochemical models (k ¼ 9:3 ; 10 � 10 cm 3 s � 1 ). Surprisingly, direct production of C3HD þ via radiative association has also been observed (kr ¼ 6:0 ; 10 � 11 cm 3 s � 1 ).ReactionsofpartlyorfullydeuteratedC3H þ þ H2 collisionsystemarestronglydependentontemperatureand thehydrogenorthotopararatio.Inaddition,itshowsverycomplicatedisotopeeffects.Forexample,inC3H þ þ HD collisions the formation of C3HD þ (k ¼ 4:6 ;10 � 10 cm 3 s � 1 ) dominates over the H-D exchange (k ¼ 5:6 ; 10 � 11 cm 3 s � 1 ) and radiative association C3H2D þ (kr ¼ 3:2 ;10 � 11 cm 3 s � 1 ). The reactions of C3H þ þ H2 are very slow for all isotope combinations. Although several questions remain open and more low-temperature experiments are needed, it is recommended that the new values are included in astrochemical databases, since they are fundamental to the correct description of the carbon chemistry in interstellar clouds.