Abstract
Abstract We present calculated cross sections and rate coefficients for the formation of the dicarbon cation ( ) by the radiative association process in collisions of a atom and a ion. Molecular structure calculations for a number of low-lying doublet and quartet states of are used to obtain the potential energy surfaces and transition dipole moments coupling the states of interest, substantially increasing the available molecular data for . Using a quantum-mechanical method, we explore a number of allowed transitions and determine those contributing to the radiative association process. The calculations extend the available data for this process down to the temperature of 100 K, where the rate coefficient is found to be about . We provide analytical fits suitable for incorporation into astrochemical reaction databases.
Highlights
The dicarbon cation C+2 is an important molecule in astrochemistry, as it is one of the species participating in hydrocarbon chemistry in, for example, interstellar clouds (Solomon & Klemperer 1972) and photon-dominated regions (Guzmán et al 2015)
The potential energy curves (PECs) and transition dipole moment (TDM) are calculated for a set of low-lying doublet and quartet electronic states that enter into the radiative association calculations
A more detailed analysis of the electronic state configurations and their effect on the TDMs should be carried out for spectroscopic applications, such as for calculations of band oscillator strengths. Such an analysis is unnecessary for our purposes, because, as we will show, the most important factors leading to significant radiative association cross sections are the lack of a barrier in the PEC and a significant TDM at large R
Summary
The dicarbon cation C+2 is an important molecule in astrochemistry, as it is one of the species participating in hydrocarbon chemistry in, for example, interstellar clouds (Solomon & Klemperer 1972) and photon-dominated regions (Guzmán et al 2015). We consider the formation of the dicarbon cation (C+2 ) by the radiative association process in collisions of a carbon atom and a carbon ion, C(3P) + C+(2Po) C2+ + hn. The process in Equation (1) is a mechanism for dicarbon cation formation, which is viable in astrochemical environments because of the applicability of two-body kinetics and because of the abundance of carbon. The rate coefficients for Equation (1) were calculated by Andreazza & Singh (1997) using a semi-classical description of the collisions in which the atom and ion approach in the quartet B 4S-u state, yielding a rate coefficient of about 3 ́ 10-18 cm s-1 at 300 K. We present calculations for a number of molecular states of C+2 , cross sections and rate coefficients for the process in Equation (1), discuss the present rate coefficients and the earlier determination, and consider in more detail the potential significance of Equation (1)
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